The Update Region
The
update region identifies the portion of a window that is out-of-date or invalid and in need
of redrawing. Windows 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. Windows adds only the invalid
portion of the window to the update region, requiring only that portion to be drawn.
When Windows 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. Windows 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_PAINT message.
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