Question

Is it possible to prioritize a message sent with PostMessage (or any of the other related methods)?

IIRC, the WM_PAINT message, for instance, is only processed when there are no other messages in the queue. Is it possible to achieve a similiar behavior with custom messages?

If I use WM_PAINT with special parameters in order to deliver a custom message to a window (of which I control the WndProc) will it have a similiar behavior?

Answer 1

I wanted a background thread to notify the main that that results where available to displayed to the user. But if you simply post a message to the main thread, it will get processed immediately.

This lead to the application not responding to paint or user input messages, because my posted message always took priority.

The trick i used was the knowledge that WM_TIMER messages have lower priority than WM_PAINT messages.

So rather than posting a message to the main form, I would set a timer.

Timer messages have lower priority than

other posted messages
WM_PAINT messages

Answer 2

For WM_PAINT the windowing code in DefWndProc just sets a flag and then checks that flag only if the queue is empty the next time GetMessage is called. Some mouse messages are also coalesced (older ones are removed when the newer ones arrive).

The real answer depends on the behaviour you re actually wanting to achieve.

~~If you are trying to avoid reentrancy just check a flag for a quick exit~~, something like:

////bool processing = false; // class/window instance variable
...
void HandleCustomMessage()
{
    ////if (processing)
    ////{
    ////    return;
    ////}

    ////processing = true;
    DoSomething();
    ////processing = false;
}

If you want an actual priority queue, there are numerous PQ implementations. Add the data item to the PQ and then post a custom message (always the same ID). The custom message handler then asks the PQ for the highest priority item.

Another option is to intercept the GetMessage loop, use a call to PeekMessage to see if there is anything to do, then call GetMessage if a message is available, or check your PQ otherwise. You don t need a custom message with this approach.

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