A customer was adding diagnostics to their application and wanted to know if there was a way to detect that the application was being slow in processing its input. These sorts of delays manifest themselves to the end user as a sluggish application which is slow to respond to input events.
They already had a watchdog timer that was reset every time their
GetMessage call returned a message, so they could capture stack traces if their application stopped processing messages for more than X milliseconds. They wanted to extend this diagnostic information to input delays.
Fortunately, there's an easy way to tell. The
GetMessageTime will tell you the time the message was added to the queue, so by subtracting that time from the current
GetTickCount, you can determine how long the input message sat in the queue. (Sent messages are not queued messages, so calling
GetMessageTime for a sent message doesn't work.)
There's a catch, here, though.
Since mouse messages are generated on demand, the
GetMessageTime for a
WM_ message is usually "Just now." (It could be older if you forced a mouse move message to be generated but left it in the queue.) To get the actual time the mouse moved, you need to use