Re: [gtk-list] Re: GTK Signal handlers return TRUE or FALSE?

[Jim Weller <asjww3 UAA ALASKA EDU>]

*2* snaps!! Put that in the tutorial~!~!


Owen Taylor writes:
  > 
  > "Scott D. Heavner" <sdh@po.cwru.edu> writes:
  >  
  > > The way I read the gtk tutorial, a signal handler returning TRUE
  > > says "I handled it, don't bother with the default handler".
  > > 
  > > In particular, I'm looking at the "select_row" signal in a gtkclist.
  > > If I return true, I would expect that the default handler wouldn't
  > > get executed and thus there would be no selection made. (Maybe hoping
  > > is a better word than expecting).
  > > 
  > > In general, are widgets being designed to act differently when
  > > a signal handler returns true vs. false.  What is supposed to be
  > > done before an overriding signal is invoked and what happens after?
  > > 
  > > As another example, what should happen if I intercept a button click
  > > event?  Should gtk go ahead and draw the button already clicked?  Am
  > > I then responsible for drawing it unclicked?  Or do I just assume
  > > that my callback/handler is just a courtesy and gtk doesn't give a
  > > flying f*ck what I return to it.
  > 
  > Here's a slighly more detailed view of what happens for event
  > signals:
  > 
  > First, note that the return from a signal emission is the 
  > return of the _last_ handler executed. Since event signals
  > are all of type GTK_RUN_LAST, this will be the default handler,
  > unless you connect with gtk_signal_connect_after().
  > 
  > Signal "emission" is the process whereby GTK+ runs all handlers
  > for a specific object and signal.
  > 
  > The way an event (say GTK_BUTTON_PRESS) is handled, is:
  > 
  >  - Start with the widget where the event occured.
  >    
  >  - Emit the generic "event" signal. If that signal emission
  >    returns TRUE, stop all processing.
  >  - Otherwise, emit a specific, "button_press_event" signal.
  >    If that returns TRUE, stop all processing.
  >  - Otherwise, go to the widget's parent, and repeat the above
  >    steps.
  >  - Continue until some emission returns TRUE, or until 
  >    you get to the toplevel widget.
  > 
  > Some consequences:
  > 
  >  - Your return value will have no effect if there is a default
  >    handler, unless you connect with gtk_signal_connect_after().
  > 
  >  - To prevent the default handler from being run, you need
  >    to use gtk_signal_emit_stop_by_name() - the return value
  >    only affects whether the signal is propagated, not the
  >    current emission.
  > 
  > So, if you want to suppress all further handling, do this
  > at the end of your handler.
  > 
  >  gtk_signal_emit_stop_by_name (GTK_OBJECT (widget), 
  >                                "button_press_event"));
  >  return FALSE;
  > 
  > Hope this makes things a bit clearer,
  >                                         Owen
  > 
  > 
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  > 
  > 
-- 
Jim Weller

  Homepage:         http://saturn.math.uaa.alaska.edu/~wellerj/
  Home Email:       asjww3@uaa.alaska.edu