Volume 6A: Motif Programming Manual

In this example, the ManagerGadget actions don't do anything unless the pointer is inside the Clear button, so the

translation is relatively safe. However, you should be sure to remember that both actions are called. If you press

Button 1 inside the PushButton and doodle around a bit before releasing it, the drawing is still done, even though the

result is hidden by the gadget. In another application, the fact that actions for both the drawing area itself and its

gadget children are both called might lead to indeterminate results.

The draw() action does not (and cannot) know if the gadget is also going to react to the button event. This problem

does not exist with the standard DrawingAreaInput() action routine used in the previous examples because that

routine is implemented by the Motif toolkit and it uses its own internal mechanisms to determine if the gadget is

activated as well. If the DrawingArea does process the event on the gadget, the DrawingAreaInput() action

knows that it should not invoke the callback function. However, this internal mechanism is not available outside of the

widget code. Reordering the action functions does not help, since there is still no way to know, without making an

educated guess, whether or not the DrawingArea acted upon an event on behalf of a gadget child.

As a result of this problem, draw() starts drawing a line, even if it starts in the middle of the PushButton, because

the DrawingArea processes all of the action functions in the list. If you drag the ...