10.9.2 GraphicsExpose and NoExpose Events

The X server can report GraphicsExpose events to clients wanting information about when a destination region could not be computed during certain graphics requests: XCopyArea() or XCopyPlane(). The X server generates this event whenever a destination region could not be computed due to an obscured or out-of-bounds source region. In addition, the X server guarantees to report contiguously all of the regions exposed by some graphics request (for example, copying an area of a drawable to a destination drawable).

The X server generates a NoExpose event whenever a graphics request that might produce a GraphicsExpose event does not produce any. In other words, the client is really asking for a GraphicsExpose event but instead receives a NoExpose event.

To receive GraphicsExpose or NoExpose events, you must first set the graphics-exposure attribute of the graphics context to True. You also can set the graphics-expose attribute when creating a graphics context using XCreateGC() or by calling XSetGraphicsExposures().

The structures for these event types contain:

typedef struct {
	int type;		/* GraphicsExpose */
	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 */
	Drawable drawable;
	int x, y;
	int width, height;
	int count;		/* if nonzero, at least this many more */
	int major_code;		/* core is CopyArea or CopyPlane */
	int minor_code;		/* not defined in the core */
} XGraphicsExposeEvent;

typedef struct {
	int type;	/* NoExpose */
	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 */
	Drawable drawable;
	int major_code;	/* core is CopyArea or CopyPlane */
	int minor_code;	/* not defined in the core */
} XNoExposeEvent;

Both structures have these common members: drawable, major_code, and minor_code. The drawable member is set to the drawable of the destination region on which the graphics request was to be performed. The major_code member is set to the graphics request initiated by the client and can be either X_CopyArea or X_CopyPlane. If it is X_CopyArea, a call to XCopyArea() initiated the request. If it is X_CopyPlane, a call to XCopyPlane() initiated the request. These constants are defined in <X11/Xproto.h>. The minor_code member, like the major_code member, indicates which graphics request was initiated by the client. However, the minor_code member is not defined by the core X protocol and will be zero in these cases, although it may be used by an extension.

The XGraphicsExposeEvent structure has these additional members: x, y, width, height, and count. The x and y members are set to the coordinates relative to the drawable'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 GraphicsExpose events to follow. If count is zero, no more GraphicsExpose events follow for this window. However, if count is nonzero, at least that number of GraphicsExpose events (and possibly more) are to follow for this window.

Next: Window State Change Events

Christophe Tronche, ch.tronche@computer.org