Tuesday, August 6, 2013

machine-wide msg queue + per-GUI-thread msg queue

(Backgrounder -- Windows GUI are event-driven. Most events, including mouse/keyboard events, come through the message pump.)

http://msdn.microsoft.com/en-us/library/ms644927(v=vs.85).aspx shows that MSWin message queues come in 2 flavours --
1) a machine-wide msg queue
2) per-thread msg queue for each GUI thread, i.e. for each thread that creates visual window(s).

The system removes the user-generated messages, one at a time, from the machine-wide message queue, examines them to determine the destination (visual) window, and then posts them to the message queue of the thread that created the window. (Your own code on) the thread removes [1] messages from its queue and sends[2] them to the appropriate window procedure.

You usually write your own message loop to remove and send messages to window procedures. Applications with multiple threads can include a message loop in each thread that creates a window.

Only one message loop is needed for a message queue, even if an application contains many windows. DispatchMessage() always dispatches the message to the window procedure of the proper (visual) window.

[1] GetMessage() -- An application can remove a message from the per-thread queue by using the GetMessage() function.
[2] DispatchMessage() -- Application (your code) can use DispatchMessage()to send the message to a window procedure.

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