Function XPAddWidgetCallback 

pub unsafe extern "C" fn XPAddWidgetCallback(
    inWidget: XPWidgetID,
    inNewCallback: XPWidgetFunc_t,
)

XPAddWidgetCallback

This function adds a new widget callback to a widget. This widget callback supercedes any existing ones and will receive messages first; if it does not handle messages they will go on to be handled by pre-existing widgets.

The widget function will remain on the widget for the life of the widget. The creation message will be sent to the new callback immediately with the widget ID, and the destruction message will be sent before the other widget function receives a destruction message.

This provides a way to ‘subclass’ an existing widget. By providing a second hook that only handles certain widget messages, you can customize or extend widget behavior.