In addition to the signal mechanism described above, there is a set of events that reflect the X event mechanism. Callbacks may also be attached to these events. These events are:
In order to connect a callback function to one of these events you use the function g_signal_connect(), as described above, using one of the above event names as the name parameter. The callback function for events has a slightly different form than that for signals:
gint callback_func( GtkWidget *widget, GdkEvent *event, gpointer callback_data );
GdkEvent is a C union structure whose type will depend upon which of the above events has occurred. In order for us to tell which event has been issued each of the possible alternatives has a type member that reflects the event being issued. The other components of the event structure will depend upon the type of the event. Possible values for the type are:
GDK_NOTHING GDK_DELETE GDK_DESTROY GDK_EXPOSE GDK_MOTION_NOTIFY GDK_BUTTON_PRESS GDK_2BUTTON_PRESS GDK_3BUTTON_PRESS GDK_BUTTON_RELEASE GDK_KEY_PRESS GDK_KEY_RELEASE GDK_ENTER_NOTIFY GDK_LEAVE_NOTIFY GDK_FOCUS_CHANGE GDK_CONFIGURE GDK_MAP GDK_UNMAP GDK_PROPERTY_NOTIFY GDK_SELECTION_CLEAR GDK_SELECTION_REQUEST GDK_SELECTION_NOTIFY GDK_PROXIMITY_IN GDK_PROXIMITY_OUT GDK_DRAG_ENTER GDK_DRAG_LEAVE GDK_DRAG_MOTION GDK_DRAG_STATUS GDK_DROP_START GDK_DROP_FINISHED GDK_CLIENT_EVENT GDK_VISIBILITY_NOTIFY GDK_NO_EXPOSE GDK_SCROLL GDK_WINDOW_STATE GDK_SETTING
So, to connect a callback function to one of these events we would use something like:
g_signal_connect (G_OBJECT (button), "button_press_event", G_CALLBACK (button_press_callback), NULL);
This assumes that button is a Button widget. Now, when the mouse is over the button and a mouse button is pressed, the function button_press_callback() will be called. This function may be declared as:
static gboolean button_press_callback( GtkWidget *widget, GdkEventButton *event, gpointer data );
Note that we can declare the second argument as type GdkEventButton as we know what type of event will occur for this function to be called.
The value returned from this function indicates whether the event should be propagated further by the GTK event handling mechanism. Returning TRUE indicates that the event has been handled, and that it should not propagate further. Returning FALSE continues the normal event handling. See the section on Advanced Event and Signal Handling for more details on this propagation process.
For details on the GdkEvent data types, see the appendix entitled GDK Event Types.
The GDK selection and drag-and-drop APIs also emit a number of events which are reflected in GTK by the signals. See Signals on the source widget and Signals on the destination widget for details on the signatures of the callback functions for these signals: