Events

Types of Event

There are many kinds of events; the GdkEvent union can represent any of them. A special event type, GdkEventAny, contains the three fields common to all events; any event can be cast to GdkEventAny. The first field in GdkEventAny is a type marker, GdkEventType; GdkEventType is also included in the GdkEvent union. Confused yet? Seeing the code should help. Here is GdkEventAny:

struct _GdkEventAny
{
  GdkEventType type;
  GdkWindow *window;
  gint8 send_event;
};

and GdkEvent:

union _GdkEvent
{
  GdkEventType              type;
  GdkEventAny               any;
  GdkEventExpose            expose;
  GdkEventNoExpose          no_expose;
  GdkEventVisibility        visibility;
  GdkEventMotion            motion;
  GdkEventButton            button;
  GdkEventKey               key;
  GdkEventCrossing          crossing;
  GdkEventFocus             focus_change;
  GdkEventConfigure         configure;
  GdkEventProperty          property;
  GdkEventSelection         selection;
  GdkEventProximity         proximity;
  GdkEventClient            client;
  GdkEventDND               dnd;
};

Every event type has the three members of GdkEventAny as its first three members. Thus, the type of an event can be referred to in many ways (assume a GdkEvent* called event):

• event->type

• event->any.type

• event->button.type

• ((GdkEventAny*)event)->type

• ((GdkEventButton*)event)->type

You'll probably see all these in GTK+ source code. Of course, each event subtype has its own unique members; the type field tells you which subtype is valid.

The window field of GdkEventAny is the GdkWindow the event was sent to. If the send_event flag is TRUE, then the event was synthesized by another (or your own) application; if FALSE, it originated with the X server. GDK does not export the X interface for sending events (XSendEvent()). However, GTK+ often "makes up" an event by declaring a static event struct, filling it in, then emitting the event's corresponding widget signal. These synthesized events will have send_event set to TRUE. the section called Receiving GDK Events in GTK+ explains how GTK+ associates events with widget signals.

There are more possible values for GdkEventType than there are members in the GdkEvent union. Many event types share the same data; for example, GDK_BUTTON_PRESS and GDK_BUTTON_RELEASE both use the button member of GdkEvent, since the same information is conveyed when mouse buttons are pressed and released. Table 2 shows all possible values of the GdkEventType enumeration, and the corresponding GdkEvent member. The meaning of each event type is described later in this section.