10.9.1 Expose Events

The X server can report Expose events to clients wanting information about when the contents of window regions have been lost. The circumstances in which the X server generates Expose events are not as definite as those for other events. However, the X server never generates Expose events on windows whose class you specified as InputOnly . The X server can generate Expose events when no valid contents are available for regions of a window and either the regions are visible, the regions are viewable and the server is (perhaps newly) maintaining backing store on the window, or the window is not viewable but the server is (perhaps newly) honoring the window's backing-store attribute of Always or WhenMapped. The regions decompose into an (arbitrary) set of rectangles, and an Expose event is generated for each rectangle. For any given window, the X server guarantees to report contiguously all of the regions exposed by some action that causes Expose events, such as raising a window.

To receive Expose events, set the ExposureMask bit in the event-mask attribute of the window.

The structure for this event type contains:

typedef struct {
	int type;		/* Expose */
	unsigned long serial;	/* # of last request processed by server */
	Bool send_event;	/* true if this came from a SendEvent request */
	Display *display;	/* Display the event was read from */
	Window window;
	int x, y;
	int width, height;
	int count;		/* if nonzero, at least this many more */
} XExposeEvent;

The window member is set to the exposed (damaged) window.

The x and y members are set to the coordinates relative to the window's origin and indicate the upper-left corner of the rectangle.

The width and height members are set to the size (extent) of the rectangle.

The count member is set to the number of Expose events that are to follow. If count is zero, no more Expose events follow for this window. However, if count is nonzero, at least that number of Expose events (and possibly more) follow for this window. Simple applications that do not want to optimize redisplay by distinguishing between subareas of its window can just ignore all Expose events with nonzero counts and perform full redisplays on events with zero counts.

Next: GraphicsExpose and NoExpose Events
Christophe Tronche, ch.tronche@computer.org