How can I intercept and cancel the minimizing of a Window?

Question

I have a Window subclass in my project, and at runtime the instance is created and shown entirely on the QML side. I know that I can prevent the window from being minimized by not including the WindowMinimizeButtonHint in the flags:, but I actually need to have the minimize button present and enabled but be able to intercept the minimize button click, cancel the actual minimizing, and do something else (FYI my client is requiring this non-standard windowing behavior, not me).

So far, the only thing I've been able to achieve is to handle the onWindowStateChanged: event, check if windowState === Qt.WindowStateMinimized and call show() from a timer (calling it inside the event handler directly does nothing). This results in the window moving down to the system tray and then suddenly coming back up to normal.

Is there any way to do this, something like an OnMinimized event that can be cancelled?

Edit: based on Benjamin T's answer, I'm at least part way to a solution for OSX:

#import <AppKit/AppKit.h>

bool NativeFilter::nativeEventFilter(const QByteArray &eventType, 
    void *message, long *result)
{
    if (eventType == "mac_generic_NSEvent") {
        NSEvent *event = static_cast<NSEvent *>(message);
        if ([event type] == NSKeyDown) {
            return true;
        }
    }
    return false;
}

In this example I'm able to intercept and cancel all NSKeyDown events (while leaving other events like mouse clicks etc. still working). The remaining problem is that I still don't know to intercept a minimize event - NSEvent.h doesn't seem to have anything that covers that. Perhaps I need to cast to a different type of event?

Edit 2 - working solution:

I was not able to find any way to intercept the minimize event proper and cancel it, so my workaround is to instead intercept the click on the window, determine if the click is over the minimize button (or the close or zoom buttons) and cancel the event if so (and send a notification to my qml window that the click occurred). I also handle the case of double-clicking the titlebar to zoom the window, and using the Command-M keys to minimize the window.

First step is to implement a QAbstractNativeEventFilter. In your header:

#include <QAbstractNativeEventFilter>

class NativeFilter : public QAbstractNativeEventFilter {
public:
    bool nativeEventFilter(const QByteArray &eventType, void *message, 
        long *result);
};

The implementation:

#import <AppKit/AppKit.h>
#import <AppKit/NSWindow.h>
#import <AppKit/NSButton.h>

bool NativeFilter::nativeEventFilter(const QByteArray &eventType, void 
    *message, long *result)
{
    if (eventType == "mac_generic_NSEvent") {

        NSEvent *event = static_cast<NSEvent *>(message);
        NSWindow *win = [event window];

        // TODO: determine whether or not this is a window whose
        // events you want to intercept. I did this by checking
        // [win title] but you may want to find and use the 
        // window's id instead.

        // Detect a double-click on the titlebar. If the zoom button 
        // is enabled, send the full-screen message to the window
        if ([event type] == NSLeftMouseUp) {
            if ([event clickCount] > 1) {
                NSPoint pt = [event locationInWindow];
                CGRect rect = [win frame];
                // event coordinates have y going in the opposite direction from frame coordinates, very annoying
                CGFloat yInverted = rect.size.height - pt.y;
                if (yInverted <= 20) {
                    // TODO: need the proper metrics for the height of the title bar

                    NSButton *btn = [win standardWindowButton:NSWindowZoomButton];
                    if (btn.enabled) {

                        // notify qml of zoom button click

                    }

                    return true;
                }
            }
        }

        if ([event type] == NSKeyDown) {

            // detect command-M (for minimize app)
            if ([event modifierFlags] & NSCommandKeyMask) {

                // M key
                if ([event keyCode] == 46) {
                    // notify qml of miniaturize button click
                    return true;
                }

            }

            // TODO: we may be requested to handle keyboard actions for close and zoom buttons. e.g. ctrl-cmd-F is zoom, I think,
            // and Command-H is hide.

        }


        if ([event type] == NSLeftMouseDown) {

            NSPoint pt = [event locationInWindow];
            CGRect rect = [win frame];

            // event coordinates have y going in the opposite direction from frame coordinates, very annoying
            CGFloat yInverted = rect.size.height - pt.y;

            NSButton *btn = [win standardWindowButton:NSWindowMiniaturizeButton];
            CGRect rectButton = [btn frame];
            if ((yInverted >= rectButton.origin.y) && (yInverted <= (rectButton.origin.y + rectButton.size.height))) {
                if ((pt.x >= rectButton.origin.x) && (pt.x <= (rectButton.origin.x + rectButton.size.width))) {

                    // notify .qml of miniaturize button click

                    return true;
                }
            }

            btn = [win standardWindowButton:NSWindowZoomButton];
            rectButton = [btn frame];

            if (btn.enabled) {
                if ((yInverted >= rectButton.origin.y) && (yInverted <= (rectButton.origin.y + rectButton.size.height))) {
                    if ((pt.x >= rectButton.origin.x) && (pt.x <= (rectButton.origin.x + rectButton.size.width))) {

                        // notify qml of zoom button click

                        return true;
                    }
                }
            }

            btn = [win standardWindowButton:NSWindowCloseButton];
            rectButton = [btn frame];
            if ((yInverted >= rectButton.origin.y) && (yInverted <= (rectButton.origin.y + rectButton.size.height))) {
                if ((pt.x >= rectButton.origin.x) && (pt.x <= (rectButton.origin.x + rectButton.size.width))) {

                    // notify qml of close button click

                    return true;
                }
            }

        }

        return false;

    }

    return false;
}

Then in main.cpp:

Application app(argc, argv);
app.installNativeEventFilter(new NativeFilter());

Answer 1

Generally speaking, you should use the event system ans not signal/slots to intercept events and changes.

The easiest way to do so is either to subclass the object you use and reimplement the appropriate event handler, or to use an event filter.

Since you are using QML, subclassing might be difficult as you don't have access to all Qt internal classes.

Here is what the code would look like when using event filtering.

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);


    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    if (engine.rootObjects().isEmpty())
        return -1;

    auto root = engine.rootObjects().first();
    root->installEventFilter(new EventFilter());

    return app.exec();
}

class EventFilter : public QObject
{
    Q_OBJECT
public:
    explicit EventFilter(QObject *parent = nullptr);
    bool eventFilter(QObject *watched, QEvent *event) override;
};

bool EventFilter::eventFilter(QObject *watched, QEvent *event)
{
    if (event->type() == QEvent::WindowStateChange) {
        auto e = static_cast<QWindowStateChangeEvent *>(event);
        auto window = static_cast<QWindow *>(watched);

        if (window->windowStates().testFlag(Qt::WindowMinimized)
                && ! e->oldState().testFlag(Qt::WindowMinimized))
        {
            // Restore old state
            window->setWindowStates(e->oldState());
            return true;
        }
    }

    // Do not filter event
    return false;
}

However, you will quickly run into the same issue that when using the signal/slot mechanism: Qt only notify you when the window has already been minimized. Meaning that restoring the window at this point will make a hide/show effect.

So you need to go deeper and you a native event filter.

The following code works on Windows, you should adapt it for macOS:

class NativeFilter : public QAbstractNativeEventFilter {
public:
    bool nativeEventFilter(const QByteArray &eventType, void *message, long *result);
};

bool NativeFilter::nativeEventFilter(const QByteArray &eventType, void *message, long *result)
{
/* On Windows we interceot the click in the title bar. */
/* If we wait for the minimize event, it is already too late. */
#ifdef Q_OS_WIN
    auto msg = static_cast<MSG *>(message);
    // Filter out the event when the minimize button is pressed.
    if (msg->message == WM_NCLBUTTONDOWN && msg->wParam == HTREDUCE)
        return true;
#endif

/* Example macOS code from Qt doc, adapt to your need */
#ifdef Q_OS_MACOS
    if (eventType == "mac_generic_NSEvent") {
        NSEvent *event = static_cast<NSEvent *>(message);
        if ([event type] == NSKeyDown) {
            // Handle key event
            qDebug() << QString::fromNSString([event characters]);
        }
}
#endif

    return false;
}

In your main():

QGuiApplication app(argc, argv);
app.installNativeEventFilter(new NativeFilter());

For more info, you can read the Qt documentation about QAbstractNativeEventFilter.

You may need to use QWindow::winId() to check to which window the native events are targeted.

As I am not a macOS developer, I do not know what you can do with NSEvent. Also it seems the NSWindowDelegate class could be useful to you: https://developer.apple.com/documentation/appkit/nswindowdelegate If you can retrieve a NSWindow from QWindow::winId(), you should be able to use it.