Signal Emission and Propagation

Signal emission is the process whereby GTK runs all handlers for a specific object and signal.

First, note that the return value from a signal emission is the return value of the last handler executed. Since event signals are all of type GTK_RUN_LAST, this will be the default (GTK supplied) handler, unless you connect with gtk_signal_connect_after().

The way an event (say "button_press_event") is handled, is:

  • Start with the widget where the event occured.

  • Emit the generic "event" signal. If that signal handler returns a value of 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 two steps.

  • Continue until some signal handler returns TRUE, or until the top-level widget is reached.

Some consequences of the above are:

  • Your handler's 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 connect with gtk_signal_connect() and use gtk_signal_emit_stop_by_name() - the return value only affects whether the signal is propagated, not the current emission.