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Message Filtering
An application can choose specific messages to retrieve from the message queue
(while ignoring other messages) by using the GetMessage or PeekMessage function to specify a message filter. The filter is a range of message
identifiers (specified by a first and last identifier), a window handle, or both. GetMessage and PeekMessage use a message filter to select which messages to retrieve from the queue.
Message filtering is useful if an application must search the message queue for
messages that have arrived later in the queue.
Any application that filters messages must ensure that a message satisfying
the message filter can be posted. For example, if an application filters for a WM_CHAR message in a window that does not receive keyboard input, the GetMessage function does not return. This effectively "hangs" the application.
To filter for keyboard, mouse, and DDE messages, an application can use the
WM_KEYFIRST and WM_KEYLAST, WM_MOUSEFIRST and WM_MOUSELAST messages, and
WM_DDE_FIRST and WM_DDE_LAST constants.
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