Expose Events

Expose events are received when a previously-obscured region of a window becomes visible. GdkWindow contents are not recorded; that is, if you draw to a GdkWindow, then the X server places another window on top of it, the graphics in the bottom window will be lost. When the top window is moved away, the bottom window will receive an expose event indicating the region that needs to be re-drawn. Expose events are also sent when a window first appears on-screen. (Incidentally, you should not draw into a GdkWindow until you receive the first expose event. The first expose event is your signal that the window is actually on-screen.)

Expose events have a unique feature: GTK+ synthesizes them for windowless widgets. This is the only kind of event GTK_NO_WINDOW widgets will receive.

Expose events are very simple:

typedef struct _GdkEventExpose GdkEventExpose;

struct _GdkEventExpose
{
  GdkEventType type;
  GdkWindow *window;
  gint8 send_event;
  GdkRectangle area;
  gint count;
};

area is the area that has been exposed and should be redrawn. count is the number of expose events that follow this one; conceivably you might want to compress successive events into a single redraw. However, GDK already makes a reasonable effort to do this, so adding another pass is unlikely to gain much. GdkRectangle is defined as follows:

typedef struct _GdkRectangle GdkRectangle;

struct _GdkRectangle
{
  gint16 x;
  gint16 y;
  guint16 width;
  guint16 height;
};

There is one other time you will receive expose events. If you call gdk_window_copy_area() to copy part of one window into another, the source window region may be partially or completely obscured. If it is, X will be unable to copy something sensible from the obscured region. By default, expose events will be generated for the areas of the destination window X was unable to copy something to. Your program's standard redraw routine can then refresh these areas of the destination window by hand. You can turn this behavior off by calling gdk_gc_set_exposures() on the graphics context passed to gdk_window_copy_area(). (Graphics contexts are discussed in the section called Graphics Contexts.)

If the source window region is not obscured, gdk_window_copy_area() can successfully draw the entire destination region. In this case, no expose events are generated; a single "no expose" event is generated instead. This event contains no information beyond the standard event fields:

typedef struct _GdkEventNoExpose GdkEventNoExpose;

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