ArcGIS Engine Developer Guide

C++ APPLICATION PROGRAMMING INTERFACE
Step 3: Manage the widgets
For the child to appear in the application, it must be managed. A single call,
XtManageChild, will do this for you, placing control of the widget in its parent’s
hands. This must be done for each widget. Place this line of code after the call to
XmStringFree, as shown.
XmStringFree(label);
XtManageChild(button);
Note: Even if a child is managed, it will not appear if its parent is not managed.
Step 4: Implement event listening and callback functions for widgets
You now have a button, but for that to be useful you must hook it to some
functionality. Widgets are attached to behavior at certain events through special
callback functions. You can add callbacks to a widget after it is created and either
before or after it is managed. As done in most of the C++ samples for Motif,
here you add the callback before it is managed.
XmStringFree(label);
XtAddCallback(button, XmNactivateCallback, ClickCallback, NULL);
XtManageChild(button);
These parameters have the following roles:
• button—the widget to add the callback to.
• XmNactivateCallback—the callback resource, defined by Motif to correspond
to certain events. Here you will pick activation of the button. For other
options, see the Motif reference at the end of this topic.
• ClickCallback—pointer to the function to call on the event.
• NULL—Client data to pass into the callback function. Here there is no data
the function will need, so NULL is passed.
Callback functions
For the callback to work, you must implement the function that is being called
on the event. Callbacks have a specific function signature, as follows:
Make sure that the data passed to client_data
will be in scope later when the callback routine
is executed.
void myCallbackName(Widget w, XtPointer client_data, XtPointer call_data)
The parameters are:
• w—the widget that was activated for this callback to be called.
• client_data—any data passed into the function, as indicated in the last parameter
of XtAddCallback.
• call_data—a structure containing data specific to the type of widget with
which the callback is associated.
For this example, place this function after main in pbExample.cpp, and have it
print to cerr the number of repeated clicks on this button (for example, “2
clicks” if double-clicked). Remember to also place a forward declaration of it
before main.
Chapter 4 • Developer environments • 237