How to detect user inactivity in Qt?

How can I detect user inactivity in a Qt QMainWindow? My idea so far is to have a QTimer that increments a counter, which, if a certain value is passed, locks the application. Any mouse or key interaction should set the timer back to 0. However I need to know how to properly handle input events which reset; I can re-implement:

virtual void keyPressEvent(QKeyEvent *event)
virtual void keyReleaseEvent(QKeyEvent *event)
virtual void mouseDoubleClickEvent(QMouseEvent *event)
virtual void mouseMoveEvent(QMouseEvent *event)
virtual void mousePressEvent(QMouseEvent *event)
virtual void mouseReleaseEvent(QMouseEvent *event)

...but won't the event handlers of all the widgets in the QMainWindow prevent events occurring in those controls from reaching the QMainWindow's? Is there a better architecture for detecting user activity as it is?

You could use a custom event filter to process all keyboard and mouse events received by your application before they are passed on to the child widgets.

class MyEventFilter : public QObject
{
  Q_OBJECT
protected:
  bool eventFilter(QObject *obj, QEvent *ev)
  {
    if(ev->type() == QEvent::KeyPress || 
       ev->type() == QEvent::MouseMove)
         // now reset your timer, for example
         resetMyTimer();

    return QObject::eventFilter(obj, ev);
  }
}

Then use something like

MyApplication app(argc, argv);
MyEventFilter filter;
app.installEventFilter(&filter);
app.exec();

This definitely works (I've tried it myself).

EDIT: And many thanks to ereOn for pointing out that my earlier solution was not very useful.