The relationship between
draw, draw_focus,
draw_default, and expose_event was discussed in
the section called
Drawing. All four class functions are
registered as default handlers for a signal of the same
name. The "draw" signal
redraws a portion of the widget; it is emitted by GTK+ or
by widget implementations.
Widget implementations should emit the "draw_focus" signal themselves, generally
in response to focus events; the signal draws or undraws
a frame indicating that the widget is focused. gtk_window_set_default() emits the "draw_default" signal for both the
widget losing default status and the widget gaining it.
(gtk_widget_grab_default()
calls
gtk_window_set_default().) Only widgets with the
GTK_CAN_FOCUS or GTK_CAN_DEFAULT flag set need
to worry about the
draw_focus and
draw_default methods. These flags should be set in
a widget's instance initialization function.
Only the draw method has a default implementation in GtkWidget; the default
implementation synthesizes an expose event covering the
widget's entire allocation. This allows you to write an
expose event handler that also handles the draw signal.
GTK+ normally emits the
"draw" signal in an idle function. That is, it
keeps a list of widgets (and regions of them) that need
to be redrawn; when no events are pending, the GTK+ main
loop runs an idle function that traverses the list and
emits the "draw" signal for
each widget. Redraw areas are merged as much as possible
to avoid multiple redraws, and the idle function is
removed after it runs once. This arrangement minimizes
the number of redraws and reduces flicker.
