Examining a Message Queue

Occasionally, an application needs to examine the contents of a thread's message queue from outside the thread's message loop. For example, if an application's window procedure performs a lengthy drawing operation, you may want the user to be able to interrupt the operation. Unless your application periodically examines the message queue during the operation for mouse and keyboard messages, it will not respond to user input until after the operation has completed. The reason for this is that the DispatchMessage function in the thread's message loop does not return until the window procedure finishes processing a message.

You can use the PeekMessage function to examine a message queue during a lengthy operation. PeekMessage is similar to the GetMessage function; both check a message queue for a message that matches the filter criteria and then copy the message to an MSG structure. The main difference between the two functions is that GetMessage does not return until a message matching the filter criteria is placed in the queue, whereas PeekMessage returns immediately regardless of whether a message is in the queue.

The following example shows how to use PeekMessage to examine a message queue for mouse clicks and keyboard input during a lengthy operation.

HWND hwnd;

BOOL fDone;

MSG msg;

// Begin the operation and continue until it is complete

// or until the user clicks the mouse or presses a key.

fDone = FALSE;

while (!fDone)

{

fDone = DoLengthyOperation(); // application-defined function

// Remove any messages that may be in the queue. If the

// queue contains any mouse or keyboard

// messages, end the operation.

while (PeekMessage(&msg, hwnd, 0, 0, PM_REMOVE))

{

switch(msg.message)

{

case WM_LBUTTONDOWN:

case WM_RBUTTONDOWN:

case WM_KEYDOWN:

//

// Perform any required cleanup.

//

fDone = TRUE;

}

}

}

Other functions, including GetQueueStatus and GetInputState, also allow you to examine the contents of a thread's message queue. GetQueueStatus returns an array of flags that indicates the types of messages in the queue; using it is the fastest way to discover whether the queue contains any messages. GetInputState returns TRUE if the queue contains mouse or keyboard messages. Both of these functions can be used to determine whether the queue contains messages that need to be processed.

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