How can I make a QWidget semitransparent to events?

Question

I've seen similar questions but no answers that fit what I need. I want an invisible widget that lives on top of my whole application (no problems here). I want this widget to catch events so that I can print stuff about them, record them, whatever. I currently have an event filter hooked up that does this just fine. Then I want it to let the event go through to whatever is behind the widget. So for instance, if I try to push a button, the invisible widget should notice that a press happened on that spot, and then the button should actually be pressed. Can this be done in a simple way, or am I going to have to write code to simulate all the events beneath the invisible widget?

I have an event filter installed on the widget, which is how I'm handling the events it receives. How can I get the filter to pass the event to widgets beneath the top one? I don't want to pass the event to the parent object, but to the widgets in the space beneath the invisible widget. — Brianide
Why don't you just handle the event and then emit a signal that is connected to your invisible widget. — dtech
I think you may be missing what I'm trying to do. The invisible widget's filter will catch the event, and then I want whatever is behind the invisible widget to also catch the event. I won't know what object(s) are back there, so I can't just hook them up to a specific slot. — Brianide
Return true from eventFilter() - this tells the event loop to forward the event further. Otherwise the event is consumed. — dtech

dtech dtech · Accepted Answer · 2013-04-18T13:17:15

From all the information you disclosed in the comments, I suggest you filter the event as previously discussed, and then use QCoreApplication::sendEvent to forward the desired events to the invisible widget. It will then propagate the event accordingly to its children.

EDIT: OK, here is quick example that includes a QObject based event filter, that will filter the events for a widget, if the event is mouse event, it will be left for the widget to handle and print the output, if the event is a key event, it will be filtered and not forwarded back to the widget:

The event filter class:

    class EventInfo : public QObject {
    Q_OBJECT
public:
    explicit EventInfo(QObject *parent = 0) : QObject(parent) {}

    bool eventFilter(QObject *, QEvent *e) {
        if (e->type() == QEvent::MouseButtonRelease){
            qDebug() << "click event not filtered";
            return false;
        }
        if (e->type() == QEvent::KeyRelease) {
            QKeyEvent *event = static_cast<QKeyEvent *>(e);
            if (event) qDebug() << "key" << event->key() << "filtered";
            return true;
        }
        return false;
    }
};

The widget:

class Widget : public QWidget {
    Q_OBJECT

public:
    Widget(QWidget *parent = 0) : QWidget(parent) {}

protected:
    void mouseReleaseEvent(QMouseEvent *e) {
        qDebug() << "widget clicked at position" << e->pos();
    }

    void keyReleaseEvent(QKeyEvent *e) {
        qDebug() << "pressed key" << e->key();
    }
};

main.cpp:

int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
    Widget w;
    EventInfo info;
    w.installEventFilter(&info);
    w.show();    
    return a.exec();
}

Testing output to show that keyboard events are filtered and mouse press events are forwarded to the widget:

click event not filtered
widget clicked at position QPoint(352,230) 

key 70 filtered 

click event not filtered
widget clicked at position QPoint(405,163) 

key 87 filtered

How can I make a QWidget semitransparent to events?

1 Answers