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Gtk+/Gnome Application Development
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Overridable Signals

You may notice that GtkWidgetClass contains two signal identifiers in addition to function pointers. These are 0 by default; otherwise, they indicate the signal to emit to "activate" the widget, or to set its scroll adjustments.

The activate_signal is emitted when the user presses the space bar or Enter key while the widget is focused; for buttons, it will be the "clicked" signal, for menu items, the "activate" signal.

The set_scroll_adjustments_signal is used by GtkScrolledWindow to set the scroll adjustments used by the widget. GtkLayout, GtkCList, and others have a signal to set the scroll adjustments.

These two hacks are necessary because GTK+ 1.2 does not support interfaces or multiple inheritance. (A future version of GTK+ may support interfaces similar in spirit to Java's interfaces.) Ideally, there would be "GtkActivatable" and "GtkScrollable" base classes or interfaces, and all widgets supporting these actions would derive from them. Including the two signal IDs in GtkWidgetClass is a short-term workaround.

Gtk+/Gnome Application Development
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