When the window resize keep the aspect ratio

Question

I am using Qt5.

When I resize the window I would like to keep the aspect ratio of the window, so the height have to be equal to the width.

I am not using layout, I have implemented my own resizeEvent in the MainWindow class, when I resize the window everything resize with it.

I have read the following stackoverflow questions, but they did not help me:

user_7136013 · Answer

We got a solution by intercepting the nativeEvent:

#include 

// Utility Functions
int rectWidth(RECT* r)
{
    return r->right - r->left;
}

int rectHeight(RECT *r)
{
    return r->bottom - r->top;
}

// these functions should have the same resulting aspect ratio
int MainWindow::heightForWidth(int width) const
{
    return width / 1 / 1; // /aspect ratio
}

// these functions should have the same resulting aspect ratio
int MainWindow::widthForHeight(int height) const
{
    return height * 1 / 1; // *aspectRatio
}

bool MainWindow::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    // Intercept Windows event
    if(eventType == "windows_generic_MSG")
    {
        MSG* msg = static_cast(message);

        if(msg->message == WM_SIZING)
        {
            RECT *rect = (RECT*)(msg->lParam);

            // Overwrite the right parameter with the calculated value:
            switch(msg->wParam)
            {
            case WMSZ_BOTTOMRIGHT:
                if(rectWidth(rect) < rectHeight(rect)) goto wmsz_bottom;
                else goto wmsz_right; // Always wanted to use goto
                break;
            case WMSZ_BOTTOM:
                wmsz_bottom:
                rect->right = rect->left + widthForHeight(rectHeight(rect));
                break;
            case WMSZ_RIGHT:
                wmsz_right:
                rect->bottom = rect->top + heightForWidth(rectWidth(rect));
                break;
            case WMSZ_TOPLEFT:
                if(rectWidth(rect) < rectHeight(rect)) goto wmsz_top;
                else goto wmsz_left; // goto away, wheeeeee
                break;
            case WMSZ_TOP:
                wmsz_top:
                rect->left = rect->right - widthForHeight(rectHeight(rect));
                break;
            case WMSZ_LEFT:
                wmsz_left:
                rect->top = rect->bottom - heightForWidth(rectWidth(rect));
                break;
            }
        }
    }

    // Pass the modified Windows Message to the original event handler
    return QMainWindow::nativeEvent(eventType, message, result);
}

Tested with Qt5.15.2 and Windows 10. But since the Windows API and Qt5 API are very stable this should work with all reasonably old versions.

When the window resize keep the aspect ratio

1 Answers