c++ - How do I implement dragging a window using its client area?

Question

I have a Win32 HWND and I'd like to allow the user to hold control and the left mouse button to drag the window around the screen. Given (1) that I can detect when the user holds control, the left mouse button, and moves the mouse, and (2) I have the new and the old mouse position, how do I use the Win32 API and my HWND to change the position of the window?

Answer 1

Implement a message handler for WM_NCHITTEST. Call DefWindowProc() and check if the return value is HTCLIENT. Return HTCAPTION if it is, otherwise return the DefWindowProc return value. You can now click the client area and drag the window, just like you'd drag a window by clicking on the caption.

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    switch (message)
    {
    case WM_NCHITTEST: {
        LRESULT hit = DefWindowProc(hWnd, message, wParam, lParam);
        if (hit == HTCLIENT) hit = HTCAPTION;
        return hit;
    }
    // etc..
}