Indicates that you are beginning the process of redrawing region. A backing store (offscreen buffer) large enough to contain region will be created. The backing store will be initialized with the background color or background surface for window. Then, all drawing operations performed on window will be diverted to the backing store. When you call Window.endPaint, the backing store will be copied to window, making it visible onscreen. Only the part of window contained in region will be modified; that is, drawing operations are clipped to region.
The net result of all this is to remove flicker, because the user sees the finished product appear all at once when you call Window.endPaint. If you draw to window directly without calling Window.beginPaintRegion, the user may see flicker as individual drawing operations are performed in sequence. The clipping and background-initializing features of Window.beginPaintRegion are conveniences for the programmer, so you can avoid doing that work yourself.
When using GTK+, the widget system automatically places calls to Window.beginPaintRegion and Window.endPaint around emissions of the expose_event signal. That is, if you’re writing an expose event handler, you can assume that the exposed area in GdkEventExpose has already been cleared to the window background, is already set as the clip region, and already has a backing store. Therefore in most cases, application code need not call Window.beginPaintRegion. (You can disable the automatic calls around expose events on a widget-by-widget basis by calling Widget.setDoubleBuffered.)
If you call this function multiple times before calling the matching Window.endPaint, the backing stores are pushed onto a stack. Window.endPaint copies the topmost backing store onscreen, subtracts the topmost region from all other regions in the stack, and pops the stack. All drawing operations affect only the topmost backing store in the stack. One matching call to Window.endPaint is required for each call to Window.beginPaintRegion.