Handling Resizing in Windows

Tue Jun 19, 2018 by Bill Farmer. Categories: Hacking .

I have a windows app where I want the app window to maintain it’s aspect ratio when it is resized. Windows provides two mechanisms for this, the WM_SIZE message and the WM_SIZING message. The WM_SIZE message is sent when a window has been resized to allow child windows to be resized, the WM_SIZING message is sent while the user is resizing it to allow the size to be adjusted. The messages are handled in the WindowProc callback function.


// Main window procedure
LRESULT CALLBACK MainWndProc(HWND hWnd,
                             UINT uMsg,
                             WPARAM wParam,
                             LPARAM lParam)
{
    switch (uMsg)
    {
    // ...
	// Size
    case WM_SIZE:
        WindowResize(hWnd, wParam, lParam);
	break;

	// Sizing
    case WM_SIZING:
	    return WindowResizing(hWnd, wParam, lParam);
	break;
    // ...
    }
}

Because a window can be zoomed vertically by double clicking on the window title bar, the WindowResize function checks the aspect ratio and resizes the window if necessary. The WIDTH and HEIGHT constants define the desired aspect ratio. If the aspect ratio is OK the child windows are resized.


// WindowResize
BOOL WindowResize(HWND hWnd, WPARAM wParam, LPARAM lParam)
{
    int width = LOWORD(lParam);
    int height = HIWORD(lParam) - toolbar.rect.bottom;

    // Get the window and client dimensions
    GetWindowRect(hWnd, &window.wind);
    GetClientRect(hWnd, &window.rect);

    if (width < (height * WIDTH) / HEIGHT)
    {
        // Calculate desired window width and height
        int border = (window.wind.right - window.wind.left) -
            window.rect.right;
        int header = (window.wind.bottom - window.wind.top) -
            window.rect.bottom;
        width = ((height * WIDTH) / HEIGHT) + border;
        height = height + toolbar.rect.bottom + header;

        // Set new dimensions
        SetWindowPos(hWnd, NULL, 0, 0,
                     width, height,
                     SWP_NOMOVE | SWP_NOZORDER);
        return true;
    }

    EnumChildWindows(hWnd, EnumChildProc, lParam);
    return true;
}

The WindowResizing function checks the offered window size and adjusts it as necessary depending on which window edge is being dragged.


// Window resizing
BOOL WindowResizing(HWND hWnd, WPARAM wParam, LPARAM lParam)
{
    PRECT rectp = (PRECT)lParam;

    // Get the window and client dimensions
    GetWindowRect(hWnd, &window.wind);
    GetClientRect(hWnd, &window.rect);

    // Edges
    int border = (window.wind.right - window.wind.left) -
        window.rect.right;
    int header = (window.wind.bottom - window.wind.top) -
        window.rect.bottom;

    // Window minimum width and height
    int width  = WIDTH + border;
    int height = HEIGHT + toolbar.rect.bottom + header;

    // Minimum size
    if (rectp->right - rectp->left < width)
	rectp->right = rectp->left + width;

    if (rectp->bottom - rectp->top < height)
	rectp->bottom = rectp->top + height;

    // Maximum width
    if (rectp->right - rectp->left > STEP + border)
        rectp->right = rectp->left + STEP + border;

    // Offered width and height
    width = rectp->right - rectp->left;
    height = rectp->bottom - rectp->top;

    switch (wParam)
    {
    case WMSZ_LEFT:
    case WMSZ_RIGHT:
        height = (((width - border) * HEIGHT) / WIDTH) +
            toolbar.rect.bottom + header;
        rectp->bottom = rectp->top + height;
        break;

    case WMSZ_TOP:
    case WMSZ_BOTTOM:
        width = ((((height - toolbar.rect.bottom) - header) *
                  WIDTH) / HEIGHT) + border;
        rectp->right = rectp->left + width;
        break;

    default:
        width = ((((height - toolbar.rect.bottom) - header) *
                  WIDTH) / HEIGHT) + border;
        rectp->right = rectp->left + width;
        break;
    }

    return true;
}

The WindowResizing function works fine, the WindowResize function handles most cases OK, but isn’t completely idiot proof. By contrast macOS makes it easy, you define what you want and it just works.

        // Set up window
        window.setContentSize(NSMakeSize(400, 480))
        window.contentMinSize = NSMakeSize(400, 480)
        window.contentAspectRatio = NSMakeSize(1.0, 1.2)
        window.showsResizeIndicator = true
        window.collectionBehavior.insert(.fullScreenNone)

Bill Farmer

Handling Resizing in Windows

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