QGraphicsView and eventFilter

Question

This has been bugging me for more than two days now, so i thought i should ask. I am using Qt 4.5.3 (compiled with VC2008) on Win7.

I have MyGraphicsView (inherits QGraphicsView) and MyFilter (inherits QObject) classes.

When i install the MyFilter object as an event filter to MyGraphicsView, Mouse events are delivered to MyFilter after they are delivered to MyGraphicsView whereas Key events are delivered to MyFilter before they are delivered to MyGraphicsView.

In the second case, i install the MyFilter object as an event filter to MyGraphicsView->viewport() (which is a standart QGLWidget), Mouse events are delivered to MyFilter before they are delivered to MyGraphicsView, whereas Key events are delivered to only MyGraphicsView.

The events are supposed to be delivered to event filters before they are delivered to the actual object, so why is this happening? What should i do to ensure this order?

Thanks in advance. Best Regards.

Answer 1

QGraphicsView is a subclass of QAbstractScrollArea which is the cause of these behaviors.

In the first case, the QAbstractScrollArea adds itself as a event filter to the MyGraphicsView when setViewport() is called. The QAbstractScrollArea's event filter captures the mouse event, first sends it through viewportEvent(), and then to the QWidget event handling which propagates to the MyGraphicsView mouse event handlers. Only after this is the QAbstractScrollArea's event filter finished and MyFilter gets to run.

In the second case, key events are delivered only to the MyGraphicsView because in setViewport() the QAbstractScrollArea sets itself as the focus proxy. If the focus proxy is reset with the following code, the key events will be delivered.

w.viewport()->setFocusProxy(0);

An alternative is to install the event filter on both the graphics view and its viewport, but modify the filter to only process key events from one object and mouse events from the other.

Change MyFilter.h

  QObject *keyObj;
  QObject *mouseObj;

public:
  MyFilter(QObject *keyObj, QObject *mouseObj, QObject *parent = NULL);

Change MyFilter.cpp

MyFilter::MyFilter(QObject *keyObj, QObject *mouseObj, QObject *parent /*= NULL*/ ) : QObject(parent), keyObj(keyObj), mouseObj(mouseObj)

and

if (obj == keyObj && e->type() == QEvent::KeyPress)
{
    qDebug()<<"Key Event recieved by MyFilter";
}
else if (obj == mouseObj && e->type() == QEvent::MouseButtonPress)
{
    qDebug()<<"Mouse Event recieved by MyFilter";
}

Change main.cpp

MyFilter *filter = new MyFilter(&w, w.viewport(), &w);

// Use this line to install to the viewport
w.viewport()->installEventFilter(filter);

//Use this line to install to MyGraphicsView
w.installEventFilter(filter);