The Update Region
The update region identifies the portion of a window that is out-of-date or invalid and in need of redrawing. The system uses the update region to generate WM_PAINT messages for applications and to minimize the time applications spend bringing the contents of their windows up to date. The system adds only the invalid portion of the window to the update region, requiring only that portion to be drawn.
When the system determines that a window needs updating, it sets the dimensions of the update region to the invalid portion of the window. Setting the update region does not immediately cause the application to draw. Instead, the application continues retrieving messages from the application message queue until no messages remain. The system then checks the update region, and if the region is not empty (non-NULL), it sends a WM_PAINT message to the window procedure.
An application can use the update region to generate its WM_PAINT messages. For example, an application that loads data from open files typically sets the update region while loading so that new data is drawn during processing of the next WM_PAINT message. In general, an application should not draw at the time its data changes, but route all drawing operations through the WM_PAINTT message.