CODEWITHSUNDEEP
c++windowswinapiwm-paint
ZeroZ30o
ZeroZ30o

Reputation: 396

Getting several WM_PAINT message with one dispatch

I'm getting several WM_PAINT messages/events in the message handler for my window while I resize it, even though I only translate+dispatch a single message.

Is this normal? Why is this happening? (I was expecting to get one WM_PAINT message per dispatch, and never more than that)

Window loop:

while (true) // only for the example
{
    std::cout << "Checking events\n";

    MSG winEvent = {};
    while (PeekMessage(&winEvent, NULL, 0, 0, PM_REMOVE))
    {
        std::cout << "ev\n";
        TranslateMessage(&winEvent);
        DispatchMessage(&winEvent);
    }
}

Message handler function:

LRESULT CALLBACK windowEvent(HWND _hwnd, UINT _uMsg, WPARAM _wParam, LPARAM _lParam)
{
    switch (_uMsg)
    {
    // extra cases removed for the example
    case WM_PAINT:
        std::cout << "PAINT EVENT\n";
        return DefWindowProc(_hwnd, _uMsg, _wParam, _lParam);
    }

    return DefWindowProc(_hwnd, _uMsg, _wParam, _lParam);
}

Console output:

Checking events
ev
PAINT EVENT
PAINT EVENT
PAINT EVENT
PAINT EVENT
PAINT EVENT
PAINT EVENT
PAINT EVENT
PAINT EVENT

Upvotes: 1

Views: 542

Answers (1)

Deduplicator
Deduplicator

Reputation: 45664

The answer is simple:

Starting the resize enters into a nested message-loop, one you didn't instrument.

Thus, you get posted messages delivered which the outer loop, which you wrote yourself, never knew about.

That the WM_PAINT message is generally generated for an empty message-queue if there was any invalidation, instead of posted, doesn't change anything relevant.

Upvotes: 3

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