Volume 6A: Motif Programming Manual

PushButton's XmN-activateCallback resource is called, the reason is XmCR_ACTIVATE. The different

values for reason make it easier to write callback routines that are called by more than one type of widget. By

testing the reason field, you can determine the appropriate action to take in the callback. Because the widget is

always passed to the callback function, you can also find out what widget caused the function to be invoked.

The event field contains the actual event that triggered the callback, which can provide a great deal of useful

information. See Volume Four, X Toolkit Intrinsics Programming Manual, for information on how to interpret the

contents of an event. That subject is not discussed at length in this book, although our examples frequently use the

events in callback structures to control processing.

3.3.7 The Event Loop

Once all of the widgets for an application have been created and managed and all of the callbacks have been

registered, it's time to start the application running. The final two function calls in hello.c perform this task:

XtRealizeWidget (toplevel);

XtAppMainLoop (app);

Realizing a widget creates the actual window for the widget. When you call XtRealizeWidget() on the top−level

widget of an application (the one returned by the call to XtVaAppInitialize()), Xt recursively traverses the

hierarchy of widgets in the application and creates a window for each widget. Before this point, the widgets existed

only as data structures on the client side of the X connection. ...