# Using Widgets

## Using Widgets

Widgets serve as the primary tools for building a user interface or application environment. The Athena widget set consists of primitive widgets that contain no children (for example, a command button) and composite widgets which may contain one or more widget children (for example, a Box widget).

The remaining chapters explain the widgets that are provided by the Athena widget set. These user-interface components serve as an interface for application programmers who do not want to implement their own widgets. In addition, they serve as a starting point for those widget programmers who, using the Intrinsics mechanisms, want to implement alternative application programming interfaces.

This chapter is a brief introduction to widget programming. The examples provided use the Athena widgets, though most of the concepts will apply to all widget sets. Although there are several programming interfaces to the X Toolkit, only one is described here. A full description of the programming interface is provided in the document X Toolkit Intrinsics - C Language Interface.

### Setting the Locale

If it is desirable that the application take advantage of internationalization (i18n), you must establish locale with `XtSetLanguageProc` before `XtDisplayInitialize` or `XtAppInitialize` is called. For full details, please refer to the document X Toolkit Intrinsics - C Language Interface, section 2.2. However, the following simplest-case call is sufficient in many or most applications.

    	XtSetLanguageProc(NULL, NULL, NULL);

Most notably, this will affect the Standard C locale, determine which resource files will be loaded, and what fonts will be required of FontSet specifications. In many cases, the addition of this line is the only source change required to internationalize Xaw programs, and will not disturb the function of programs in the default "C" locale.

### Initializing the Toolkit

You must call a toolkit initialization function before invoking any other toolkit routines (besides locale setting, above). `XtAppInitialize` opens the X server connection, parses the command line, and creates an initial widget that will serve as the root of a tree of widgets created by this application.

      Widget XtAppInitialize
      XtAppContext app_context_return
      String application_class
      XrmOptionDescRec options
      Cardinal num_options
      int *argc_in_out
      String *argv_in_out
      String *fallback_resources
      ArgList args
      Cardinal num_args

app_con_return

  Returns the application context of this application, if non-NULL.

application_class

  Specifies the class name of this application, which is usually the generic name for all instances of this application. A useful convention is to form the class name by capitalizing the first letter of the application name. For example, the application named ``xman'' has a class name of ``Xman''.

options

  Specifies how to parse the command line for any application-specific resources. The options argument is passed as a parameter to `XrmParseCommand` . For further information, see Xlib - C Language Interface.

num_options

  Specifies the number of entries in the options list.

argc_in_out

  Specifies a pointer to the number of command line parameters.

argv_in_out

  Specifies the command line parameters.

fallback_resources

  Specifies resource values to be used if the site-wide application class defaults file cannot be opened, or NULL.

args

  Specifies the argument list to use when creating the Application shell.

num_args

  Specifies the number of arguments in args.

This function will remove the command line arguments that the toolkit reads from argc_in_out, and argv_in_out. It will then attempt to open the display. If the display cannot be opened, an error message is issued and XtAppInitialize terminates the application. Once the display is opened, all resources are read from the locations specified by the Intrinsics. This function returns an ApplicationShell widget to be used as the root of the application's widget tree.

### Creating a Widget

Creating a widget is a three-step process. First, the widget instance is allocated, and various instance-specific attributes are set by using `XtCreateWidget`. Second, the widget's parent is informed of the new child by using `XtManageChild`. Finally, X windows are created for the parent and all its children by using `XtRealizeWidget` and specifying the top-most widget. The first two steps can be combined by using `XtCreateManagedWidget`. In addition, `XtRealizeWidget` is automatically called when the child becomes managed if the parent is already realized.

To allocate, initialize, and manage a widget, use `XtCreateManagedWidget .`

      Widget XtCreateManagedWidget
      String name
      WidgetClass widget_class
      Widget parent
      ArgList args
      Cardinal num_args

name

  Specifies the instance name for the created widget that is used for retrieving widget resources.

widget_class

  Specifies the widget class pointer for the created widget.

parent

  Specifies the parent widget ID.

args

  Specifies the argument list. The argument list is a variable-length list composed of name and value pairs that contain information pertaining to the specific widget instance being created. For further information, see Section 2.7.2.

num_args

  Specifies the number of arguments in the argument list. If the num_args is zero, the argument list is never referenced.

When a widget instance is successfully created, the widget identifier is returned to the application. If an error is encountered, the `XtError` routine is invoked to inform the user of the error.

For further information, see X Toolkit Intrinsics - C Language Interface.

### Common Resources

Although a widget can have unique arguments that it understands, all widgets have common arguments that provide some regularity of operation. The common arguments allow arbitrary widgets to be managed by higher-level components without regard for the individual widget type. Widgets will ignore any argument that they do not understand.

The following resources are retrieved from the argument list or from the resource database by all of the Athena widgets:

Name | Class | Type | Default Value
--- | --- | --- | ---
accelerators | Accelerators | AcceleratorTable | NULL
ancestorSensitive | AncestorSensitive | Boolean | True
background | Background | Pixel | XtDefaultBackground
backgroundPixmap | Pixmap | Pixmap | XtUnspecifiedPixmap
borderColor | BorderColor | Pixel | XtDefaultForeground
borderPixmap | Pixmap | Pixmap | XtUnspecifiedPixmap
borderWidth | BorderWidth | Dimension | 1
colormap | Colormap | Colormap | Parent's Colormap
depth | Depth | int | Parent's Depth
destroyCallback | Callback | XtCallbackList | NULL
height | Height | Dimension | widget dependent
mappedWhenManaged | MappedWhenManaged | Boolean | True
screen | Screen | Screen | Parent's Screen
sensitive | Sensitive | Boolean | True
translations | Translations | TranslationTable | widget dependent
width | Width | Dimension | widget dependent
x | Position | Position | 0
y | Position | Position | 0

The following additional resources are retrieved from the argument list or from the resource database by many of the Athena widgets:

Name | Class | Type | Default Value
--- | --- | --- | ---
callback | Callback | XtCallbackList | NULL
cursor | Cursor | Cursor | widget dependent
foreground | Foreground | Pixel | XtDefaultForeground
insensitiveBorder | Insensitive | Pixmap | GreyPixmap

### Resource Conversions

Most resources in the Athena widget set have a converter registered that will translate the string in a resource file to the correct internal representation. While some are obvious (string to integer, for example), others need specific mention of the allowable values. Three general converters are described here:

- Cursor

- Pixel

- Bitmap

Many widgets have defined special converters that apply only to that widget. When these occur, the documentation section for that widget will describe the converter.

#### Cursor Conversion

The value for the `cursorName` resource is specified in the resource database as a string, and is of the following forms:

- A standard X cursor name from `< X11/cursorfont.h >`. The names in `cursorfont.h` each describe a specific cursor. The resource names for these cursors are exactly like the names in this file except the `XC_` is not used. The cursor definition `XC_gumby` has a resource name of `gumby`.

- Glyphs, as in FONT font-name glyph-index [[ font-name ] glyph-index ]. The first font and glyph specify the cursor source pixmap. The second font and glyph specify the cursor mask pixmap. The mask font defaults to the source font, and the mask glyph index defaults to the source glyph index.

- A relative or absolute file name. If a relative or absolute file name is specified, that file is used to create the source pixmap. Then the string "Mask" is appended to locate the cursor mask pixmap. If the "Mask" file does not exist, the suffix "msk" is tried. If "msk" fails, no cursor mask will be used. If the filename does not start with '/' or './' the the bitmap file path is used (see section 2.4.3).

#### Pixel Conversion

The string-to-pixel converter takes any name that is acceptable to XParseColor (see Xlib - C Language Interface). In addition this routine understands the special toolkit symbols `XtDefaultForeground' and `XtDefaultBackground', described in X Toolkit Intrinsics - C Language Interface. In short the acceptable pixel names are:

- Any color name for the rgb.txt file (typically in the directory /usr/lib/X11 on POSIX systems).

- A numeric specification of the form #<red><green><blue> where these numeric values are hexadecimal digits (both upper and lower case).

- The special strings `XtDefaultForeground' and `XtDefaultBackground'

#### Bitmap Conversion

The string-to-bitmap converter attempts to locate a file containing bitmap data whose name is specified by the input string. If the file name is relative (i.e. does not begin with / or ./), the directories to be searched are specified in the `bitmapFilePath` resource--class `BitmapFilePath`. This resource specifies a colon (:) separated list of directories that will be searched for the named bitmap or cursor glyph (see section 2.4.1). The `bitmapFilePath` resource is global to the application, and may `not` be specified differently for each widget that wishes to convert a cursor to bitmap. In addition to the directories specified in the `bitmapFilePath` resource a default directory is searched. When using POSIX the default directory is `/usr/include/X11/bitmaps .`

### Realizing a Widget

The `XtRealizeWidget` function performs two tasks:

- Calculates the geometry constraints of all managed descendants of this widget. The actual calculation is put off until realize time for performance reasons.

- Creates an X window for the widget and, if it is a composite widget, realizes each of its managed children.

        void XtRealizeWidget
        Widget w

- Specifies the widget.

For further information about this function, see the X Toolkit Intrinsics - C Language Interface.

### Processing Events

Now that the application has created, managed and realized its widgets, it is ready to process the events that will be delivered by the X Server to this client. A function call that will process the events is `XtAppMainLoop`.

      void XtAppMainLoop
      XtAppContext app_context

app_context

  Specifies the application context of this application. The value is normally returned by `XtAppInitialize`.

This function never returns: it is an infinite loop that processes the X events. User input can be handled through callback procedures and application defined action routines. More details are provided in X Toolkit Intrinsics - C Language Interface.

### Standard Widget Manipulation Functions

After a widget has been created, a client can interact with that widget by calling one of the standard widget manipulation routines provided by the Intrinsics, or a widget class-specific manipulation routine.

The Intrinsics provide generic routines to give the application programmer access to a set of standard widget functions. The common widget routines let an application or composite widget perform the following operations on widgets without requiring explicit knowledge of the widget type.

- Control the mapping of widget windows

- Destroy a widget instance

- Obtain an argument value

- Set an argument value

#### Mapping Widgets

By default, widget windows are mapped (made viewable) automatically by `XtRealizeWidget`. This behavior can be disabled by using `XtSetMappedWhenManaged`, making the client responsible for calling `XtMapWidget` to make the widget viewable.

      void XtSetMappedWhenManaged
      Widget w
      Boolean map_when_managed

w

  Specifies the widget.

map_when_managed

  Specifies the new value. If map_when_managed is `True`, the widget is mapped automatically when it is realized. If map_when_managed is `False`, the client must call `XtMapWidget` or make a second call to `XtSetMappedWhenManaged` to cause the child window to be mapped.

The definition for `XtMapWidget` is:

      void XtMapWidget
      Widget w

w

  Specifies the widget.

When you are creating several children in sequence for a previously realized common parent it is generally more efficient to construct a list of children as they are created (using `XtCreateWidget`) and then use `XtManageChildren` to request that their parent managed them all at once. By managing a list of children at one time, the parent can avoid wasteful duplication of geometry processing and the associated ``screen flash''.

      void XtManageChildren
      WidgetList children
      Cardinal num_children

children

  Specifies a list of children to add.

num_children

  Specifies the number of children to add.

If the parent is already visible on the screen, it is especially important to batch updates so that the minimum amount of visible window reconfiguration is performed.

For further information about these functions, see the X Toolkit Intrinsics - C Language Interface.

#### Destroying Widgets

To destroy a widget instance of any type, use `XtDestroyWidget`

      void XtDestroyWidget
      Widget w

w

  Specifies the widget.

`XtDestroyWidget` destroys the widget and recursively destroys any children that it may have, including the windows created by its children. After calling `XtDestroyWidget ,` no further references should be made to the widget or any children that the destroyed widget may have had.

#### Retrieving Widget Resource Values

To retrieve the current value of a resource attribute associated with a widget instance, use `XtGetValues .`

      void XtGetValues
      Widget w
      ArgList args
      Cardinal num_args

w

  Specifies the widget.

args

  Specifies a variable-length argument list of name and `address` pairs that contain the resource name and the address into which the resource value is stored.

num_args

  Specifies the number of arguments in the argument list.

The arguments and values passed in the argument list are dependent on the widget. Note that the caller is responsible for providing space into which the returned resource value is copied; the `ArgList` contains a pointer to this storage (e.g. x and y must be allocated as Position). For further information, see the X Toolkit Intrinsics - C Language Interface.

#### Modifying Widget Resource Values

To modify the current value of a resource attribute associated with a widget instance, use `XtSetValues .`

      void XtSetValues
      Widget w
      ArgList args
      Cardinal num_args

w

  Specifies the widget.

args

  Specifies an array of name and `value` pairs that contain the arguments to be modified and their new values.

num_args

  Specifies the number of arguments in the argument list.

The arguments and values that are passed will depend on the widget being modified. Some widgets may not allow certain resources to be modified after the widget instance has been created or realized. No notification is given if any part of a `XtSetValues` request is ignored.

For further information about these functions, see the X Toolkit Intrinsics - C Language Interface. The argument list entry for `XtGetValues` specifies the address to which the caller wants the value copied. The argument list entry for `XtSetValues ,` however, contains the new value itself, if the size of value is less than sizeof(XtArgVal) (architecture dependent, but at least sizeof(long)); otherwise, it is a pointer to the value. String resources are always passed as pointers, regardless of the length of the string.

### Using the Client Callback Interface

Widgets can communicate changes in their state to their clients by means of a callback facility. The format for a client's callback handler is:

      void CallbackProc
      Widget w
      XtPointer client_data
      XtPointer call_data

w

  Specifies widget for which the callback is registered.

client_data

  Specifies arbitrary client-supplied data that the widget should pass back to the client when the widget executes the client's callback procedure. This is a way for the client registering the callback to also register client-specific data: a pointer to additional information about the widget, a reason for invoking the callback, and so on. If no additional information is necessary, NULL may be passed as this argument. This field is also frequently known as the closure.

call_data

  Specifies any callback-specific data the widget wants to pass to the client. For example, when Scrollbar executes its `jumpProc` callback list, it passes the current position of the thumb in call_data.

Callbacks can be registered either by creating an argument containing the callback list described below or by using the special convenience routines `XtAddCallback` and `XtAddCallbacks`. When the widget is created, a pointer to a list of callback procedure and data pairs can be passed in the argument list to `XtCreateWidget .` The list is of type `XtCallbackList :`

    typedef struct {
    	XtCallbackProc callback;
    	XtPointer closure;
    } XtCallbackRec, *XtCallbackList;

The callback list must be allocated and initialized before calling `XtCreateWidget .` The end of the list is identified by an entry containing NULL in callback and closure. Once the widget is created, the client can change or de-allocate this list; the widget itself makes no further reference to it. The closure field contains the client_data passed to the callback when the callback list is executed.

The second method for registering callbacks is to use `XtAddCallback` after the widget has been created.

      void XtAddCallback
      Widget w
      String callback_name
      XtCallbackProc callback
      XtPointer client_data

w

  Specifies the widget to add the callback to.

callback_name

  Specifies the callback list within the widget to append to.

callback

  Specifies the callback procedure to add.

client_data

  Specifies the data to be passed to the callback when it is invoked.

`XtAddCallback` adds the specified callback to the list for the named widget.

All widgets provide a callback list named `destroyCallback` where clients can register procedures that are to be executed when the widget is destroyed. The destroy callbacks are executed when the widget or an ancestor is destroyed. The call_data argument is unused for destroy callbacks.

### Programming Considerations

This section provides some guidelines on how to set up an application program that uses the X Toolkit.

#### Writing Applications

When writing an application that uses the X Toolkit, you should make sure that your application performs the following:

1. Include `< X11/Intrinsic.h >` in your application programs. This header file automatically includes `< X11/Xlib.h >,` so all Xlib functions also are defined. It may also be necessary to include `< X11/StringDefs.h >` when setting up argument lists, as many of the XtNsomething definitions are only defined in this file.

2. Include the widget-specific header files for each widget type that you need to use. For example, `< X11/Xaw/Label.h >` and `< X11/Xaw/Command.h >.`

3. Call the `XtAppInitialize` function before invoking any other toolkit or Xlib functions. For further information, see Section 2.1 and the X Toolkit Intrinsics - C Language Interface.

4. To pass attributes to the widget creation routines that will override any site or user customizations, set up argument lists. In this document, a list of valid argument names is provided in the discussion of each widget. The names each have a global symbol defined that begins with `XtN` to help catch spelling errors. For example, `XtNlabel` is defined for the `label` resource of many widgets.

5. For further information, see Section 2.9.2.2.

6. When the argument list is set up, create the widget with the `XtCreateManagedWidget` function. For further information, see Section 2.2 and the X Toolkit Intrinsics - C Language Interface.

7. If the widget has any callback routines, set by the `XtNcallback` argument or the `XtAddCallback` function, declare these routines within the application.

8. After creating the initial widget hierarchy, windows must be created for each widget by calling `XtRealizeWidget` on the top level widget.

9. Most applications now sit in a loop processing events using `XtAppMainLoop ,` for example:

10.     XtCreateManagedWidget(name, class, parent, args, num_args);
        XtRealizeWidget(shell);
        XtAppMainLoop(app_context);

11. For information about this function, see the X Toolkit Intrinsics - C Language Interface.

12. Link your application with `libXaw` (the Athena widgets), `libXmu` (miscellaneous utilities), `libXt` (the X Toolkit Intrinsics), `libSM` (Session Management), `libICE` (Inter-Client Exchange), `libXext` (the extension library needed for the shape extension code which allows rounded Command buttons), and `libX11` (the core X library). The following provides a sample command line:

13.     cc -o application application.c \-lXaw \-lXmu \-lXt \
        \-lSM \-lICE \-lXext \-lX11

#### Changing Resource Values

The Intrinsics support two methods of changing the default resource values; the resource manager, and an argument list passed into XtCreateWidget. While resources values will get updated no matter which method you use, the two methods provide slightly different functionality.

Resource Manager

  This method picks up resource definitions described in Xlib - C Language Interface from many different locations at run time. The locations most important to the application programmer are the fallback resources and the app-defaults file, (see X Toolkit Intrinsics - C Language Interface for the complete list). Since these resource are loaded at run time, they can be overridden by the user, allowing an application to be customized to fit the particular needs of each individual user. These values can also be modified without the need to rebuild the application, allowing rapid prototyping of user interfaces. Application programmers should use resources in preference to hard-coded values whenever possible.

Argument Lists

  The values passed into the widget at creation time via an argument list cannot be modified by the user, and allow no opportunity for customization. It is used to set resources that cannot be specified as strings (e.g. callback lists) or resources that should not be overridden (e.g. window depth) by the user.

##### Specifying Resources

It is important for all X Toolkit application programmers to understand how to use the X Resource Manager to specify resources for widgets in an X application. This section will describe the most common methods used to specify these resources, and how to use the X Resource manager.

`Xrdb`

  The `xrdb` utility may be used to load a file containing resources into the X server. Once the resources are loaded, the resources will affect any new applications started on the display that they were loaded onto.

Application Defaults

  The application defaults (app-defaults) file (normally in /usr/lib/X11/app-defaults/classname) for an application is loaded whenever the application is started.

The resource specification has two colon-separated parts, a name, and a value. The value is a string whose format is dependent on the resource specified by name. Name is constructed by appending a resource name to a full widget name.

The full widget name is a list of the name of every ancestor of the desired widget separated by periods (.). Each widget also has a class associated with it. A class is a type of widget (e.g. Label or Scrollbar or Box). Notice that class names, by convention, begin with capital letters and instance names begin with lower case letters. The class of any widget may be used in place of its name in a resource specification. Here are a few examples:

xman.form.button1

  This is a fully specified resource name, and will affect only widgets called button1 that are children of widgets called form that are children of applications named xman. (Note that while typically two widgets that are siblings will have different names, it is not prohibited.)

Xman.Form.Command

  This will match any Command widget that is a child of a Form widget that is itself a child of an application of class Xman.

Xman.Form.button1

  This is a mixed resource name with both widget names and classes specified.

This syntax allows an application programmer to specify any widget in the widget tree. To match more than one widget (for example a user may want to make all Command buttons blue), use an asterisk (*) instead of a period. When an asterisk is used, any number of widgets (including zero) may exist between the two widget names. For example:

Xman*Command

  This matches all Command widgets in the Xman application.

Foo*button1

  This matches any widget in the Foo application that is named button1.

The root of all application widget trees is the widget returned by `XtAppInitialize`. Even though this is actually an ApplicationShell widget, the toolkit replaces its widget class with the class name of the application. The name of this widget is either the name used to invoke the application (`argv[0]`) or the name of the application specified using the standard -name command line option supported by the Intrinsics.

The last step in constructing the resource name is to append the name of the resource with either a period or asterisk to the full or partial widget name already constructed.

*foreground:Blue

  Specifies that all widgets in all applications will have a foreground color of blue.

Xman*borderWidth:10

  Specifies that all widgets in an application whose class is Xman will have a border width of 10 (pixels).

xman.form.button1.label:Testing

  Specifies that a particular widget in the xman application will have a label named Testing.

An exclamation point (!) in the first column of a line indicates that the rest of the line should be treated as a comment.

`Final Words`

The Resource manager is a powerful tool that can be used very effectively to customize X Toolkit applications at run time by either the application programmer or the user. Some final points to note:

- An application programmer may add new resources to their application. These resources are associated with the global application, and not any particular widget. The X Toolkit function used for adding the application resources is `XtGetApplicationResources`.

- Be careful when creating resource files. Since widgets will ignore resources that they do not understand, any spelling errors will cause a resource to have no effect.

- Only one resource line will match any given resource. There is a set of precedence rules, which take the following general stance.

- - More specific overrides less specific, thus period always overrides asterisk.

  - Names on the left are more specific and override names on the right.

  - When resource specifications are exactly the same, user defaults will override program defaults.

For a complete explanation of the rules of precedence, and other specific topics see X Toolkit Intrinsics - C Language Interface and Xlib - C Language Interface.

##### Creating Argument Lists

To set up an argument list for the inline specification of widget attributes, you may use any of the four approaches discussed in this section. Each resource name has a global symbol associated with it. This global symbol has the form XtNresource name. For example, the symbol for ``foreground'' is `XtNforeground`. For further information, see the X Toolkit Intrinsics - C Language Interface.

Argument are specified by using the following structure:

    typedef struct {
    	String name;
    	XtArgVal value;
    } Arg, *ArgList;

The first approach is to statically initialize the argument list. For example:

    static Arg arglist[] = {
    	{XtNwidth, (XtArgVal) 400},
    	{XtNheight, (XtArgVal) 300},
    };

This approach is convenient for lists that do not need to be computed at runtime and makes adding or deleting new elements easy. The `XtNumber` macro is used to compute the number of elements in the argument list, preventing simple programming errors:

    XtCreateWidget(name, class, parent, arglist, XtNumber(arglist));

The second approach is to use the `XtSetArg` macro. For example:

    Arg arglist[10];
    XtSetArg(arglist[1], XtNwidth, 400);
    XtSetArg(arglist[2], XtNheight, 300);

To make it easier to insert and delete entries, you also can use a variable index:

    Arg arglist[10];
    Cardinal i=0;
    XtSetArg(arglist[i], XtNwidth,  400);       i++;
    XtSetArg(arglist[i], XtNheight, 300);       i++;

The i variable can then be used as the argument list count in the widget create function. In this example, `XtNumber` would return 10, not 2, and therefore is not useful. You should not use auto-increment or auto-decrement within the first argument to `XtSetArg .` As it is currently implemented, `XtSetArg` is a macro that dereferences the first argument twice.

The third approach is to individually set the elements of the argument list array:

    Arg arglist[10];
    arglist[0].name  = XtNwidth;
    arglist[0].value = (XtArgVal) 400;
    arglist[1].name  = XtNheight;
    arglist[1].value = (XtArgVal) 300;

Note that in this example, as in the previous example, `XtNumber` would return 10, not 2, and therefore would not be useful.

The fourth approach is to use a mixture of the first and third approaches: you can statically define the argument list but modify some entries at runtime. For example:

    static Arg arglist[] = {
    	{XtNwidth, (XtArgVal) 400},
    	{XtNheight, (XtArgVal) NULL},
    };
    arglist[1].value = (XtArgVal) 300;

In this example, `XtNumber` can be used, as in the first approach, for easier code maintenance.

### Example Programs

The best way to understand how to use any programming library is by trying some simple examples. A collection of example programs that introduces each of the widgets in that Athena widget set, as well as many important toolkit programming concepts, is available in the X11R6 release as distributed by the X Consortium. It can be found in the distribution directory `contrib/examples/mit/Xaw`, but see your site administrator for the exact location of these files on your system. See the README file from that directory for a guide to the examples.