Issues with eventFilter



  • Hi

    Since I have added an eventFilter on the Application object to catch all mouse/touch events to filter them I am having some issues . Basically, the eventFilter works. For debug purpose I just show the event and then forward it.

    However, with the eventFilter in place the system feels kind of irresponsive on the mouse/touch. What bugs even more is that the some functions seem not to fire always and have to be submitted multiple times until they really fire.
    i.e. something like that has sometimes to be called multiple times until the ui really changes:
    @
    ui->webGUI->hide();
    ui->WebBrowser->setGeometry(0, 0, 1280, 800);
    ui->WebBrowser->show();
    @

    If I remove the eventFilter, it's back to work as supposed.

    Any hint what I missed?
    Thanks
    McL

    b.t.w.: I am on Qt 4.6.3 on eLinux


  • Lifetime Qt Champion

    Hi,

    Printing e.g. using qDebug is an expensive operation so printing something for every event of the application will give you a big performance hit



  • Thanks for trying to help.
    Printing is only temporary to see what happens. Even if disabled, I have the issue that the code like in the sample above is not fired always.

    The code segment above is started by an external serial command. The external command is received and the code segment is called - I set a response on the serial port to check that. There is no debug printing involved here except for if the code really executes I do get 1 MouseMove event which I assume is normally due to the ui switch.

    My eventFilter currently does forward all events and looks like below. In the end, I need just to catch all mouse/touch events and filter(discard) depending on the active ui.
    @
    if (event->type() >= QEvent::MouseButtonPress &&
    event->type() <= QEvent::MouseMove )
    {
    qDebug("Touch/Mouse event type: %d", event->type());
    //return true;
    return QObject::eventFilter(obj, event);
    }
    else if (event->type() == QEvent::KeyPress)
    {
    QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
    qDebug("Key %d pressed, Event type: %d", keyEvent->key(), event->type());
    //return true;
    return QObject::eventFilter(obj, event);
    }
    else
    {
    return QObject::eventFilter(obj, event); // fwd to std event processing
    }

    @

    So, there must be something else that creates this issue.
    Any further suggestions are highly appreciated.
    Thanks.


  • Moderators

    when checking for an enum value it's recommended to use a switch-case statement rather than an if-else-constrcut. Switch-case statements are much faster since the compiler can optimize with a lookup-table/hash-list.

    This (without qDebug prints) probably won't solve your issue, but worth a shot:
    @
    bool eventFilter(QObject* watched, QEvent* event)
    {
    switch( event->type() )
    {
    case QEvent::MouseButtonPress:
    case QEvent::MouseMove:
    case QEvent::MouseButtonRelease:
    //...
    break;
    case QEvent::KeyPress:
    //...
    break;
    }

    return false;
    }
    @

    Is the code you've posted really the full code of your eventFilter method?



  • Thanks for the hint. I changed my code accordingly, and yes it's all. I just started to implement the event filter and "real" functionality is not yet in.
    @
    bool QTGUI_MainWindow::eventFilter(QObject *obj, QEvent *event)
    {
    switch( event->type() )
    {
    case QEvent::MouseButtonPress:
    case QEvent::MouseButtonRelease:
    case QEvent::MouseMove:
    case QEvent::MouseButtonDblClick:
    {
    qDebug("Touch/Mouse event type: %d", event->type());
    //return true;
    return QObject::eventFilter(obj, event);
    break;
    }
    case QEvent::KeyPress:
    {
    QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
    qDebug("Key %d pressed, Event type: %d", keyEvent->key(), event->type());
    //return true;
    return QObject::eventFilter(obj, event);
    break;
    }
    default:
    return QObject::eventFilter(obj, event); // fwd to std event processing
    }
    return false;
    }
    @


  • Moderators

    i don't see how this no-op eventFilter should slow down your application...
    The issue must be somewhere else in your code.

    Another hint:
    you don't need to call QObject::eventFilter() in every case branch.
    Returning false once at the end of the eventFilter has the same effect ;)



  • OK, thanks :)
    As far as I know the rest is ok as well:
    @
    class QTGUI_MainWindow : public QMainWindow
    {
    Q_OBJECT

    protected:
    bool eventFilter(QObject *obj, QEvent *event);
    ...
    @

    @
    int main(int argc, char *argv[])
    {
    QApplication program(argc, argv);
    ...
    QTGUI_MainWindow eventFilter;
    program.installEventFilter(&eventFilter);
    ...
    @

    I'm out of ideas ...



  • Since the eventFilter on the QApplication objects generally slows down the event handling (according doc), is there another way to catch all mouse/touch events?


  • Moderators

    yes, you can subclass QApplication and reimplement "notify()":http://qt-project.org/doc/qt-4.8/qapplication.html#notify.
    This has the advantage that this gets only called once for each event and not multiple times like the event filter would do since the events will get propagated...



  • I did not really understand everything you wrote. Probably because I am not (yet) very experienced with Qt nor with C++ - I am a "C" guy.
    Sure I can dig up some more on that ... thanks anyway :)


  • Moderators

    Code is a language every programmer understands... so here you go ;)

    @
    class MyApplication : public QApplication
    {
    public:
    MyApplication(...) : QApplication(...)
    {
    }

    virtual bool notify ( QObject * receiver, QEvent * event )
    {
            //same code like in your previous eventFilter() implementation
            if( event-should-be-filtered-out )
                  return true;
    
             QApplication::notify(receiver,event);
    }
    

    };

    int main(int argc, char *argv[])
    {
    MyApplication program(argc, argv);
    ...
    QTGUI_MainWindow mainWin;

    ...
    }
    @



  • Thanks a million!
    It works ... and solves my issue!
    Responsiveness is back and everything works as supposed.

    Best regards (from aprox. 500 km west)



  • I am extending it ... and got stuck.
    I seem not to be able to call a function that is defined in another class from notify().
    What is the correct way to do this?


  • Moderators

    [quote author="McLion" date="1389690173"]
    I seem not to be able to call a function that is defined in another class from notify().[/quote]
    what exactly is preventing you from calling a method of another class?!



  • Getting:
    @./release\moc_qtgui_mainwindow.o:moc_qtgui_mainwindow.cpp:(.text$_ZN13MyApplication6notifyEP7QObjectP6QEvent[MyApplication::notify(QObject*, QEvent*)]+0x76): undefined reference to `writeToTCOport(QByteArray)'
    collect2: ld returned 1 exit status@

    The other class:
    @class QTGUI_MainWindow : public QMainWindow
    {
    Q_OBJECT

    public:
    explicit QTGUI_MainWindow(QWidget *parent = 0);
    ~QTGUI_MainWindow();
    void writeToTCOport(QByteArray dataBA);
    ....@


  • Moderators

    in that order:

    clean your project

    rerun qmake

    rebuild project

    or more likely i assume your are just calling the method without an object (your mainwindow instance) on or connecting a signal to it, right? (post the code of the call to writeToTCOport() inside the notify() implementation).

    if the error remains:
    Are the implementations of the QTGUI-MainWindow and MyApplication classes in the same project? Or e.g. loaded in a library?



  • Cleaning and rebuilding did not help.

    [quote author="raven-worx" date="1389691039"]
    ... or more likely i assume your are just calling the method without an object (your mainwindow instance) on or connecting a signal to it, right? (post the code of the call to writeToTCOport() inside the notify() implementation).
    [/quote]

    I get:
    @..\src/qtgui_mainwindow.h: In member function 'virtual bool MyApplication::notify(QObject*, QEvent*)':
    ..\src/qtgui_mainwindow.h:86: error: cannot call member function 'void QTGUI_MainWindow::writeToTCOport(QByteArray)' without object@
    when I call it like:
    @QTGUI_MainWindow::writeToTCOport(sendData);@

    And calling without QTGUI_MainWindow:: returns the error posted before.
    I seem to miss some of the basic c ++ concepts ..


  • Moderators

    @error: cannot call member function 'void QTGUI_MainWindow::writeToTCOport(QByteArray)' without object@
    says it clearly: you need an object/instance to call it on.

    The call
    @QTGUI_MainWindow::writeToTCOport(sendData);@
    would be fine when you define the method "static". You can define a method static if you don't need to access object specific members within it. If that's ok in this method do it.

    Otherwise i would suggest something like this for example:
    @
    class MyApplication : public QApplication
    {
    public:
    MyApplication(...) : QApplication(...)
    {
    }

        virtual bool notify ( QObject * receiver, QEvent * event )
        {
                //same code like in your previous eventFilter() implementation
                if( event-should-be-filtered-out )
                      return true;
     
                 if( m_MainWindow )
                      m_MainWindow->writeToTCOport(...);
    
                 QApplication::notify(receiver,event);
        }
    
        void setMainWindowObject(QTGUI_MainWindow* mainWin)
        {
                m_MainWindow = mainWin;
        }
    
    protected:
              QPointer<QTGUI_MainWindow> m_MainWindow;
    };
     
     
    int main(int argc, char *argv[])
    {
        MyApplication program(argc, argv);
    ...
        QTGUI_MainWindow mainWin;
        program.setMainWindowObject(&mainWin);
    ...
    }
    

    @



  • Thanks - tried the static solution. Compiles as long as the function is empty. As soon as I add port->write(); like in many other places I get back the same error.
    Going to read your second suggestion ... as well as some C++ tutorials ;)


  • Moderators

    as i said:
    [quote author="raven-worx" date="1389692609"]You can define a method static if you don't need to access object specific members within it.[/quote]

    i guess "port" is a member of the class?

    [quote author="McLion" date="1389697075"]
    Going to read your second suggestion ... as well as some C++ tutorials ;)[/quote]

    yes, good idea. ;)



  • Need to pick this up again.
    The following should filter all MouseEvents while !bTouchActive, i.e. the GUI is hidden in my app. With the code below, the GUI still reflects the touches and another object has the focus after the GUI is turned back on. While the GUI is inactive it currently just should debug the cursor position where it has been touched but not "react" on the GUI. Funny enough, it does change the focus to a different object but it does not execute its press/release MouseEvent.
    What is wrong in the code below?

    @class MyApplication : public QApplication
    {
    public:
    MyApplication(int &argc, char **argv ) : QApplication(argc, argv)
    {
    }

    virtual bool notify ( QObject *receiver, QEvent *event )
    {
      switch( event->type() )
      {
        case QEvent::MouseButtonPress:
        case QEvent::MouseButtonRelease:
        case QEvent::MouseMove:
        case QEvent::MouseButtonDblClick:
        {
          if(bTouchActive)
          { qDebug("Touch/Mouse event type: %d", event->type());
          }
          else
          { QMouseEvent *TEvent = static_cast<QMouseEvent *>(event);
            QPoint cursorPos = TEvent->globalPos();
            qDebug("TouchInActive: TouchXpos %d, TouchYpos %d", cursorPos.rx(), cursorPos.ry());
            return true;  // return with true filters the event
          }
          break;
        }
        case QEvent::KeyPress:
        case QEvent::KeyRelease:
        { QKeyEvent *keyEvent = static_cast<QKeyEvent *>(event);
          qDebug("Key %d pressed, Event type: %d", keyEvent->key(), event->type());
          break; // break does not filter and forwards the event to notify()
        }
        default:
          break;
      }
      QApplication::notify(receiver,event);
      return false;
    }
    

    };
    @


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