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Window Update Lock
A window update lock is a temporary suspension of drawing in a window. Windows uses the lock to
prevent other windows from drawing over the tracking rectangle whenever the user
moves or sizes a window. Applications can use the lock to prevent drawing if
they carry out similar moving or sizing operations with their own windows.
An application uses the LockWindowUpdate function to set or clear a window update lock, specifying the window to lock.
The lock applies to the given window and all of its child windows. When the
lock is set, the GetDC and BeginPaint functions return a display DC with a visible region that is empty. Given
this, the application can continue to draw in the window, but all output is
clipped. The lock persists until the application clears it by calling LockWindowUpdate, specifying NULL for the window. Although LockWindowUpdate forces a window's visible region to be empty, the function does not make the
given window invisible and does not clear the WS_VISIBLE style bit.
After the lock is set, the application can use the GetDCEx function, with the DCX_LOCKWINDOWUPDATE value, to retrieve a display DC to
draw over the locked window. This allows the application to draw a tracking
rectangle when processing keyboard or mouse messages. Windows uses this method when
the user moves and sizes windows. GetDCEx retrieves the display DC from the display DC cache, so the application must
release the DC as soon as possible after drawing.
While a window update lock is set, the system creates an accumulated bounding
rectangle for each locked window. When the lock is cleared, Windows uses this
bounding rectangle to set the update region for the window and its child
windows, forcing an eventual WM_PAINT message. If the accumulated bounding rectangle is empty (that is, if no
drawing has occurred while the lock was set), the update region is not set.
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