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UNIX Unleashed, Internet Edition

- 2 -

Graphical User Interfaces for Programmers

by Cameron Laird and Kamran Husain

The purpose of this chapter is to educate in a number of ways. This chapter is first a tutorial in writing small Motif applications; after working through it, you will be comfortable creating your own Graphical User Interfaces (GUIs). More than that, though, the exercise of developing Motif applications is good experience for engineering any contemporary user interface in the WIMP (Windows, Icons, Menus, and Pointer) category. In fact, one of us generally advises that there are superior alternatives to Motif for most specific uses in day-to-day programming, for technical reasons described below. Even when Motif is not the best choice for a particular development project, though, the knowledge of how to use it is very valuable for individual developers, as its concepts re-appear in so many other technologies.

A secondary benefit of this chapter is that it provides a summary reference to the books and vendors that are likely to be important to UNIX GUI builders.

The highlights of this chapter are the opportunity to learn:

  • How to write a correct Motif program

  • Use of specific common Motif widgets, including Buttons, Text displays, Scrollbars, Menus, Dialogs, drawing areas, and more

  • Naming conventions of X and Motif

  • Elements of designing layouts for user interfaces

  • How to program mouse events

  • Computer models for fonts and colors

  • Which technologies extend or replace Motif to meet specific requirements

A Range of Technologies

Describing GUIs relevant to UNIX is an enormous task, because there are many of them, each with its own commercial and technical history, portability, programming model, ancillary GUI builders, and other dimensions. This chapter mentions a few of the highlights, and invites interested readers to pursue the references at the end of this chapter.

OSF/Motif is a collection of guidelines for user interfaces that the Open Software Foundation (http://www.osf.org) first released in 1989. It is a "look and feel" built on top of the X Toolkit (Xt) mechanisms. While Xt is freely available from the X Consortium (http://www.x.org), OSF charges licensing fees for Motif. Motif is available for essentially all UNIXes, and also for OpenVMS and Windows NT. Its natural bindings are to C.

An API (Application Programming Interface) for the Microsoft Foundation Classes is available for most UNIX releases from Bristol (http://bristol.com). There are few technical reasons to begin developing with MFC under UNIX, but this can be an important technology for organizations already strong in MFC.

Tk ( http://sunscript.sun.com/TclTkCore/) provides an interesting alternative to Motif. Its portability is unsurpassed, as it is freely available for UNIX, Open VMS, Mac OS, Windows 95, Windows NT, Amiga DOS, OS/2, and several specialized OSes. Bindings are available not only for Tcl, the language with which it was first released, but also C (see Chapter 6, "The C and C++ Programming Languages)," Perl (see Chapter 5, Perl"), Python, LUA, Limbo, and Java. It has demonstrated robustness and performance that are at least comparable to those of Motif, and is much easier to use for beginners.

Java (http://java.sun.com) evangelists promote Java's Abstract Windowing Toolkit as the GUI technology of the future. As of Summer 1997, though, it is less portable, less stable, less robust, and more tedious to use than the previously mentioned alternatives. Many capable people are working on Java, and these faults in AWT are all likely to be corrected. Its re-entrance, thread-safety, and strong object-oriented programming model should serve it well in the future.

These four approaches illustrate how diverse the possibilities are among no-cost or bundled GUIs; commercial products for UNIX, including XVT, Neuron Data, SL-GMS, MetaCard, and many more, multiply the complexity. It's important to understand that many of these products co-exist. They have overlapping functionality, but also unique differentiators. There are products specially designed to support particular networking protocols, or mainframe hosting, or portability requirements, or a multitude of other possibilities. This overwhelming complexity reinforces the approach of this chapter: to teach the first few steps of Motif engineering thoroughly and understandably, so that readers have a solid foundation from which to launch their own explorations and accomplishments.

Writing Motif Applications

This chapter introduces event-driven programming. A principal milestone in learning Motif programming is understanding event-driven models. A conventional C application starts its control flow with main() and continues, more or less sequentially, through to an exit(). When it needs information, it seeks it from such sources as a file or the keyboard, and gets the information almost as soon as it asks for it. In event-driven programming, in contrast, applications are executed on an asynchronous basis. The order and time of arrival of events are not determined by the programmer. Instead, the application waits for an event to occur and then proceeds based on that event.

X Window programming relies on an input queue of events. Execution of an application is a sequence of responses to the events which appear in the input queue. It's conventional, in the X world, to see this as an instance of client-server programming: servers wait to receive events from clients, then report results of those events back to their clients.

X implements this client-server functionality in three layers. In the middle, the X Toolkit Intrinsics, abbreviated Xt, is an object-oriented C API that manages abstract notions of event-handling and widget interaction. Widgets are user-interface building blocks. Practical construction of GUIs is invariably done in a layer built atop Xt; Motif, Athena, Tk, and other development APIs are sometimes called widget sets, because they define the collections of buttons, labels, drawing areas, and so on, that are useful in building applications. There is also a layer below Xt: Xlib is the API to the X11 communications protocol. This chapter will have little need to refer to Xlib.

Typical Xt applications run in an endless event loop of events and responses to events. An application launches itself into this loop by calling the function XtAppMainLoop(). The fundamental role of the main body of the application is to wait for events, and either respond to them or, as is most often the case, dispatch them on to widgets that are equipped to respond.

Widgets are programming objects with knowledge about how to respond to events. Widgets typically have graphical behavior--they show up as pictures on a screen--and implement their event responses by registering a collection of callback functions. For example, some widgets redraw themselves when a Pointer button is clicked in their display area. To achieve this, they register a redraw callback function on a button click. Xt also supports actions, which allow applications to register a function with Xt. An action is called when one or more sequences of specific event types are received. For example, Ctrl+X might bind to the exit() function. It is one of the responsibilities of Xt to maintain a translation table that maps actions to events. Event handlers are these functions bound to specific events.

Naming Conventions

Most Xlib functions begin with the letter "X", but there are enough exceptions to confuse. X, like English spelling, is a technology with a history, and is comparably inconsistent. Several macros and functions, such as BlackColor and WhiteColor, do not begin with X. In general, if a name in Xlibrary begins with X, it's a function. If a name begins with any other capital letter, it's a macro.

In Xt proper, naming conventions show more deliberate engineering, with few exceptions. Xt does not distinguish macros from functions.


TIP: Do not rely on the name of a toolkit function to tell you whether it's a macro or not. Read the manual.

In Motif, almost all declarations begin with Xm. XmC refers to a class. XmR refers to a resource. XmN refers to a name. XtN refers to Xt resources used by Motif.

In Motif, declarations ending with the words WidgetClass define the base class for a type of widget. Other conventions to remember about parameters passed in most X library function calls are: width is always to the left of (or before) height. x is to the left of y. Source is to the left of destination. Display is usually the first parameter.

With practice, you will be able to identify the types of parameters to pass and which toolkit a function belongs to, and you'll be able to "guess" what parameters an unknown function might expect.

Writing Your First Motif Application

In just a few minutes more, you'll be a successful beginning Motif programmer. This section includes everything you need to generate from source code and execute your first application.

Preparing the Source

See 2_1.c on the CD-ROM for a complete listing showing the basic format for a Motif application.

The listing shows an application in which a button attaches itself to the bottom of a form. No matter how you resize the window, the button will always be on the bottom. The application does the following:

  • Initializes the toolkit to get a Shell widget.

  • Makes a Form widget.

  • Manages all widgets as they are created.

  • Makes a Button widget and puts it on top of the Form widget.

  • Attaches a callback function to the button.

  • Realizes the widget (that is, makes the hierarchy visible).

  • Goes into its event loop.

Let's look at the application in more detail. The #include files in the beginning of the file are required for most applications. Note the following files:

#include <X11/Intrinsic.h>
#include <Xm/Xm.h>

These declare the definitions for XtIntrinsics and Motif, respectively. Some systems may not require the first inclusion, but it's harmless to put it in there because multiple inclusions of Intrinsic.h are permitted. In addition, each Motif widget requires its own header file. In Listing 2.1, the two widgets Form and PushButton require the following header files:

#include <Xm/Form.h>
#include <Xm/PushB.h>

The variables in the program are declared in the following lines:

Widget top;
XtAppContext app;
Widget aForm;
Widget aButton;
int    n;

The top, aForm, and aButton represent widgets. Notice that, while they are different types of widgets (application top, Form, and Button, respectively), it is conventional to code their references in C all in terms of Widget.

XtAppContext is an opaque type, which means that a Motif programmer does not have to be concerned about how the type is set up. Widgets are opaque types as well, because only the items that are required by the programmer are visible.

The first executable line of the program calls the XtAppInitialize() function. This initializes the Xt toolkit and creates an application shell and context for the rest of the application. This value is returned to the widget "top" (for top level shell). This widget provides the interface between the window manager and the rest of the widgets in this application.

The application then creates a Form widget on this top-level widget. A Form widget places other widgets on top of itself. It is a Manager widget because it manages other widgets. There are two steps for displaying a widget: managing it and realizing it.

Managing a widget allows it to be visible. If a widget is unmanaged, it will never be visible. By managing a widget, the program gives the viewing control over to the windowing system so it can display it. If the parent widget is unmanaged, any child widgets remain invisible even if managed.

Realizing a widget actually creates all the subwindows under an application and displays them. Normally only the top-level widget is realized after all the widgets are managed. This call realizes all the children of this widget.

Note that realizing a widget takes time. A typical program manages all the widgets except the topmost widget. This way the application only calls XtRealizeWidget on the topmost parent when the entire tree has to be displayed. You have to realize a widget at least once, but you can manage and unmanage widgets as you want to display or hide them.

In the past, the way to create and manage a widget was to call XtCreate and XtManageChild in two separate calls. However, this text uses the following single call to create and manage a widget:

XtVaCreateManagedWidget

Note the parameters to this call to create the Form widget:

aForm = XtVaCreateManagedWidget("Form1", xmFormWidgetClass, top, XmNheight,90, XmNwidth,200,
NULL);

The first parameter names the new widget. The second parameter describes the class of the widget being created. Recall that this is simply the widget name sandwiched between xm and WidgetClass. In this case, it's xmFormWidgetClass. Note the lowercase x for the class pointer. The Form.h header files declares this class pointer .


TIP: As another example, a label's class pointer would be called xmLabelWidgetClass and would require the Label.h file. Motif programmers have to be especially wary of the case-sensitivity of all variables and filenames.

The next argument is the parent widget of this new widget. In this case top is the parent of Form1. The top widget was returned from the call to XtAppInitialize.

The remaining arguments specify the parameters of this widget. In this case you are setting the width and height of this widget. This list is terminated by a NULL parameter.

After the form is created, a button is placed on top of it. A Form widget facilitates placement of other widgets on top of it. In this application, you will cause the button to "attach" itself to the bottom of the form.

The class definition of this button, xmPushButtonWidgetClass, appears in the PushB.h file. The name of this widget is also the string that is displayed on the face of the button. Note that the parent of this button is the aForm widget. Thus, the hierarchy is: top is the parent of aForm is the parent of aButton.

The next step is to add a callback function to respond when the button is pressed. This is done with the following call:

XtAddCallback( aButton, XmNactivateCallback, bye, (XtPointer) NULL);

In this call:

  • aButton is the pushbutton widget.

  • XmNactivateCallback is the action that will trigger this function.

  • bye is the name of the function called. You must declare this function before making this function call.

  • NULL is a pointer. This pointer could point to some structure meaningful to function bye.

This registers the callback function bye for the widget. Now the topmost widget, top, is realized. This causes all managed widgets below top to be realized. The application then goes into a forever loop while waiting for events.

The bye function of this program simply exits the application.

Generating an Executable Application

Read the compiler documentation for your machine. Almost all vendor-supplied compilers now conform to the ANSI C standard (see chapter 6 of this volume); the most notable exception is the thousands of SunOS hosts that have declined to migrate to Sun's supported OS, Solaris. If your compiler is not ANSI compatible, your likely choices are to

  • Upgrade some combination of your OS and development environment with commercial products.

  • Install copylefted gcc.

  • Rewrite the source code to match the expectations of your compiler.

The authors have used each of these expedients, in different circumstances.

Next, check the location of the libraries in your system. Check the /usr/lib/X11 directory for the following libraries: libXm.*, libXt.*, and libX11.*. Library architecture in the UNIX world has splintered over the last several years; there's no simple, correct way to describe how these files might appear on your system. The Xm library, for example, might be present as libXm.a, libXm.so, libXm.sl, libXm.so.1, or even under other names, or a combination of these. This divergence mostly bears on whether the libraries are to be linked statically or as shared objects. The most important fact to remember in this regard is that linking with "... -L/usr/lib/X11 -lXm -lXt -lX11 ..." is likely to generate a correct executable on any system, and that's where newcomers to Motif should begin. For a discussion of the subtleties of shared-linking, particularly in the context of deployment of an application to hosts other than a development machine, see Chapter 32, "SVR4 FAQs," and also the discussion in http://starbase.neosoft.com/%7Eclaird/comp.unix.programmer/linking-unix.html.

This command generates the executable application from the source:

CC list1.c -o list1 -LLIB -lXm -lXt -lX11

Substitute for CC your version of the ANSI compiler: gcc, acc, cc, or whatever, and for LIB substitute the name of the directory where you find your X libraries; this is likely to be /usr/lib/X11, /usr/openwin/lib, /usr/home/X11/lib, or some variant. It is best to create a makefile (see Chapter 7, "The make Utility," for an explanation of this tool), although this command is brief enough that those who prefer can invoke it from the command line.

The Elements of Motif Programming

Deep understanding of Motif requires knowledge of its widgets, events, drawing primitives, and font and color management. Complete references to these elements are published in thick, multi-volume books. It's possible, though, to present the essentials of what is necessary for simple applications in a single chapter; that's what we do here.

The Widget Hierarchy

The Motif widget set is a hierarchy of widget types. (See Figure 2.1.) Any resources provided by a widget are inherited by all its derived classes. Consider the three most important base classes: Core, XmPrimitive, and XmManager.

Figure 2.1
The widget hierarchy.

Abstract Classes Core

The Core widget class provides the basis for all classes. It provides at least the following variables:

  • XmNx, XmNy: This is a widget's position on the display.

  • XmNheight, XmNwidth: This is a widget's size.

  • XmNborderWidth: This is set to 1 by default.

  • XmNsensitive: This is a Boolean resource that specifies whether this widget can receive input.

  • XmNcolorMap: This is the default colormap.

  • XmNbackground: This is the background color.

Several of the Motif manuals listed as references detail these attributes completely.

XmPrimitive

The XmPrimitive widget class inherits all the resources from Core and adds more functionality.

  • XmNforeground: The foreground color.

  • XmNhighlightOnEnter: Changes color when the pointer is within the window of the widget.

  • XmNhighlightThickness: If XmNhighlightOnEnter is True, changes the border to this thickness.

  • XmNhighlightColor: The color to use when drawing the highlighted border.

  • XmNshadowThickness: The number of pixels used to draw the pseudo-3D look for which Motif is celebrated. This defaults to 2.

  • XmNtopShadowColor and XmNbottomShadowColor: Sets the color for top and bottom lines around a widget.

  • XmNuserData: A pointer available for use to the programmer.

The XmPrimitive widget also provides the XmNdestroyCallback resource. This can be set to a function that does clean-up when a widget is destroyed. In Motif 1.2 or later, the XmPrimitive class also provides a XmNhelpCallback resource that is called when the F1 key is pressed in the widget's window. This facilitates context-sensitive help information for a widget.

XmManager

The XmManager class provides support for all Motif widgets that contain other widgets. This is never used directly in an application and works in a similar manner to the XmPrimitive class.

Strings in Motif

A compound string is Motif's natural idiom of representing a string. For a typical C program, a null-terminated character array adequately represents a string. In Motif, a string is also defined by the character set it uses. Strings in Motif are referred to as compound strings and are kept in opaque data structures called XmString.

In order to get a compound string from a regular C string, use the following function call:

XmString XmStringCreate( char *text, char *tag);

This returns an equivalent compound string, given a pointer to a null-terminated C string and a tag. The tag specifies which font list to use and defaults to XmFONTLIST_DEFAULT_TAG.

New lines in C strings have to be handled by special separators in Motif. To create a string and preserve the new lines, use this call:

XmString XmStringCreateLtoR( char *text, char *tag);

The compound strings have to be created and destroyed just like any other object. They persist long after the function call that created them returns. Therefore, it's a good idea to free all locally used XmStrings in a function before returning, or else all references to the strings, and the memory they occupy, will be lost.

Here's the declaration of a call to free XmString resources:

XmStringFree( XmString s);

You can run operations on strings similar to those you would under ASCII programming, except that they are called by different names. Use Boolean XmStringByteCompare( XmString s1, XmString s2); for a strict byte-for-byte comparison. For just the text comparison, use XmStringCompare( XmString s1, XmString s2);.

To check if a string is empty, use the following:

Boolean XmStringEmpty( XmString s1);

To concatenate two strings together, use the following:

XmString XmStringConcat( XmString s1, XmString s2);

It creates a new string by concatenating s2 to s1. This new resource has to be freed just like s1 and s2.

If you want to use sprintf, use it on a temporary buffer and then create a new string. For example:

char str[32];
XmString xms;
......
sprintf(str," pi = %lf, Area = %lf", PI, TWOPI*r);
xms =  XmStringCreateLtoR( str,  XmFONTLIST_DEFAULT_TAG); ......
n = 0;
XtSetArg(arg[n],XmNlabelString,xms); n++; XtSetValues(someLabel, arg, n); XmStringFree(xms);

If a string value appears to corrupt itself over time, check to see if the widget is not making a copy of the passed XmString for its own use. Sometimes a widget may only be keeping a pointer to the XmString. If that string was "freed," the widget may wind up pointing to bad data.

One good way to check is to set an XmString resource. Then use the XtGetValues function to get the same resource from the widget. If the values of the XmStrings are the same, the widget is not making a copy for itself. If they aren't the same, it's safe to free the original because the widget is making a local copy. The default course of action is to assume that a widget makes copies of such resources for itself.

A similar test could be used to determine if a widget returns a copy of its resource to a pointer to it. Use the preceding listing, but this time use XTgetValues to get the same resource twice. Then do the comparison.

/**
*** This is a sample partial listing of how to check if the
*** data returned on an XtGetValues and an XtSetValues
*** call is a copy or a reference.
***/
#include "Xm/Text.h"
..
Widget w;
XmString x1, x2, x3;
x3 = XmStringCreateLtoR("test", XmFONTLIST_DEFAULT_TAG); XmTextSetString(w,x3);
...
x1 = XmTextGetString(w);
x2 = XmTextGetString(w);
XtWarning(" Checking SetValues");
if (x1 != x3)
     XtWarning("Widget keeps a copy ! Free original!");
else
     XtWarning("Widget does not keep a copy! Do NOT free original");
XtWarning(" Checking GetValues");
if (x1 == x2)
     XtWarning("Widget returns a copy! Do NOT free");
else
     XtWarning("Widget does not return a copy! You should free it ");

The XtWarning() message is especially useful for debugging the execution of programs. The message is relayed to the stderr of the invoking application. If this is an xterm, you will see an error message on that terminal window. If no stderr is available for the invoker, the message is lost.


TIP: The XtSetArg macro is defined as:

#define XtSetArg(arg,n,d) \
    ((void)((arg).name = (n).(arg).value = (XtArgVal)))

Do not use XtSetArg(arg[n++], ... because this will increment n twice.


Label

The Label widget is used to display strings or pixmaps. Include the Xm/Label.h file in your source file before you use this widget. The resources for this widget include:

  • XmNalignment: This resource determines the alignment of the text in this widget. The allowed values are XmALIGNNMENT_END, XmALIGNMENT_CENTER, and XmALIGNMENT_BEGIN, for right-, center-, and left-justification, respectively.

  • XmNrecomputeSize: A Boolean resource. If set to TRUE, the widget will be resized when the size of the string or pixmap changes dynamically. This is the default. If set to FALSE, the widget will not attempt to resize itself.

  • XmNlabelType: The default value of this type is XmSTRING to show strings. However, it can also be set to XmPIXMAP when displaying a pixmap specified in the XmNpixmap resource.

  • XmNlabelPixmap: This is used to specify which pixmap to use when the XmNlabelType is set to XmPIXMAP.

  • XmNlabelString: This is used to specify which XmString compound string to use for the label. This defaults to the name of the label. See the section "Strings in Motif: Compound Strings" later in this chapter.

To acquaint yourself with left- and right-justification on a label, see file 2_2 on the CD-ROM. This listing also shows how the resources can be set to change widget parameters, programmatically and through the .Xresource files.

Avoid using the \n in the label name. If you have to create a multi-string widget, use the XmStringCreateLtoR call to create a compound string. Another way to set the string is to specify it in the resource file and then merge the resources.

The listing shows the label to be right-justified. You could easily center the string horizontally by not specifying the alignment at all and letting it default to the center value. Alternatively, try setting the alignment parameter to XmALIGNMENT_BEGINNING for a left-justified label.

List

The List widget displays a list of items from which the user can select. The list is created from a list of compound strings. Users can select either one item or many items from this list. The resources for this widget include:

  • XmNitemCount: This determines the number of items in the list.

  • XmNitems: An array of compound strings. Each entry corresponds to an item in the list. Note that a List widget makes a copy of all items in its list when using XtSetValues; however, it returns a pointer to its internal structure when returning values to an XtGetValues call. So do not free this pointer from XtGetValues.

  • XmNselectedItemCount: The number of items currently selected.

  • XmNselectedItems: The list of selected items.

  • XmNvisibleItemCount: The number of items to display at one time.

  • XmNselectionPolicy: This is used to set single or multiple selection capability. If set to XmSINGLE_SELECT, the user will be able to select only one item. Each selection will invoke XmNsingleSelectionCallback. Selecting one item will deselect another previously selected item. If set to XmEXTENDED_SELECT, the user will be able to select a block of contiguous items in a list. Selecting one or more new items will deselect other previously selected items. Each selection will invoke the XmNmultipleSelection callback.

If set to XmMULTIPLE_SELECT, the user will be able to select multiple items in any order. Selecting one item will not deselect another previously selected item. Each selection will invoke the XmNmultipleSelection callback.

If the resource is set to XmBROWSE_SELECT, the user can move the pointer across all the selections with the button pressed, but only one item will be selected. This will invoke XmbrowseSelectionCallback when the button is finally released on the last item browsed. Unlike with the XmSINGLE_SELECT setting, the user does not have to press and release the button to select an item.

It is easier to create the List widget with a call to XmCreateScrolledList(), because this will automatically create a scrolled window for you. Also, the following convenience functions will make working with List widgets easier. However, they may prove to be slow when compared to XtSetValues() calls. If you feel that speed is important, consider using XtSetValues(). You should create the list for the first time by using XtSetValues.

  • XmListAddItem(Widget w, XmString x, int pos): This will add the compound string x to the List widget w at the 1-relative position pos. If pos is 0, the item is added to the back of the list. This function is very slow. Do not use it to create a new list, because it rearranges the entire list before returning.

  • XmListAddItems(Widget w, XmString *x, int count, int pos): This will add the array of compound strings, x, of size count, to the List widget w from the position pos. If pos is 0, the item is added to the back of the list. This function is slow too, so do not use it to create a new list.

  • XmDeleteAllItems(Widget w): This will delete all the items in a list. It's better to write a convenience function:
n = 0;
XtSetArg(arg[n], XmNitems, NULL); n++;
XtSetArg(arg[n], XmNitemCount, 0); n++;
XtSetValues(mylist,arg,n);
  • XmDeleteItem(Widget w, XmString x): Deletes the item x from the list. This is a slow function.

  • XmDeleteItems(Widget w, XmString *x, int count): Deletes all count items in x from the list. This is an even slower function. You might be better off installing a new list.

  • XmListSelectItem(Widget w, XmString x, Boolean Notify): Programmatically selects x in the list. If Notify is TRUE, the appropriate callback function is also invoked.

  • XmListDeselectItem(Widget w, XmString x): Programmatically deselects x in the list.

  • XmListPos( Widget w, XmString x): Returns the position of x in the list; 0 if not found.

See file 2_6c on the CD-ROM.

Scrollbar

The Scrollbar widget allows the user to select a value from a range. Its resources include:

  • XmNvalue: The value representing the location of the slider.

  • XmNminimum and XmNmaximum: The range of values for the slider.

  • XmNshowArrows: The Boolean value if set shows arrows at either end.

  • XmNorientation: Set to XmHORIZONTAL for a horizontal bar or XmVERTICAL (default) for a vertical bar.

  • XmNprocessingDirection: Set to either XmMAX_ON_LEFT or XmMAX_ON_RIGHT for XmHORIZONTAL, or XmMAX_ON_TOP or XmMAX_ON_BOTTOM for XmVERTICAL orientation.

  • XmNincrement: The increment per move.

  • XmNpageIncrement: The increment if a button is pressed in the arrows or the box. This is defaulted to 10.

  • XmNdecimalPoint: Specifies depth of the decimal point, counting from the right. Note that all values in the Scrollbar widget's values are given as integers. Look at the radio station selection example in file 2_8c on the CD-ROM. Note that the Push to Exit button for the application is offset on the left and right by 20 pixels. This is done by offsetting the XmATTACH_FORM value for each side (left or right) through the value in XmNleftOffset and XmNrightOffset. See the "Forms" section for more details.

For the case of FM selections, you would want the bar to show odd numbers. A good exercise for you would be to allow only odd numbers in the selection. Hint: Use XmNvalueChangedCallback as follows:

XtAddCallback(aScale, XmNvalueChangedCallback, myfunction);

The callback will send a pointer to the structure of type XMScaleCallbackStruct. where myfunction is defined as:

/**
*** Partial listing for not allowing even numbers for FM selection.
**/
#define MAX_SCALE 1080
#define MIN_SCALE 800
static void
myfunction(Widget w, XtPointer dclient,  XmScaleCallbackStruct *p)
{
int k;
k = p->value;
if ((k & 0x1) == 0)  /** % 2  is zero ** check limits & increase **/
     {
     k++;
     if (k >= MAX_SCALE) k = MIN_SCALE + 1;
     if (k <= MIN_SCALE) k = MAX_SCALE - 1;
XmScaleSetValue(w,k);  /** this will redisplay it too
**/
}
}

Text

The Text widget allows the user to type in text. This text can be multi-line, and the Text widget provides full text-editing capabilities. If you are sure you want only single-line input from the user, you can specify the TextField widget. This is simply a scaled-down version of the Text widget. The resources for both are the same unless explicitly stated. These include:

  • XmNvalue: A character string, just like in C. This is different from Motif 1.1 or older, where this value was a compound string. If you have Motif 1.2 or later, this will be C string.

  • XmNmarginHeight and XmNmarginWidth: The number of pixels between the widget border and the text. The default is 5 pixels.

  • XmNmaxLength: This sets the limit on the number of characters in the XmNvalue resource.
  • XmNcolumns: The number of characters per line.

  • XmNcursorPosition: The number of characters at the cursor position from the beginning of the text file.

  • XmNeditable: The Boolean value that, if set to TRUE, will allow the user to insert text.

The callbacks for this widget are:

  • XmNactivateCallback: Called when the user presses the Return key.

  • XmNfocusCallback: Called when the widget receives focus from the pointer.

  • XmNlosingFocusCallback: Called when the widget loses focus from the pointer.

There are several convenience functions for this widget:

  • XmTextGetString(Widget w) returns a C string (char *).

  • XmTextSetString(Widget w, char *s) sets a string for a widget.

  • XmTextSetEditable(Widget w, Boolean trueOrFalse) sets the editable string of the widget.

  • XmTextInsert(Widget w, XmTextPosition pos, char *s): Sets the text at the position defined by pos. This XmTextPosition is an opaque item defining the index in the text array.

  • XmTextShowPosition(Widget w, XmTextPosition p): Scrolls to show the rest of the string at the position p.

  • XmTextReplace(Widget w, XmTextPosition from, XmTextPosition to, char *s): Replaces the string starting from the location from inclusive to the position to, with the characters in string s.

  • XmTextRemove(Widget w): Clears the text in a string.

  • XmTextCopy(Widget w, Time t): Copies the currently selected text to the Motif clipboard. The Time t value is derived from the most recent XEvent (usually in a callback), which is used by the clipboard to take the most recent entry.

  • XmTextCut(Widget w, Time t): Similar to XmTextCopy, but it removes the selected text from the text's buffer.

  • XmTextPaste(Widget w): Pastes the contents of the Motif clipboard onto the text area at the current cursor (insertion) position.

  • XmTextClearSelection(Widget w, XmTextPosition p, XmTextPosition q, Time t): Selects the text from location p to location q.

In the following example, you could construct a sample editor application with the Text widget. For the layout of the buttons, you would use widgets of the XmManager class to manage the layout for you. These manager widgets are:

  • XmBulletinBoard

  • XmRowColumn

  • XmForm

Buttons

Buttons are ubiquitous in the point-and-click world. Special variations include radiobuttons and checkbuttons; most important, though, are pushbuttons.

PushButton

XmPushButton is perhaps the most frequently used widget in Motif. Listings 2.1 and 2.2 showed the basic usage for this class. When a button is pressed in the pushbutton area, the button goes into an "armed" state. The color of the button changes to reflect this state. This color can be set by using XmNarmColor. This color is shown when the XmNfillOnArm resource is set to TRUE.


TIP: If the XmNarmcolor for a pushbutton does not seem to be working, try setting the XmNfillOnArm resource to TRUE.

The callback functions for a pushbutton are:

  • XmNarmCallback: Called when a pushbutton is armed.

  • XmNactivateCallback: Called when a button is released in the widgets area while the widget is armed. This is not invoked if the pointer is outside the widget when the button is released.

  • XmNdisarmCallback: Called when a button is released with the pointer outside the widget area while the widget is armed.


TIP: If a callback has more than one function registered for a widget, all the functions will be called but not necessarily in the order they were registered. Do not rely on the same order being preserved on other systems. If you want more than one function performed in a particular sequence during a callback, sandwich them in one function call.

In Listing 2.2, you saw how a callback function was added to a pushbutton with the XtAddCallback function. The same method can be used to call other functions for other actions, such as the XmNdisarmCallback.

Toggle Button

The toggle button class is a subclass of the XmLabel widget class. There are two types of buttons: N of many and one of many. When using N of many, users can select many options. When using one of many, the users must make one selection from many items. Note the way the buttons are drawn; N of many buttons are shown as boxes and one of many buttons are shown as diamonds.

The resources for this widget include:

  • XmNindicatorType: This determines the style and can be set to XmN_OF_MANY or XmONE_OF_MANY (the default).

  • XmNspacing: The number of pixels between the button and its label.

  • XmNfillOnSelect: The color of the button changes to reflect a "set" when the XmNfillOnArm resource is set to TRUE.

  • XmNfillColor: The color to show when "set."

  • XmNset: A Boolean resource indicating whether the button is set or not. If this resource is set from a program, the button will automatically reflect the change.

It's easier to use the convenience function XmToggleButtonGetState(Widget w) to get the Boolean state for a widget, and to use XmToggleButtonSetState(Widget w, Boolean b) to set the value for a toggle button widget.

Like the pushbutton class, the toggle button class has three similar callbacks:

  • XmNarmCallback: Called when the toggle button is armed.

  • XmNvalueChangedCallback: Called when a button is released in the widget area while the widget is armed. This is not invoked if the pointer is outside the widget when the button is released.

  • XmNdisarmCallback: Called when a button is released with the pointer outside the widget area while the widget is armed.

For the callbacks, data appear in a structure of type:

typedef struct {
     int  reason;
     XEvent  *event;
     int  set;
} XmToggleButtonCallbackStruct;

The reason for the callback is one of the following: XmCR_ARM, XmCR_DISARM, or XmCR_ACTIVATE. The event is a pointer to XEvent that caused this callback. The set value is 0 if the item is not set and non-zero if it's set. The buttons are arranged in one column through the RowColumn widget discussed later in this chapter. See file 2_3c on the CD-ROM for an example of how to use the toggle button.

By defining the DO_RADIO label, you can make this into a radio button application. That is, only one of the buttons can be selected at one time.

Convenience Functions

Usually, the way to set resources for a widget is to do it when you create the widget. This is done with either the XtVaCreateManaged call or the XmCreateYYY call, where YYY is the widget you're creating. The text uses the variable argument call to create and manage widgets. If you use the XmCreateYYY call, you have to set the resource settings in a list of resource sets. An example of creating a Label widget is shown in file 2_4c on the CD-ROM. This is a function that creates a Label widget on a widget given the string x.

Or you could use the variable argument lists to create this label, as shown in file 2_5c.

Either approach is legitimate; through experience and study of others' work, you'll develop judgment on which is preferable for a particular situation. The label created with the variable lists is a bit easier to read and maintain. But what about setting values after a widget has been created? This would be done through a call to XtSetValue with a list and count of resource settings. For example, to change the alignment and text of a label, you would use the following:

n = 0;
XtSetArg(arg[n], XmNalignment, XmALIGNMENT_BEGIN); n++;
XtSetArg(arg[n], XmNlabelString, x); n++;
XtSetValues(lbl,arg,n);

Similarly, to get the values for a widget you would use XtGetValues:

Cardinal n; /* usually an integer or short... use Cardinal to be safe
*/ int align;
XmString x;
...
n = 0;
XtSetArg(arg[n], XmNalignment, &align); n++;
XtSetArg(arg[n], XmNlabelString, &x); n++; XtGetValues(lbl,arg,n);

In the case of other widgets, such as the Text widget, this setting scheme is hard to read, quite clumsy, and prone to typos. For example, to get a string for a Text widget, do you use x or address of x?

For this reason, Motif provides convenience functions. In the ToggleButton widget class, for example, rather than use the combination of XtSetValue and XtSetArg calls to get the state, you would use one call, XmToggleButtonGetState(Widget w), to get the state. These functions are valuable code savers when you're writing complex applications. In fact, you should write similar convenience functions whenever you cannot find one that suits your needs.

Design Widgets

Bulletin boards, row-columns, and forms are handy geometric constructs, widgets which facilitate lay-out of an interface on a computer screen.

Bulletin Board

The Bulletin Board widget allows the programmer to lay out widgets by specifying their XmNx and XmNy resources. These values are relative to the top left corner of the Bulletin Board widget. The Bulletin Board widget will not move its children widgets around on itself. If a widget resizes, it's the application's responsibility to resize and restructure its widgets on the Bulletin Board.

The resources for the widget are:

  • XmNshadowType: Specifies the type of shadow for this widget. It can be set to XmSHADOW_OUT (the default), XmSHADOW_ETCHED_IN, XmSHADOW_ETCHED_OUT, or XmSHADOW_IN.

  • XmNshadowThickness: The number of pixels for the shadow. This is defaulted to 0 (no shadow).

  • XmNallowOverlap: Allows the children to be overlapped as they are laid on the widget. This is a Boolean resource and defaults to TRUE.

  • XmNresizePolicy: Specifies the resize policy for managing itself. If set to XmRESIZE_NONE, it will not change its size. If set to XmRESIZE_ANY, it will grow or shrink to attempt to accommodate all its children automatically. This is the default. If set to XmRESIZE_GROW, it will grow, but never shrink, automatically.

  • XmNbuttonFontList: Specifies the font for all XmPushButton children.

  • XmNlabelFontList: Specifies the default font for all widgets derived from XmLabel.

  • XmNtextFontList: Specifies the default font for all Text, TextField, and XmList children.

It also provides the callback XmNfocusCallback, which is called when any children of the Bulletin Board receives focus.

RowColumn

The RowColumn widget class orders its children in a row or columnar fashion. This is used to set up menus, menu bars, and radio buttons. The resources provided by this widget include:

  • XmNorientation: XmHORIZONTAL for a row major layout of its children; XmVERTICAL for a column major layout.

  • XmNnumColumns: Specifies the number of rows for a vertical widget and the number of columns for a horizontal widget.

  • XmNpacking: Determines how the children are packed. XmPACK_TIGHT allows the children to specify their own size. It fits children in a row (or column if XmHORIZONTAL), and then starts a new row if no space is available. XmPACK_NONE forces Bulletin Board-like behavior. XmPACK_COLUMN forces all children to be the size of the largest column. This uses the XmNnumColumns resource and places all its children in an organized manner.

  • XmNentryAlignment: Specifies which part of the children to use in its layout alignment. Its default is XmALIGNMENT_CENTER, but it can be set to XmALIGNMENT_BEGINNING for the left side or XmALIGNMENT_END for the right side. This is on a per column basis.

  • XmNverticalEntryAlignment: Specifies the alignment on a per row basis. It can be assigned a value of XmALIGNMENT_BASELINE_BOTTOM, XmALIGNMENT_BASELINE_TOP, XmALIGNMENT_CONTENTS_BOTTOM, XmALIGNMENT_CONTENTS_TOP, or XmALIGNMENT_CENTER.

  • XmNentryBorder: The thickness of a border drawn around all children, and is defaulted to 0.

  • XmNresizeWidth: A Boolean variable that, if set to TRUE, will allow the RowColumn widget to resize its width when necessary.

  • XmNresizeHeight: A Boolean variable that, if set to TRUE, will allow the RowColumn widget to resize its height when necessary.

  • XmNradioBehavior: Works with toggle buttons only. It allows only one toggle button in a group of buttons to be active at a time. The default is FALSE.

  • XmNisHomogeneous: If set to TRUE, this specifies that only children of the type Class in XmNentryClass can be children of this widget. The default is FALSE.

  • XmNentryClass: Specifies the class of children allowed in this widget if XmNisHomogeneous is TRUE. A sample radio button application was shown in file 2_5c. To see another example of the same listing but with two columns, see file 2_8c on the CD-ROM.

Form

The beginning of the chapter introduced you to the workings of the Form widget. This is the most flexible and most complex widget in Motif. Its resources include:

  • XmNtopAttachment
  • XmNleftAttachment
  • XmNrightAttachment
  • XmNbottomAttachment

These values specify how a child is placed. The following values correspond to each side of the widget:

XmATTACH_NONE: Do not attach this side to Form.

XmATTACH_FORM: Attach to corresponding side on Form.

XmATTACH_WIDGET: Attach this side to opposite side of a reference widget. For example, attach the right side of this widget to the left side of the reference widget. A reference widget is another child on the same form.

XmATTACH_OPPOSITE_WIDGET: Attach this side to same side of a reference widget. This is rarely used.

XmATTACH_POSITION: Attach a side by the number of pixels shown in XmNtopPosition, XmNleftPosition, XmNrightPosition, and XmNbottomPosition resources, respectively.

XmATTACH_SELF: Use XmNx, XmNy, XmNheight, and XmNwidth.

The following resources are set to the corresponding widgets for each side for the XmATTACH_WIDGET setting in an attachment:

  • XmNtopWidget
  • XmNleftWidget
  • XmNrightWidget
  • XmNbottomWidget

The following resources are the number of pixels a side of a child is offset from the corresponding Form side. The offset is used when the attachment is XmATTACH_FORM.

  • XmNtopOffset
  • XmNleftOffset
  • XmNrightOffset
  • XmNbottomOffset

Sometimes it is hard to get the settings for a Form widget just right, or the Form widget does not lay out the widgets in what seems to be the proper setting for a child widget. In these cases, lay the children out in ascending or descending order from the origin of the Form widget. That is, create the top left widget first and use it as an "anchor" to create the next child, then the next one to its right, and so on. There is no guarantee that this will work, so try using the bottom right, bottom left, or top right for your anchor positions.

If this technique does not work, try using two forms on top of the form you're working with. Forms are cheap, and your time is not. It's better to just make a form when two or more widgets have to reside in a specific layout.

When you're trying a new layout on a Form widget, if you get error messages about failing after 10,000 iterations, it means you have conflicting layout requests to one or more child widgets. Check the attachments very carefully before proceeding. This error message results from the Form widget trying different layout schemes to accommodate your request.


TIP: At times, conflicting requests to a form will cause your application to slow down while it's trying to accommodate your request, not show the form, or both.

Designing Layouts

When you're designing layouts, think about the layout before you start writing code. Let's try an album search front-end example. See file 2_9c on the CD-ROM.

The application is shown in Figure 2.2. Notice how the labels do not line up with the Text widget. There is a problem in the hierarchy of the setup. See the hierarchy of the application in Figure 2.3.

Figure 2.2.
The output of Listing 2.2.

Figure 2.3
The hierarchy of Listing 2.2.

The Form widgets are created to maintain the relative placements of all widgets that correspond to a type of function. The RowColumn widgets allow items to be placed on them. The best route to take in this example is to lay one text and one label on one RowColumn widget and have three RowColumn widgets in all, one for each instance up to NUM_ITEMS. This will ensure that each label lines up with its corresponding Text widget.

A couple of points to note about laying out applications:

  • Think about what you want the form or dialog to do. Draw it on paper if you have to. Coding is the easy part; determining what to do is much harder.

  • Be consistent. Users will love you for it. If Alt+x is a shortcut for "Exit" in one screen, do not make it a cut operator in another. Keep controls on the same side of all dialog boxes and forms. Use separators to separate different functions on the same window.

  • Choose a color scheme for your end users. What may be cool to you may be grotesque to the end user. They may not even be using a color monitor in some rare cases. A combination of white, gray, and black might be your best bet if you don't want to deal with different color schemes in your code.

  • Colors on your monitor might not be the same on the end user's monitor.

  • Do not assume that the user's monitor has the same resolution as yours. Keep fonts (and buttons) big enough for a large cursor. Allow windows to be resizeable as much as possible to allow the user to customize his desktop.

  • Assume nothing. If the user can size your window to an unworkable size, either limit the size in resizeCallback to the lowest size or don't allow sizing at all.

  • Offer some help for the user. In the future, Help will be required as a standard option on menu bars, so plan ahead.

  • Avoid clutter. Too many options and entries on one huge form tend to confuse and baffle the user. Consider two tiers or more. Default everything as much as possible.

  • Make the program more forgiving. Sometimes an "Are you sure?" dialog with an option to change a list of parameters can be endearing to a user. On the other hand, some users hate this type of checking.

Menus

The way you design widget hierarchies is especially important when you're working with Motif menus. Motif menus are a collection of widgets, so there is no "menu" widget for a menu. You create menus using a hierarchy of different types of widgets: RowColumn, PushButton, CascadeButton, ToggleButton, Label, and Separator.

There are three kinds of menus in Motif:

  • Popup: This appears as a list of items when a pointer button is pressed on a widget.

  • Pulldown: This appears when a button on an existing menu is pressed.

  • Option: This allows the user to select from a list of options, with the current selection visible at all times.

The procedure to create a menu is different for each type of menu.

Popup

To create a Popup menu, do the following:

1. Include the correct header files. You will need the header files for the menu:
Label.h
RowColumn.h
PushB.h
Separator.h
BulletinB.h
CascadeB.h
2. Create the menu pane with a call to XmCreatePopupMenu. This is a convenience call to create a RowColumn widget and a MenuShell widget with the proper settings.

3. Create the buttons on the menu pane. Use XmPushbuttons, XmToggleButtons, XmSeparator, and XmCascadeButtons.

4. Attach callback functions to the widgets.

See file 2_10c on the CD-ROM for a listing that sets up a pop-up menu.

Note three important items about this listing: You can use printf functions within Motif applications. The output goes to the controlling terminal by default. This is invaluable in debugging. The menu is not visible by itself. An event handler on the parent of the menu is registered before the menu can be displayed. This allows the menu to be displayed any time a button is pressed. The XmMenuPosition call sets the position of the Popup menu. It is then managed (after placement).

Menu Bar

A menu bar is a horizontal bar that is continually available to the user. Motif uses the RowColumn widget as a bar, with cascading buttons for each option.

The procedure for creating a menu bar is as follows:

1. Include the correct header files. You will need the header files for the menu:

Label.h RowColumn.h
PushB.h Separator.h
BulletinB.h CascadeB.h

2. Create the menu bar with a call to XmCreateMenuBar().

3. Create the pull-down menu panes with a call to XmCreatePulldownMenu().

4. For each pull-down pane, create a cascade button on the menu bar. Use the menu bar as the parent. A cascade button is used to link the items in a menu with the menu bar itself.

5. Attach the menu pane to its corresponding cascade button. Use the XmNsubMenuId resource of the cascade button on the appropriate menu pane.

6. Create the menu entries in the menu panes.

File 2_1k on the CD-ROM shows how to set up a menu bar and pull-down menus.

Note that the Motif programming style requires you to make the Help button (if you have one) right-justified on the menu bar. This Help cascade button should then be set to the XmNmenuHelpWidget of a menu bar. The menu bar will automatically position this widget at the right side of the visible bar.

File 2_12c on the CD-ROM is another example of setting up a menu bar and pull-down menus.

Options

The Options menu allows the user to select from a list of items, and displays the most recently selected item. The procedure for creating an Options menu is similar to creating a menu bar.

1. Include the correct header files. You will need these header files for the menu:
Label.h Separator.h

RowColumn.h BulletinB.h

PushB.h CascadeB.h

2. Create the menu bar with a call to XmCreateOptionMenu().

3. Create the pull-down menu panes with a call to XmCreatePulldownMenu().

4. For each pull-down pane, create a cascade button on the menu bar.

5. Attach the menu pane to its corresponding cascade button. Use the XmNsubMenuId resource of the cascade button on the appropriate menu pane.

6. Create the menu entries in the menu panes.

Accelerators and Mnemonics

A menu item's accelerator is a keystroke that invokes the callback for that particular item. For example, to open a file you could use Ctrl+O. The resource for this accelerator could be set in the resource file as the following:

*Open*accelerator: Ctrl<Key>O

The corresponding menu item should read "Open Ctrl+O" to make the user aware of this shortcut. You can also set this resource through the following command in the .Xresources file:

*Open*acceleratorText: "Ctrl+O"

Using the .Xresource file is the preferred way of setting these resources.

Mnemonics are a shorthand for letting users select menu items without using the mouse. For example, you could use <meta>F for invoking the File menu. These are usually set in the .Xresources file as well. The syntax for the File menu to use the <meta>F key would be as follows:

*File*mnemonic: F

Dialog Boxes

A dialog box conveys information about something to the user, and receives one of a limited number of responses. For example, a dialog box could read "Go Ahead and Print" with three buttons--OK, Cancel, and Help. The user would then select one of the three buttons.

A typical dialog box displays an icon, a message string, and (usually) three buttons. Motif provides predefined dialog boxes for the following categories: Errors, information, warnings, working, and question.

Each of the preceding dialog box types displays a different icon: a question mark for the Question dialog box, an exclamation mark for the Information dialog box, and so on. Convenience functions ease creation of dialog boxes:

  • XmCreateErrorsDialog
  • XmCreateInformationDialog
  • XmCreateWarningDialog
  • XmCreateWorkingDialog
  • XmCreateQuestionDialog

The notorious "Really Quit?" dialog box can be implemented as shown in Listing 2.13. There is another example in file 2_17c on the CD-ROM.

Append this to the end of any source to get instant verification before you actually quit the application.

Note that the quitDlg dialog box is set to NULL when the function is first called. It is only managed for subsequent calls to this function.

Modes of a Dialog Box

A dialog box can have four modes of operation, called modalities. The mode is set in the XmNdialogStyle resource. The possible values are as follows:

  • Non-Modal: The user can ignore the dialog box and work with any other window on the screen. The resource value is XmDIALOG_MODELESS.

  • Primary Application Modal: All input to the window that invoked the dialog box is locked out. The user can use the rest of the windows in the application. The resource value is XmDIALOG_PRIMARY_APPLICATION_MODAL.

  • Full Application Modal: All input to all the windows in the application that invoked the dialog box is locked out. The user cannot use the rest of the windows in the application. The resource value is XmDIALOG_FULL_APPLICATION_MODAL.

  • System Modal: All input is directed to the dialog box. The user cannot interact with any other window in the system. The resource value is XmDIALOG_SYSTEM_MODAL.

The dialog boxes provided by Motif are based on the XmMessageBox widget. Sometimes it is necessary to get to the widgets in a dialog. This is done by a call to the following:

Widget XmMessageBoxGetChild( Widget dialog, typeOfWidget);

Here, typeOfWidget can be one of these:

XmDIALOG_HELP_BUTTON    XmDIALOG_CANCEL_BUTTON
XmDIALOG_SEPARATOR      XmDIALOG_MESSAGE_LABEL
XmDIALOG_OK_BUTTON      XmDIALOG_SYMBOL_LABEL

The dialog box may have more widgets that can be addressed. Check the man pages for the descriptions of these widgets.

For example, to hide the Help button in a dialog box, use this call:

XtUnmanageChild(XmMessageBoxGetChild(dlg, XmDIALOG_HELP_BUTTON));

In the case of adding a callback, use this sequence:

XtAddCallback(XmMessageBoxGetChild(dlg, XmDIALOG_OK_BUTTON),
XmNactivateCallback, yourFunction);

A typical method of creating custom dialog boxes is to use existing ones. Then, using the XmMessageBoxGetChild function, you can add or remove any function you want. For example, replace the Message String widget with a Form widget and you have a place to lay out widgets however you need.

Events

An event is a message sent from the X server to the application that some condition in the system has changed. This could be a button press, a keystroke, a request for information from the server, or a timeout. An event is always relative to a window and starts from the bottom up. It propagates up the window hierarchy until it gets to the root window, where the root window application makes the decision whether to use or discard it. If an application in the hierarchy does use the event or does not allow propagation of events upward, the message is used at the window itself. Only device events (keyboard or mouse) are propagated upward, not configuration events.

An application must request an event of a particular type before it can begin receiving events. Each Motif application calls XtAppInitialize to make this request automatically.

Events contain at least the following information:

  • The type of event
  • The display where it happened
  • The event window
  • The serial number of the last event processed by the server

Look in the file <X11/Xlib.h> for a description of the union called XEvent, which allows access to these values. The file <X11/X.h> contains the descriptions of constants for the types of events.

All event types share this prologue definition:

typedef struct {
     int type;
     unsigned long serial;   /* # of last request processed by server */
     Bool send_event;        /* true if this came from a SendEvent request */
      Display *display;/* Display the event was read from */
     Window window;  /* window on which event was requested in event mask */ } XAnyEvent;

The types of events include:

KeyPress KeyRelease ButtonPress
ButtonRelease MotionNotify EnterNotify
LeaveNotify FocusIn FocusOut
KeymapNotify Expose GraphicsExpose
NoExpose VisibilityNotify CreateNotify
DestroyNotify UnmapNotify MapNotify
MapRequest ReparentNotify ConfigureNotify
ConfigureRequest GravityNotify ResizeRequest
CirculateNotify CirculateRequest PropertyNotify
SelectionClear SelectionRequest SelectionNotify
ColormapNotify ClientMessage MappingNotify
Expose

The server generates an Expose when a window that has been covered by another is brought to the top of the stack, or even partially exposed.

The structure for this event type is as follows:

typedef struct {
     int type;     /* Type of event */
     unsigned long serial;     /* # of last request processed by server */
     Bool send_event;     /* true if this came from a SendEvent request */
     Display *display;     /* Display the event was read from */
     Window window;
     int x, y;
     int width, height;
     int count;     /* if non-zero, at least this many more */
} XExposeEvent;

Note how the first five fields are shared between this event and XAnyEvent. Expose events are guaranteed to be in sequence. An application may get several Expose events from one condition. The count field keeps a count of the number of Expose events still in the queue when the application receives this one. Thus, it can be up to the application to wait to redraw until the last Expose event is received (count == 0).

Pointer Events

A pointer event is generated by a mouse button press or release, or by any mouse movement. This type of event is called XButtonEvent. Recall that the leftmost button is Button1, but it can be changed (see the section "Left-Handed Users" in Chapter 1). The structure returned by a button press and release is the following:

typedef struct {
     int type;     /* of event */
     unsigned long serial;     /* # of last request processed by server */
     Bool send_event;     /* true if this came from a SendEvent request */
     Display *display;     /* Display the event was read from */
     Window window;     /* "event" window it is reported relative to */
     Window root;     /* root window that the event occured on */
     Window subwindow;     /* child window */
     Time time;     /* milliseconds */
     int x, y;     /* pointer x, y coordinates in event window */
     int x_root, y_root;     /* coordinates relative to root */
     unsigned int state;     /* key or button mask */
     unsigned int button;     /* detail */
     Bool same_screen;     /* same screen flag */
} XButtonEvent;
typedef XButtonEvent XButtonPressedEvent;
typedef XButtonEvent XButtonReleasedEvent;

The event for a movement is called XMotionEvent, with the type field set to MotionNotify.

typedef struct {
     int type;     /* MotionNotify */
     unsigned long serial;     /* # of last request processed by server */
     Bool send_event;     /* true if this came from a SendEvent request */
     Display *display;     /* Display the event was read from */
     Window window;     /* "event" window reported relative to */
     Window root;     /* root window that the event occured on */
     Window subwindow;     /* child window */
     Time time;     /* milliseconds */
     int x, y;     /* pointer x, y coordinates in event window */
     int x_root, y_root;     /* coordinates relative to root */
     unsigned int state;     /* key or button mask */
     char is_hint;     /* detail */
     Bool same_screen;     /* same screen flag */
} XMotionEvent;
typedef XMotionEvent XPointerMovedEvent;

Keyboard Events

A keyboard event is generated when the user presses or releases a key. Both types of events, KeyPress and KeyRelease, are returned in an XKeyEvent structure.

typedef struct {
     int type;     /* of event */
     unsigned long serial;     /* # of last request processed by server */
     Bool send_event;     /* true if this came from a SendEvent request */
     Display *display;     /* Display the event was read from */
     Window window;     /* "event" window it is reported relative to */
     Window root;     /* root window that the event occured on */
     Window subwindow;     /* child window */
     Time time;     /* milliseconds */
     int x, y;     /* pointer x, y coordinates in event window */
     int x_root, y_root;     /* coordinates relative to root */
     unsigned int state;     /* key or button mask */
     unsigned int keycode;     /* detail */
     Bool same_screen;     /* same screen flag */
} XKeyEvent;
typedef XKeyEvent XKeyPressedEvent;
typedef XKeyEvent XKeyReleasedEvent;

The keycode field presents the information on whether the key was pressed or released. These constants are defined in <X11/keysymdef.h> and may be vendor-specific. These are called KeySym and are generic across all X servers. For example, the F1 key could be described as XK_F1.

The function XLookupString converts a KeyPress event into a string and a KeySym (a portable key symbol). Here's the call:

int XLookupString(XKeyEvent *event,
               char *returnString,
               int max_length,
               KeySym  *keysym,
               XComposeStatus *compose);

The returned ASCII string is placed in returnString for up to max_length characters. The KeySym contains the key symbol. Generally, the compose parameter is ignored.

Window-Crossing Events

The server generates crossing EnterNotify events when a pointer enters a window, and LeaveNotify events when a pointer leaves a window. These are used to create special effects for notifying the user that the window has focus. The XCrossingEvent structure looks like this:

typedef struct {
     int type;               /* of event */
     unsigned long serial;     /* # of last request processed by server */
     Bool send_event;     /* true if this came from a SendEvent request */
     Display *display;     /* Display the event was read from */
     Window window;          /* "event" window reported relative to */
     Window root;          /* root window that the event occured on */
     Window subwindow;     /* child window */
     Time time;          /* milliseconds */
     int x, y;               /* pointer x, y coordinates in event window */
     int x_root, y_root;     /* coordinates relative to root */
     int mode;               /* NotifyNormal, NotifyGrab, NotifyUngrab */
     int detail;
     /*
          * NotifyAncestor, NotifyVirtual, NotifyInferior,
          * NotifyNonlinear,NotifyNonlinearVirtual
          */
     Bool same_screen;     /* same screen flag */
     Bool focus;          /* boolean focus */
     unsigned int state;     /* key or button mask */
} XCrossingEvent;
typedef XCrossingEvent XEnterWindowEvent;
typedef XCrossingEvent XLeaveWindowEvent;

These are generally used to change a window's color when the user moves the pointer in and out of it.

Event Masks

An application requests events of a particular type by calling a function XAddEventHandler.

XAddEventHandler( Widget ,
                    EventMask ,
                    Boolean maskable,
XtEventHandler handlerfunction,
                    XtPointer clientData);

The handler function is of this form:

void handlerFunction( Widget w, XtPointer clientData,
                         XEvent *ev, Boolean *continueToDispatch);

The first two arguments are the client data and widget passed in XtAddEventHandler. The ev argument is the event that triggered this call. The last argument allows this message to be passed to other message handlers for this type of event. This should be defaulted to TRUE.

You would use the following call on a widget (w) to be notified of all pointer events of the type ButtonMotion and PointerMotion on this widget.

extern void handlerFunction( Widget w, XtPointer clientData,
               XEvent *ev, Boolean *continueToDispatch); ..
XAddEventHandler( w, ButtonMotionMask | PointerMotionMask, FALSE, handlerFunction, NULL );

The possible event masks are the following:

NoEventMask

KeyPressMask

KeyReleaseMask

ButtonPressMask

ButtonReleaseMask

EnterWindowMask

LeaveWindowMask

PointerMotionMask

PointerMotionHintMask

Button1MotionMask

Button2MotionMask

Button3MotionMask

Button4MotionMask

Button5MotionMask

ButtonMotionMask

KeymapStateMask

ExposureMask

VisibilityChangeMask

StructureNotifyMask

ResizeRedirectMask

SubstructureNotifyMask

SubstructureRedirectMask

FocusChangeMask

PropertyChangeMask

ColormapChangeMask

OwnerGrabButtonMask

File 2_14c on the CD-ROM is a sample application that shows how to track the mouse position.

Managing the Queue

The XtAppMainLoop() function handles all the incoming events through the following functions:

  • XtAppPending: Checks the queue to see if any events are pending.

  • XtAppNextEvent: Removes the next event from the queue.

  • XtDispatchEvent: Passes the message to the appropriate window.

The loop can do something else between checking and removing messages through the replacement code segment:

while (!done)
          {
          while (XtAppPending( applicationContext))
               {
               XtAppNextEvent( applicationContext, &ev));
               XtDispatchEvent( &ev));
               }
          done = interEventFunction();
          }

There are warnings that belong with this scheme:

  • This is a non-blocking function. It must be fed at all times with events or it will take over all other applications' time.

  • There is no guarantee when your inter-event function will be run if the queue is flooded with events.

  • Note the while loop for checking messages. It's more efficient to flush the queue first and then call your function, rather than calling it once per check for messages.

  • The inter-event function must be fast or you will see the user interface slow down. If you want to give your user feedback about what's going on during a long inter-event function, you can handle just the Expose events through a call to XmUpdateDisplay( Display *). This will handle only the Expose events in the queue so that you can update a status display.


Caution: Consider using the select() call to handle incoming events on file descriptors. This is a call that allows an application to wait for events from various file descriptors (in AIX, on UNIX message queues) on read-ready, write-ready, or both. This is done by setting the bits in a 32-bit wide integer for up to 16 files (and 16 more message queues in AIX) to wait on input from. The setup scheme for select calls is different on different UNIX systems. Check the man pages for the select() function on your system. The pseudo-code to handle select calls follows.

Check your system's man pages for this code.

Open all the files with an open call.

Get the file descriptor for the event queue. Use the Select macros to set up the parameters for select call ret = return from the select function.

switch (ret)
   case0:
          process the event queue.
     case 1:     ...
          process the file descriptor



			

Work Procedures

These are functions called by the event handler loop whenever no events are pending in the queue. The function is expected to return a Boolean value indicating whether it has to be removed from the loop once it is called. If TRUE, it will be removed. If FALSE, it will be called again. For example, you could set up a disk file transfer to run in the "background" that will keep returning FALSE until it is done, at which time it will return TRUE.

The work procedures are defined as

XtWorkProc yourFunction(XtPointer clientdata);

The way to register a work procedure is to call

XtWorkProcId  XtAppAddWorkProc ( XtAppContext app,
XtWorkProc   functionPointer, XtPointer    clientData);

The return ID from this call is the handle to the work procedure. It is used to remove the work procedure with a call to

XtRemoveWorkProc( XtWorkProcId id);

Using Timeouts

A timeout is used to perform some task at (almost) regular intervals. Applications set up a timer callback function, which is called when a requested time interval has passed. This function is defined as the following:

XtTimerCallbackProc thyTimerCallback( XtPointer clientdata, XtInterval *tid);

Here, clientdata is a pointer to client-specific data.

The setup function for the timeout returns the timer ID and is defined as the following:

XtIntervalId XtAppAddTimeOut ( XtAppContext app,
int milliseconds, XtTimerCallback TimerProcedure, XtPointer clientdata);

This call sets up a timer to call the TimerProcedure function when the requested milliseconds have passed. It will do this only once. If you want cyclic timeouts, say, in a clock application, you have to explicitly set up the next function call in the timer handler function itself. So generally the last line in a timer handler is a call to set a timeout for the next time the function wants to be called.

UNIX was not originally designed for real-time applications and you cannot expect a deterministic time interval between successive timer calls. Some heavy graphics updates can cause delays in the timer loop. For user interface applications, the delays are probably not a big drawback. However, consult your vendor before you attempt to write a time-critical control application. Your mileage may vary depending on your application.

File 2_15c on the CD-ROM is a program that sets a cyclic timer.

Other Sources

The XtAddInput function is used to handle inputs from sources other than the event queue. Here is the definition:

XtInputId XtAddInput( XtAppContext app,
     int UNIXfileDescriptor,
     XtPointer  condition,
     XtInputCallback inputHandler,
     XtPointer clientdata);

The return value from this call is the handle to the inputHandler function. This is used to remove the call through the call:

XtAppAddInput( XtInput Id);

The input Handler function itself is defined as:

XtImportCallbackProc InputHandler(XtPointer clientdata, int *fd,
     XtInputId *id);

Unlike timers, you must register this function only once. Note that a pointer to the file descriptor is passed into the function. The file descriptor must be a UNIX file descriptor. You do not have support for UNIX IPC message queues or semaphores through this scheme. The IPC mechanism is considered dated, and is limited to one machine. Consider using sockets instead.

Handling Output

How does an application communicate its results back to its user? Motif provides a number of primitives for drawing and animation.

Graphics Context

Each widget draws itself on the screen using its set of drawing parameters, called the graphics context. For drawing on a widget, you can use the X primitive functions if you have its window and its graphics context. It's easier to limit your artwork to the DrawingArea widget, which is designed for this purpose. You can think of the GC as your paintbrush and the widget as the canvas. The colors and the thickness of the paintbrush are just two of the factors that determine how the paint is transferred to the canvas. The GC is your paintbrush.

Here is the function call to create a GC:

GC XCreateGC (Display dp, Drawable d, unsigned long mask, XGCValue *values);

For use with a widget, w, this call becomes the following:

GC gc;
XGCVvalue gcv;
unsigned long mask;
gc = XCreate(XtDisplay(w), XtWindow(w),
     mask, gcv);

Also, you can create a GC for a widget directly with a call to XtGetGC:

gc = XtGetGC (Widget w, unsigned long mask, XGCValue *values);

The values for the mask are defined as follows:

GCFunction GCPlaneMask GCForeground
GCBackground GCLineWidth GCLineStyle
GCCapStyle GCJoinStyle GCFillStyle
GCFillRule GCTile GCStipple
GCTileStipXOrigin GCTileStipYOrigin GCFont
GCSubWindowMode GCGraphicsExposures GCClipXOrigin
GCClipYOrigin GCClipMask GCDashOffset
GCDashList GCArcMode

The data structure for setting graphics context is shown here:

typedef struct {
     int function;     /* logical operation */
     unsigned long plane_mask;/* plane mask */
     unsigned long foreground;/* foreground pixel */
     unsigned long background;/* background pixel */
     int line_width;     /* line width */
     int line_style;     /* LineSolid, LineOnOffDash, LineDoubleDash */
     int cap_style;     /* CapNotLast, CapButt,
        CapRound, CapProjecting */
     int join_style;     /* JoinMiter, JoinRound, JoinBevel */
     int fill_style;     /* FillSolid, FillTiled,
               FillStippled, FillOpaeueStippled */
     int fill_rule;     /* EvenOddRule, WindingRule */
     int arc_mode;     /* ArcChord, ArcPieSlice */
     Pixmap tile;     /* tile pixmap for tiling operations */
     Pixmap stipple;     /* stipple 1 plane pixmap for stipping */
     int ts_x_origin;     /* offset for tile or stipple operations */
     int ts_y_origin;
     Font font;     /* default text font for text operations */
     int subwindow_mode;     /* ClipByChildren, IncludeInferiors */
     Bool graphics_exposures;/* boolean, should exposures be generated */
     int clip_x_origin;     /* origin for clipping */
     int clip_y_origin;
     Pixmap clip_mask;     /* bitmap clipping; other calls for rects */
     int dash_offset;     /* patterned/dashed line information */
     char dashes;
} XGCValues;

If you want to set a value in a GC, you have to take two steps before you create the GC:

1. Set the value in the XGCValue structure.

2. Set the mask for the call GCFunction. This determines how the GC paints to the screen. The dst pixels are the pixels currently on the screen, and the src pixels are those that your application is writing by using the GC.
GXclear  dst = 0
GXset    dst = 1
GXand    dst = src AND dst
GXor     dst = src OR dst

GXcopy   dst = src

GXnoop   dst = dst
GXnor    dst = NOT(src OR dst)

GXxor    dst = src XOR dst

GXinvert dst = NOT dst

GxcopyInverted dst = NOT src

The function for a GC is changed through a call XSetFunction (Display *dp, GC gc, int function), where function is set to one of the values above. The default value is GXcopy.

There are several other masks that you can apply. They are listed in the <X11/X.h> file.

  • GCPlaneMask: The plane mask sets which planes of a drawable can be set by the GC. This is defaulted to AllPlanes, thereby allowing the GC to work with all planes on a window.

  • GCForeground and GCBackground: These are the values of the pixels to use for the foreground and background colors, respectively. Here is the call to manipulate these:
XSetForeground(Display *dp, GC gc, Pixel pixel)
XSetBackground(Display *dp, GC gc, Pixel pixel)
  • GCLineWidth: This is the number of pixels for the width of all lines drawn through the GC. It is defaulted to 0, which is the signal to the server to draw the thinnest line possible.

  • GCLineStyle, GCDashOffset, and GCDashList: This determines the style of the line drawn on the screen. LineSolid draws a solid line using the foreground color, LineOnOffDash draws an intermittent line with the foreground color, and LineDoubleDash draws a line that is composed of interlaced segments of the foreground and background colors. The GCDashOffset and GCDashList values determine the position and length of these dashes.

  • GCCapStyle: This determines how the server draws the ends of lines. CapNotLast draws up to, but not including, the end point pixels of a line. CapButt draws up to the end points of a line (inclusive). CapRound tries to round off the edges of a thick line (3 or more pixels wide). CapProjecting projects the end point out a little.

  • GCJoinStyle: This is used to draw the end points of a line. It can be set to JointMiter for a 90-degree joint, JoinBevel for a beveled joint, or JoinRound for a rounded joint.

  • GCFillStyle, GCTile, and GCStipple: The fill style can be set to FillSolid, which specifies the foreground color as the fill color. FillTiled specifies a pattern set in the tile attribute. FillStipple specifies a pattern in the stipple attribute. It uses the foreground color when a bit is set to 1 and nothing when a bit is set to 0, whereas FillOpaqueStippled uses the foreground when a bit is set to 1 and the background when a bit is set 0.

  • GCFont: This specifies the font list to use. See the section "Using Fonts and Font Lists" later in this chapter.

  • GCArcMode: This defines the way an arc is drawn on a screen. See the next section, "Drawing Lines, Points, Arcs, and Polygons."

Drawing Lines, Points, Arcs, and Polygons

Motif applications can access all the graphics primitives provided by Xlib. All Xlib functions must operate on a window or a pixmap; both are referred to as drawable. A widget has a window after it is realized, and you can access this window with a call to XtWindow(). An application can crash if Xlib calls are made to a window that is not realized. The way to check is through a call to XtIsRealized() on the widget, which returns TRUE if it's realized and FALSE if it's not. Use the XmDrawingArea widget's callbacks for rendering your graphics, because it is designed for this purpose. The following callbacks are available to you:

  • XmNresizeCallback: Invoked when the widget is resized.

  • XmNexposeCallback: Invoked when the widget receives an Expose event.

  • XmNinputCallback: Invoked when a button or key is pressed on the widget.

All three functions pass a pointer to XmDrawingAreaCallbackStruct.

Drawing a Line

To draw a point on a screen, use the XDrawLine or XDrawLines function call. Consider the example shown on the CD-ROM in file 2_16c.

The following code is an example of the primitives required to draw one line on the widget. Note the number of GCValues that have to be set to achieve this purpose. The XDrawLine function definition is shown here:

XDrawLine( Display *dpy,
     Drawable d,
     GC gc,
     int x1,
     int y1,
     int x2,
     int y2);

It's more efficient to draw multiple lines in one call. Use the XDrawLines function with a pointer to an array of points and its size.

The mode parameter can be set to:

  • CoorModeOrigin: Use the values relative to the drawable's origin.

  • CoorModePrevious: Use the values as deltas from the previous point. The first point is always relative to the drawable's origin.

To draw boxes, use the XDrawRectangle function:

XDrawRectangle( Display *display,
Drawable dwindow,
          GC       gc,
          int      x,
          int      y,
          unsigned int width,
          unsigned int height);

This will draw a rectangle at (x, y) of geometry (width, height). To draw more than one box at one time, use the XDrawRectangles() function. This is declared as the following:

XDrawRectangles( Display *display,
               Window  dwindow,
               GC      gc,
               XRectangle *xp,
               int     number);

Here, xp is a pointer to an array of "number" rectangle definition structures.

For filled rectangles, use the XFillRectangle and XFillRectangles calls, respectively.

Drawing a Point

o draw a point on a screen, use the XDrawPoint or XDrawPoints function call. These are similar to line-drawing functions. Look at Listing 2.16.

Drawing Arcs

To draw circles, arcs, and so on, use the XDrawArc function:

XDrawArc(Display *display,
          Window  dwindow,
          GC   gc,
          int  x,
          int  y,
          unsigned int    width;
          unsigned int    height;
          int     a1,
          int  a2);

This function is very flexible. It draws an arc from angle a1, starting at the 3 o'clock position, to angle a2. The unit of measurement for angles is 1/64 of a degree. The arc is drawn counterclockwise. The largest value is 64 x 360 units because the angle arguments are truncated at one full rotation. The width and height define the bounding rectangle for the arc.

The XDrawArcs() function is used to draw multiple arcs, given pointers to the array.

XDrawArcs (Display *display,
          Window  dwindow,
          GC   gc,
          XArc *arcptr,
          int  number);

To draw polygons, use the call:

XDrawSegments( Display *display, Window dwindow,
          GC   gc,
          XSegment *segments,
          int     number);

The XSegment structure includes four "short" members, x1, y1, x2, and y2, which define the starting and ending points of all segments. For connected lines, use the XDrawLines function shown earlier. For filled polygons, use the XFillPolygon() function call.

Using Fonts and Font Lists

Fonts are perhaps the trickiest aspect of Motif to master. See the section on "Fonts" in the previous chapter before you read this section to familiarize yourself with font definitions. The function XLoadQueryFont(Display *dp, char *name) returns an XFontStruct structure. This structure defines the extents for the character set. This is used to set the values of the Font field in a GC.

To draw a string on the screen, use the following:

XDrawString ( Display *dp, Drawable dw, GC gc,
     int x, int y, char *str, int len);

This only uses the foreground color. To draw with the background and foreground, use this:

XDrawImageString ( Display *dp, Drawable dw, GC gc,
          int x, int y, char *str, int len);

The X Color Model

The X color model is based on an array of colors called a colormap. Applications refer to a color by its index in this colormap. The indexes are placed in an application's frame buffer, which contains an entry for each pixel of the display. The number of bits in the index define the number of bitplanes. The number of bitplanes define the number of colors that can be displayed on a screen at one time. For example, one bit per pixel displays two colors, four bits per pixel displays 16 colors, and eight bits per pixel displays 256 colors.

An application generally inherits the colormap of its parent. It can also create its own colormap by using the XCreateColormap call. The call is defined as:

Colormap XCreateColormap( Display *display,
               Window   dwindow,
               Visual   *vp,
               int       allocate);

This allocates the number of allocate color entries in a window's colormap. Generally, the visual parameter is derived from this macro:

DefaultVisual (Display *display, int screenNumber);

Here screenNumber = 0 in almost all cases. See the section "Screens, Displays, and Windows" in Chapter 1 for a definition of screens.

Colormaps are a valuable resource in X and must be freed after use. This is done through this call:

XFreeColormap(Display *display, Colormap c);

Applications can discover the standard colormap from the X server by using the XGetStandardColormap() call, and can set it through the XSetStandardColormap() call. These are defined as

XGetStandardColormap( Display *display,
          Window  dwindow,
          XStandardColormap *c,
         Atom      property);

and

XSetStandardColormap( Display *display,
     Window  dwindow, XStandardColormap *c, Atom      property);

Once applications have a colormap to work with, you have to take two steps:

1. Define the colormap entries.

The property atom can take the values of RGB_BEST_MAP, RGB_GRAY_MAP, or RGB_DEFAULT_MAP. These are names of colormaps stored in the server. They are not colormaps themselves.

2. Set the colormap for a window through this call:
XSetWindowColormap ( Display *display,
          Window  dwindow,
          Colormap c );
For allocating a color in the colormap, use the XColor structure defined in <X/Xlib.h>.

To see a bright blue color, use the segment:
XColor color;
color.red = 0;
color.blue = 0xffff;
color.green = 0;
Then add the color to the colormap using the call to the function:
XAllocColor(Display *display,
     Window dwindow,
     XColor *color );

A sample function that sets the color of a widget is shown in file 2_17c on the CD-ROM.

The default white and black pixels are defined as the following:

Pixel BlackPixel( Display *dpy, int screen); Pixel WhitePixel( Display *dpy, int screen);

These will work with any screen as a fallback.

The index (Pixel) returned by this function is not guaranteed to be the same every time the application runs. This is because the colormap could be shared between applications that each request colors in a different order. Each entry is allocated on the basis of next available entry. Sometimes if you overwrite an existing entry in a cell, you may actually see a change in a completely different application. So be careful.

Applications can query the RGB components of a color by calling this function:

XQueryColor( Display *display,
          Colormap *cmp,
          XColor  *clr);

For many colors at one time, use this:

XQueryColors( Display *display,
          Colormap *cmp,
          XColor  *clr,
          int number);

At this time, the application can modify the RGB components and then store them in the colormap with this call:

XStoreColor( Display *display,
          Colormap *cmp,
          XColor  *clr);

Recall that X11 has some strange names for colors in /usr/lib/rgb.txt. Applications can get the RGB components of these names with a call to this:

XLookupColor( Display *display,
          Colormap cmp,
          char     *name,
          XColor  *clr
          XColor  *exact);

The name is the string to search for in the rgb.txt file. The returned value clr contains the next closest existing entry in the colormap.

The exact color entry contains the exact RGB definition in the entry in rgb.txt. This function does not allocate the color in the colormap. To do that, use this call:

XAllocNamedColor( Display *display,
          Colormap cmp,
          char    *name,
          XColor  *clr
          XColor  *exact);

Pixmaps, Bitmaps, and Images

A pixmap is like a window but is off-screen, and is, therefore, invisible to the user. It is usually the same depth as the screen. You create a pixmap with this call:

XCreatePixmap (Display *dp,
Drawable dw, unsigned int width, unsigned int height, unsigned int depth);

A drawable can be either a window (on-screen) or a pixmap (off-screen). Bitmaps are pixmaps with a depth of one pixel. Look in /usr/include/X11/bitmaps for a listing of some of the standard bitmaps.

The way to copy pixmaps from memory to the screen is through this call:

XCopyArea( Display dp,
     Drawable Src,
     Drawable Dst,
     GC   gc,
     int  src_x,
     int  src_y,
     unsigned int width,
     unsigned int height,
     int  dst_x,
     int  dst_y);

The caveat with this call is that the Src and Dst drawables have to be of the same depth. To show a bitmap with a depth greater than one pixel on a screen, you have to copy the bitmap one plane at a time. This is done through the following call:

XCopyPlane( Display dp,
     Drawable Src,
     Drawable Dst,
     GC   gc,
     int  src_x,
     int  src_y,
     unsigned int width,
     unsigned int height,
     int  dst_x,
     int  dst_y,
     unsigned long plane);

The plane specifies the bit plane that this one-bit-deep bitmap must be copied to. The actual operation is largely dependent on the modes set in the GC.

For example, to show the files in the /usr/include/bitmaps directory, which have three defined values for a sample file called gumby.h:

  • gumby_bits: Pointer to an array of character bits

  • gumby_height: Integer height

  • gumby_width: Integer width

First create the bitmap from the data using the XCreateBitmapFromData() call. To display this one-plane-thick image, copy the image from this plane to plane 1 of the display. You can actually copy to any plane in the window.

A sample call could be set for copying from your pixmap to the widget's plane 1 in the following manner:

XCopyPlane( XtDisplay(w), yourPixmap, XtWindow(w), gc,
0,0, your_height, your_width, 0,0,1);

It copies from the origin of the pixmap to the origin of plane 1 of the window.

There are other functions for working with images in X. These include the capability to store device-dependent images on disk and the Xpm format.

Xpm was designed to define complete icons and is complicated for large pixmaps. The format for an Xpm file is as follows:

char *filename[] =
{
"Width Height numColors CharacterPerPixel",
"character colortypes"
..PIXELS..
};

A string of "8 8 2 1" defines an 8x8 icon with two colors and one character per color. The PIXELS are strings of characters: the number of strings equals the number of rows. The number of characters per string equals the number of columns.

The character represents a color. Colortypes are a type followed by a color name. So "a c red m white" would show a red pixel at every "a" character on color screens, and a white pixel on monochrome screens. See the following example:

char *someFig[ ] = {
"8 8 2 1",
"a c red m white",
". c blue m black",
"aa....aa",
"aa....aa",
"aa....aa",
"aaaaaaaa","aaaaaaaa",
"aa....aa",
"aa....aa",
"aa....aa"
};

See the man pages for more details on using Xpm files. Look for the functions XpmReadFileToPixmap and XpmWriteFileToPixmap for information on reading these images from or storing them to disk.

Variations on the Theme: Complements and Alternatives to Motif

Motif is essentially a universal GUI toolkit for UNIX in the sense that almost any GUI that might be written can be done with Motif. There are many projects, though, for which Motif is only a possible choice of technology, not the best one. Consider these three categories: tools to use instead of Motif; tools to use with Motif; and tools to use above Motif.

Toolkits That Substitute for Motif

There are several other widget sets a UNIX GUI developer should consider, as mentioned previously in "The Technologies". Apart from OpenStep (http://www.next.com/OPENSTEP/), the most important ones are based on X. One of us particularly recommends Tk as an alternative to Motif. A full comparison is beyond the scope of this chapter, but we'll mention three aspects that are easiest to explain: licensing, language neutrality, and ease-of-first-use. Tk is freely available, at no charge. While commercial organizations exist to support it, they're generally hard put to match the responsiveness and intelligence that characterize the relevant comp.lang.* Usenet newsgroups. Second, Tk bindings specific to several languages are available; in particular, tkperl is quite a success among the Perl community (see Chapter 5 for more on Perl). Finally, it's much less intimidating starting with Tk than with Motif. A beginner in Motif needs to understand where libraries are, how to compile and link, how to launch an event loop, and the XtSetArg() idiom. Once a Tk interpreter has been properly installed, though, scripting as simple as


button .mybutton -text "Push me" -command exit
     pack .mybutton

creates a tiny, but complete--and readable!--application.

Supplements to Motif

GUI interface builders are used with Motif (among other toolkits). They ease layout chores, but require toolkit expertise for completing an application.

They do save time even if you don't intend to use the code generated by the builder. They streamline lay out of widgets and assignment of appropriate placements to achieve desired effects (colors, X,Y positions, and so on).

One of the failings of such software packages is that no backups are kept of the code that a developer has elaborated from the callback stubs. Refer to the sections on using and writing Motif widgets for more information about callbacks. This software simply generates code from the interface that the user has designed. This code includes all stubs for the widgets that the user has designated. Therefore, regenerating code from an edited interface overwrites any modifications to any previously edited stubs. Some builders do this, but some don't. Check with your vendor.

Environments tend to lock you into a specific programming mode. For some developers, this may equate to lack of freedom, and may turn them away from what might well mean time to market. The time to try an environment out and test its flexibility is before you buy.

Code generated by GUI builders might not be the most efficient for your particular application. You should be able to easily modify the generated code.

Check to see if functionality can be added without going through special hoops (such as precompilers). For example, how easy is it to add your own C++ classes?

Does the builder generate native code, or do you have to use his libraries? If shared libraries are supplied, be careful to check the licensing agreements or see if static versions are available.

Among the suppliers of adjuncts to Motif are

  • DataViews's (800-732-3300 or www.dvcorp.com) X-Designer generates C, C++, or UIL (User Interface Language) source code, and also provides a path for porting to Microsoft Windows.

  • Imperial Software Technology Ltd., Reading, England, through VI Corporation (800-732-3200), offers a GUI builder, X-Designer, which has built-in OpenLook-to-Motif conversion.

  • Integrated Computer Solutions (800-683-5023 or www.ics.com) maintains several products including Builder Xcessory.

  • KL Group (800-663-4723 or www.klg.com) is one of the leaders in selling widgets that supplement Motif functionality with carefully crafted charting, data-entry, tab, table, and other widgets.

  • LIANT (800-237-1873 / 800-762-6265 or www.liant.com) offers a C++/Views visual programming tool that ports Motif applications to DOS text, OS/2, Windows, and so on.
  • SL Corporation's (415-927-1724 or www.sl.com) SL-GMS is a toolkit with market success in projects in avionics, network management, and process control.

  • XVT Design (800-678-7988) offers an Interactive Design Tool and the XVT Portability Toolkit, which ports Motif applications to DOS text, OS/2, Windows, and so on.

  • Zinc Software (801-785-8900 or www.zinc.com) offers Zinc Designer and Applications Framework to build and port Motif applications to DOS text, OS/2, Windows, and so on.

Motif-Based GUI Development Environments

A third category of product is those GUI builders that are built on top of Motif. They present a development interface that is complete in the sense that there is no intrinsic need to program directly in Motif. Among the products in this category are those from

  • Bluestone's (609-727-4600 or www.bluestone.com) db-UIM/X is a mature, successful GUI builder that emphasizes database connectivity, client-server deployment, and its object-oriented engineering.

  • Kinesix (713-953-8300 or www.kinesix.com) provides Sammi, an integrated GUI building environment. Sammi has the reputation of being easy-to-use, and integrates well with other Motif adjuncts. It's also available for Windows NT and Windows 95, and includes networking and special-purpose widgets that make it popular in process control.

  • LOOX Software (800-684-LOOX or www.loox.com) is a Motif development environment known for its remarkably capable adjunct animation, editing, interactive, charting, and other widgets.

  • Neuron Data (800-876-4900 or www.neurondata.com) offers a GUI development environment, Open Interface, that is ideal for Motif development in a number of respects. It's possible to do all work directly in the higher-level Open Interface approach, or to code entirely in standard Motif, or to combine the two freely. The results are then available on an amazing 40 platforms.

  • Computer Aid (800-444-WKSH) is the current vendor for the Windowing Korn Shell. WKSH is an outlier. The other commercial products we profile show a lot of similarities among themselves in their marketing, training requirements, integration with other standards, and high price. WKSH's story is different. It's designed for UNIX developers who are most comfortable with command-line-oriented ksh scripting (see Chapter 11 of the System Administrator's Edition, "The Korn Shell"), but who have an ancillary requirement for a point-and-click interface. This is a narrow niche, but WKSH fills it precisely.

  • Thomson Software Products' (619-457-2700 or www.thomsoft.com) TeleUSE is based on CDE and CORBA standards. It integrates well with algorithms coded in C, C++, and Ada, several database management systems, and, in its Windows versions, native controls. It's a comfortable environment for individual developers, but also one of the best at facilitating the teamwork of large projects.

  • Visual Edge Software's (514-332-6430 or www.vedge.com) UIM/X is another mature product with strong presence in specific markets.

Readers should also be aware of UIL, a layout-specification language bundled with a Motif license. It is in wide use, but introducing it is beyond the scope of this chapter.

Please remember in reading these profiles how dynamic is the software industry. We've selected a few of the many vendors and technologies available based on our judgment as of April 1997, that these are likely to be of most interest to our readers, but there can be no guarantee that they'll occupy the same positions in the market in future years. Also, while it's important to distinguish the three categories we present in this section, it's also a fact that most vendors offer multiple products, and cross the boundaries. We've slotted them based, again, on our judgment of the benefits that are most likely to manifest for our readers.

Finally, we want readers outside the U.S.A., and those without World Wide Web access, to understand that we mean no offense by listing North American "800" telephone numbers and corporate URLs. Any reader who has difficulty contacting a particular vendor is welcome to send e-mail to claird@NeoSoft.com for internationally-accessible telephone and FAX numbers.

Acknowledgments

Kamran Husain is indebted to Metro Link software for providing its version of Motif 1.2, which he used to develop the routines and test the sources in this book. Metro Link's software installed cleanly with no hassles on a Linux (1.02) system running on a 386DX. All libraries worked great and presented no compatibility problems in porting to Sun and AIX. There was no reason to call the Metro Link support line, so he could not evaluate it. The price for all binaries and the development system is $208, which includes overnight shipping and a choice of Volume 3 or Volume 6 from the O'Reilly X Window System User's Guide manual set.

You can contact Metro at (305) 938-0283.

References

Quercia, Valerie and O'Reilly, Tim. The Definitive Guides to the X Window System, X Window System User's Guide, Volume Three, Motif Edition. O'Relly, March 1992.

Johnson, Eric F. and Reichard, Kevin. Advanced X Window Applications Programming. MIS: Press, 1990.

Johnson, Eric F. and Reichard, Kevin. Power Programming ... Motif Second Edition. MIS: Press, 1993.

OSF/Motif Programmers Guide. Prentice Hall, 1993.

OSF/Motif Style Guide. Prentice Hall, 1990.

Taylor, Dave. Teach Yourself UNIX in a Week. Sams Publishing, 1994.

Rost, Randi J. X and Motif Quick Reference Guide. Digital Press, 1990.

Young, Doug. The X Window System Programming and Applications with Xt, OSF/Motif Edition, 1994.

Several informative references are freely available online. Motif newcomers should particularly study:

MOTIF Frequently Asked Questions: http://www.cen.com/mw3/faq/motif-faq.html

comp.windows.x.intrinsics Frequently Asked Questions: http://www.cs.ruu.nl/wais/html/na-dir/Xt-FAQ.html

comp.windows.x Frequently Asked Questions: http://www.cs.ruu.nl/wais/html/na-dir/x-faq/.html

User Interface Software Tools: http://www.cs.cmu.edu/afs/cs.cmu.edu/user/bam/www/toolnames.html

Technical X Window System and OSF/Motif WWW Sites: http://www.rahul.net/kenton/xsites.html

Summary

The body of these two volumes makes clear that UNIX is fundamentally an architecture for organizing computational and communications tasks. The user interface for delivering the results of those tasks has always been of secondary importance to the mainstream of UNIX workers. Motif has emerged from this marginal position, though, as the canonical answer to questions about how to implement WIMP GUIs for UNIX. In 1997, graphical user interfaces for UNIX are either based on Motif, or are defined in terms of their ability to compete with Motif. Even the erupting technologies of World-Wide Web browsers and Java share the bulk of their concepts and vocabulary with Motif.

Programming GUIs for UNIX, therefore, starts in programming Motif. After working through this chapter, you have a thorough knowledge of all the elements of authoring and generating small working Motif applications. You know how to exploit the common widgets that suffice for the most common applications. Perhaps most importantly, you know enough of the idioms and habits of Motif programming to maintain and enhance with efficiency what others have written. Finally, you have a perspective and the essential references to judge when it's best to move beyond Motif.

Motif is fun for us, and it's gratifying to satisfy our users with what we create. We hope you have the same success.

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