qtqeventqgadget

How to get human-readable event type from QEvent?


I want to debug event handling code and would like to convert QEvent::Type enum's value to a human-readable string. QEvent has a Q_GADGET macro, so presumably there's a way of pulling that off?


Solution

  • Recent versions of Qt do the right thing when outputting events to the debug stream, so the below isn't neccessary. If you get an error similar to warning C4273: 'operator <<' : inconsistent dll linkage, it means that your version of Qt already supports this without need for the code below.

    The Q_GADGET macro adds a QMetaObject staticMetaObject member to the class. The static metaobject's definition is generated by moc, and it - in the case of QEvent - contains the enumeration information.

    Below is an example of how to leverage that to give a more reasonable QDebug output of events.

    #include <QEvent>
    #include <QMetaEnum>
    #include <QDebug>   
    
    /// Gives human-readable event type information.
    QDebug operator<<(QDebug str, const QEvent * ev) {
       static int eventEnumIndex = QEvent::staticMetaObject
             .indexOfEnumerator("Type");
       str << "QEvent";
       if (ev) {
          QString name = QEvent::staticMetaObject
                .enumerator(eventEnumIndex).valueToKey(ev->type());
          if (!name.isEmpty()) str << name; else str << ev->type();
       } else {
          str << (void*)ev;
       }
       return str.maybeSpace();
    }
    

    Use example:

    void MyObject::event(QEvent* ev) {
      qDebug() << "handling an event" << ev;
    }