ID Number: Q40669
2.x 3.00
WINDOWS
Summary:
The object-oriented nature of Windows programming can create a
situation in which an application posts a message to itself. When such
an application is designed, care must be taken to avoid posting
messages so frequently that system messages to the application are not
processed. This article discusses two methods of using the PeekMessage
function to combat this situation.
More Information:
In the first method, a PeekMessage loop is used to check for system
messages to the application. If none are pending, the SendMessage
function is used from within the PeekMessage loop to send a message to
the appropriate window. The following code demonstrates this
technique:
while (fProcessing)
{
if (PeekMessage(&msg, NULL, 0, 0, PM_REMOVE))
{
if (msg.message == WM_QUIT)
break;
/* process system messages */
}
else
{
/* perform other processing */
...
/* send WM_USER message to window procedure */
SendMessage(hWnd, WM_USER, wParam, lParam);
}
}
In the second method, two PeekMessage loops are used, one to look for
system messages and one to look for application messages. PostMessage
can be used from anywhere in the application to send the messages to
the appropriate window. The following code demonstrates this
technique:
while (fProcessing)
{
if (PeekMessage(&msg, NULL, 0, WM_USER-1, PM_REMOVE))
{
if (msg.message == WM_QUIT)
break;
/* process system messages */
}
else if (PeekMessage(&msg, NULL, WM_USER, WM_USER, PM_REMOVE))
/* process application messages */
}
An application should use a PeekMessage loop for as little time as
possible. To be compatible with battery-powered computers and to
optimize system performance, every Windows application should inform
Windows that it is idle as soon and as often as possible. An
application is idle when the GetMessage or WaitMessage function is
called and no messages are waiting in the application's message queue.