C++ MFC button disappears at window resize

Question

I have a Dialog in MFC C++ that has a CButton attached. I want to modify OnSize() so that the button will anchor to bottom-left.

if (btn.m_hWnd) {
        CRect winRect;
        GetWindowRect(&winRect);

        int width = winRect.right - winRect.left;
        int height = winRect.bottom - winRect.top;

        int x = width - wndWidth;
        int y = height - wndHeight;


        CRect rect;
        btn.GetWindowRect(&rect);
        rect.left += x;
        rect.top += y;
        btn.MoveWindow(&rect);
        ScreenToClient(&rect);
        btn.ShowWindow(SW_SHOW);
    }

x and y are the difference of how much the window has changed and will be added to the button's start coordinates.

I am not sure about the last 2 commands (I might as well delete them) but then I run the program the button disappears.

I need to know a way to move the button by x and y.

Answer 1

Basically, the answer should be to do ScreenToClient before MoveWindow!

Some more detail: You need to familiarize yourself with what function returns or uses client coordinates (and relative to what these client coordinates are) and which screen coordinates; this is one of the parts of MFC which can be really confusing. As to your question:

CWnd::GetWindowRect returns screen coordinates; CWnd::MoveWindow expects coordinates relative to the parent CWnd (or to the screen if it's a top level window). So before calling MoveWindow, you have to convert the rect returned by GetWindowRect to client coordinates of your parent window; suppose pParent is the CWnd * to the parent window of btn, then your moving code should look like this:

CRect rect;
btn.GetWindowRect(&rect);
pParent->ScreenToClient(&rect);  // this line and it's position is important
rect.left += x;
rect.top += y;
btn.MoveWindow(&rect);

If you're in a method of the parent window (as I think you are, since you mention OnSize of the dialog), then just leave out the pParent->. The way you do ScreenToClient at the moment, it has no effect on MoveWindow, since it's executed after it!

Answer 2

The original code was using the wrong coordinate system for both the parent dialog, and the button.

A correct way to dock to the bottom left would be like this:

if (btn.m_hWnd) { 
    CRect winRect; 
    GetClientRect(&winRect); 

    CRect rect; 
    btn.GetWindowRect(&rect); 
    ScreenToClient(&rect); 

    int btnWidth = rect.Width();
    int btnHeight = rect.Width();
    rect.left = winRect.right-btnWidth; 
    rect.top  = winRect.bottom-btnHeight;
    rect.right = winRect.right;
    rect.bottom = winRect.bottom; 
    btn.MoveWindow(&rect); 
}

OR

if (btn.m_hWnd) { 
    CRect winRect; 
    GetClientRect(&winRect); 

    CRect rect; 
    btn.GetWindowRect(&rect); 
    ScreenToClient(&rect); 

    int btnWidth = rect.Width();
    int btnHeight = rect.Width();
    btn.SetWindowPos(NULL,winRect.right-btnWidth,winRect.bottom-btnHeight,0,0,SWP_NOSIZE|SWP_NOZORDER);
}