Use WM_WINDOWPOSCHANGED to react to window state changes

Raymond Chen

The documentation for the WM_SHOWWINDOW message points out that the message is not sent under certain circumstances. But what if you want to know when the window is shown, including in the cases where you don’t get WM_SHOWWINDOW?

The WM_WINDOWPOSCHANGED message is sent at the end of the window state change process. It sort of combines the other state change notifications, WM_MOVE, WM_SIZE, and WM_SHOWWINDOW. But it doesn’t suffer from the same limitations as WM_SHOWWINDOW, so you can reliably use it to react to the window being shown or hidden. The handler would go something like this:

void OnWindowPosChanged(HWND hwnd, const WINDOWPOS *pwp)
{
    if (pwp->flags & SWP_SHOWWINDOW) {
       window_was_shown();
    }
    if (pwp->flags & SWP_HIDEWINDOW) {
       window_was_hidden();
    }
    if (!(pwp->flags & SWP_NOMOVE)) {
       window_moved_to(pwp->x, pwp->y);
    }
    if (!(pwp->flags & SWP_NOSIZE)) {
       window_resized_to(pwp->cx, pwp->cy);
    }
}
HANDLE_MSG(hwnd, WM_WINDOWPOSCHANGED, OnWindowPosChanged);

Note also that if you don’t pass the WM_WINDOWPOSCHANGED message to DefWindowProc, then you won’t get WM_MOVE or WM_SIZE messages, since it is DefWindowProc that converts WM_WINDOWPOSCHANGED into the WM_MOVE and WM_SIZE messages.

“If WM_WINDOWPOSCHANGED is redundant with WM_MOVE, WM_SIZE, and WM_SHOWWINDOW, then why do we have those other messages anyway?”

The WM_WINDOWPOSCHANGED message wasn’t invented until Windows 3.1. Prior to that, you had no choice but to react to those other messages. You can think of those other three messages as legacy messages now. There’s nothing wrong with them, but they’re kind of old-fashioned now.

Next time, we’ll look at the companion message WM_WINDOWPOSCHANGING.

Postscript: This entry was inspired by an actual customer question regarding the cases where WM_SHOWWINDOW message is not sent if the program is run with the SW_SHOWMAXIMIZED state. Unfortunately, one detail I missed in the customer’s question was the remark that they need to know when the window is shown because it is “critical for the application to initialize its state.” I didn’t follow up on that little remark, but I should have, because it’s very strange to do initialization work when a window is shown. What if the window is never shown? Does this mean that the program will never initialize itself? (For example, somebody might have run your program with the SW_HIDE state.) The WM_NCCREATE and WM_CREATE are the more traditional places to do window initialization.

Raymond Chen

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