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Window Procedure
A window procedure is a function that receives and processes all messages sent
to the window. Every window class has a window procedure, and every window
created with that class uses that same window procedure to respond to messages.
The system sends a message to a window procedure by passing the message data
as arguments to the procedure. The window procedure then performs an appropriate
action for the message; it checks the message identifier and, while processing
the message, uses the information specified by the message parameters.
A window procedure does not usually ignore a message. If it does not process a
message, it must send the message back to the system for default processing.
The window procedure does this by calling the DefWindowProc function, which performs a default action and returns a message result. The
window procedure must then return this value as its own message result. Most
window procedures process just a few messages and pass the others on to the system
by calling DefWindowProc.
Because a window procedure is shared by all windows belonging to the same
class, it can process messages for several different windows. To identify the
specific window affected by the message, a window procedure can examine the window
handle passed with a message. For more information about window procedures, see Window Procedures.
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