hpux man page for xteventhandler

XtEventHandler()														  XtEventHandler()

Name
  XtEventHandler - interface definition for event handler procedure.

Synopsis
  typedef void (*XtEventHandler)(Widget, XtPointer, XEvent *, Boolean *);
	 Widget w;
	 XtPointer client_data;
	 XEvent *event;
	 Boolean *continue_to_dispatch_return

Inputs
  w	    Specifies the widget for which the handler was registered.

  client_data
	    Specifies data registered with this event handler.

  event     Specifies the event that triggered this call.

Outputs
  continue_to_dispatch_return
	    Returns a Boolean indicating whether to call the remaining event handlers that are registered for the current event.

Description
  An XtEventHandler is registered with XtAddEventHandler(), XtAddRawEventHandler(), XtInsertEventHandler(), or XtInsertRawEventHandler().  It
  is called when one of the events that it was registered to handle occurs on the widget it was registered for.

  An XtEventHandler should do whatever processing is necessary for the widget or application to handle the event event that occurred on  wid-
  get w.  The client_data argument can be arbitrary data registered with the event handler procedure.  It is cast to an XtPointer when regis-
  tered and should be cast back to the appropriate type within the event handler.

  The continue_to_dispatch_return is the address of a Boolean variable which is intialized to True by the Intrinsics before the event handler
  is  called.	If  a  handler	sets  this variable to False, then no more handlers will be dispatched for the event.  Doing this may lead to
  portability problems because implementations of the Intrinsics are allowed to add event handlers for any widget at any time.	If  you  pre-
  vent these potential "invisible" event handlers from receiving events, the Intrinsics are not guaranteed to behave as expected.

Usage
  Most widgets and applications do not need to use event handlers explicitly.  Instead they can use translation tables and action procedures.

Example
  The  procedure  below is an event handler from the xmag client.  It is a special handler that follows the location of the mouse while it is
  dragged with the button down and a pointer grab is in effect.   When the button is released,	it  magnifies  a  new  area  of  the  screen,
  releases the pointer grab, and calls XtRemoveEventHandler() on itself.

     /*
      * ResizeEH() -- Event Handler for resize of selection box.
      */
     static void
     ResizeEH(w, closure, event, continue_to_dispatch)	     /* ARGSUSED */
	  Widget w; XtPointer closure; XEvent *event; Boolean *continue_to_dispatch;
     {
       hlPtr data = (hlPtr)closure;
       switch (event->type) {
       case MotionNotify:
	 data->x = event->xmotion.x_root;
	 data->y = event->xmotion.y_root;
	 break;
       case ButtonRelease:
	 GetImageAndAttributes(FindWindow(event->xmotion.x_root,
			     event->xmotion.y_root),
		  min(data->homeX,event->xbutton.x_root),
		  min(data->homeY,event->xbutton.y_root),
		  abs(data->homeX - event->xbutton.x_root),
		  abs(data->homeY - event->xbutton.y_root),
		  data);
	 if (data->newScale)
	   PopupNewScale(data);
	 else
	   SWSetImage(data->scaleInstance, data->image);
	 XtUngrabPointer(w, CurrentTime);
	 XtRemoveEventHandler(w, PointerMotionMask ButtonReleaseMask,
			      True, ResizeEH, (XtPointer)data);
	 data->selectMode = done;
	 break;
       }
     }

  This event handler is registered with the following code, invoked when the user presses mouse button 2.

	 XtAddEventHandler(w, PointerMotionMask ButtonReleaseMask,
			   True, ResizeEH, (XtPointer)data);

See Also
  XtAddEventHandler(1), XtAddRawEventHandler(1), XtAppAddActions(1), XtRemoveEventHandler(1), XtRemoveRawEventHandler(1).

Xt - Event Handling														  XtEventHandler()