CODEWITHSUNDEEP
c++windowsmessage-queue
lancery
lancery

Reputation: 678

WindowProc Invocation Timing

I am new to win32 messages. I'd like to know that when a WindowProc is in the middle of processing a message, is it possible for it to be interrupted to process another message? Take the example below, if both [A] and [B] are executed, is it possible for the WindowProc to be interrupted to process the new WM_ACTIVATE and/or WM_ACTIVATEAPP message (as a result of the ShowWindow call) between [A] and [B]? If this is possible, are there any specific circumstances that make it so?

LRESULT CALLBACK WindowProc(
  _In_ HWND   hwnd,
  _In_ UINT   uMsg,
  _In_ WPARAM wParam,
  _In_ LPARAM lParam
{
    ...
    case WM_ACTIVATEAPP:

            if (wParam == FALSE)
            {
                // [A] Do something that would trigger a WM_ACTIVATE or WM_ACTIVATEAPP message to be queued to the message queue
                ShowWindow(hwnd, SW_MINIMIZE);

                // [B] Modify Window Styles
                SetWindowLong(hwnd, GWL_EXSTYLE, WS_EX_CLIENTEDGE);
            }

            break;
    ...
}

Thanks for any comments!

Upvotes: 0

Views: 427

Answers (2)

MSalters
MSalters

Reputation: 179819

No. That is the point of the GUI Thread - it's a single thread doing one thing at a time. If you're processing messages, you're not calling GetMessage.

Upvotes: 1

YePhIcK
YePhIcK

Reputation: 5856

The system will never call this callback function while it is processing another message simply because all the messages are being put into a queue and (either your code or the framework you use, like MFC) are dequeued one by one with no overlaps. Unless one message is finished being processed the control doesn't return back to the dequeueing block.
In fact you can safely treat your window code as single-threaded. if you have multiple windows running in a single application they are all serialized and are still running in a single thread.

In fact you need to create separate threads if you want to run your windows (or workers) in parallel to each other.

Upvotes: 0

Related Questions