How to port QApplication::x11EventFilter to XCB QAbstractNativeEventFilter

Gluttony

Hi,

We are currently porting an application from Qt4 to Qt5 and there is not much documentation on how to replace MyApplication::x11EventFilter implementation to a QAbstractNativeEventFilter installed with QCoreApplication::installNativeEventFilter (as explained in https://doc.qt.io/qt-5/sourcebreaks.html#changes-to-qcoreapplication).
We just started the implementation of

MyXcbEventFilter::nativeEventFilter(const QByteArray &eventType, void *message, long *)

as explained in https://doc.qt.io/qt-5/qabstractnativeeventfilter.html#nativeEventFilter in "Linux example" section.
And, with what we found on the web, for example replacing this in MyApplication:

bool MyApplication::x11EventFilter(XEvent* p_event)
{
    int eventType = p_event->type;

    QSharedPointer<QEvent> eventToSend;

    if (eventType == ButtonPress)
    {
        XButtonPressedEvent& bpe = p_event->xbutton;
        const QPoint pos(bpe.x_root,bpe.y_root);
        QWidget* widget = qApp->widgetAt(pos);
        if (widget && widget->window() != qApp->activeWindow())
        {
            QPoint local_pos = widget->mapFromGlobal(pos);
            eventToSend = QSharedPointer<QEvent>((QEvent*)new QMouseEvent(QEvent::MouseButtonPress, local_pos, pos, Qt::LeftButton, Qt::LeftButton, Qt::NoModifier));
        }
    }

    return false;
}

To something like this (with installNativeEventFilter(m_myXcbEventFilter) called in MyApplication constructor):

bool MyXcbEventFilter::nativeEventFilter(const QByteArray& p_eventType, void* p_message, long*)
{
    if (p_eventType == "xcb_generic_event_t")
    {
        xcb_generic_event_t* ev = static_cast<xcb_generic_event_t *>(p_message);
        uint16_t xcbEventType = ev->response_type & ~0x80;

        if (xcbEventType == XCB_BUTTON_PRESS)
        {
            xcb_button_press_event_t* buttonPressEvent = static_cast<xcb_button_press_event_t*>(p_message);
            // Do stuff
        }
    }
}

But we never fall in xcbEventType == XCB_BUTTON_PRESS, it seems instead there is (at least for button press/release) a XCB_GE_GENERIC that we can cast to xcb_ge_generic_event_t that has an event_type member that contains for example a XCB_BUTTON_PRESS, the code then changed to:

bool MyXcbEventFilter::nativeEventFilter(const QByteArray& p_eventType, void* p_message, long*)
{
    if (p_eventType == "xcb_generic_event_t")
    {
        xcb_generic_event_t* ev = static_cast<xcb_generic_event_t *>(p_message);
        uint16_t xcbEventType = ev->response_type & ~0x80;

        if (xcb_eventType == XCB_GE_GENERIC)
        {
            xcb_ge_generic_event_t* genericEvent = static_cast<xcb_ge_generic_event_t*>(p_message);
            xcbEventType = genericEvent->event_type;
        }

        if (xcbEventType == XCB_BUTTON_PRESS)
        {
            xcb_button_press_event_t* buttonPressEvent = static_cast<xcb_button_press_event_t*>(p_message);
            // Do stuff
        }
    }
}

Now we fall in xcbEventType == XCB_BUTTON_PRESS on button press but casting to xcb_button_press_event_t seems to have no meaning since root_x, root_y, event_y and event_y members have always the same values anywhere we click.

Anyone knows how to properly implement this ?

Thanks

Christian Ehrlicher

Since Qt is opensource you can take a look to the place where the eventFilter is called:
https://code.woboq.org/qt5/qtbase/src/plugins/platforms/xcb/qxcbconnection.cpp.html#_ZN14QXcbConnection14handleXcbEventEP19xcb_generic_event_t

Gluttony

@Christian-Ehrlicher Thanks, but if I follow what is happening from the *QXcbConnection::handleXcbEvent(xcb_generic_event_t event) function you pointed, I can see first there is the same

uint response_type = event->response_type & ~0x80

From which I never get a XCB_BUTTON_PRESS, and, if I follow XCB_GE_GENERIC I get, it goes to:

QXcbConnection::xi2HandleEvent(xcb_ge_event_t *event)

That, for XCB_BUTTON_PRESS calls an event filter got from:

QXcbWindowEventListener *QXcbConnection::windowEventListenerFromId(xcb_window_t id)
{
    return m_mapper.value(id, 0);
}

And here I'm lost since QXcbWindowEventListener are added to m_mapper with QXcbConnection::addWindowEventListener I can't seen where it is called, I tried searching where this function is called in other Qt classes and I either found dead ends or classes calling functions just returning false.

Gluttony

And in addition, I just tried to implement key press filter and it works fine (we directly got the XCB_KEY_PRESS event type from event->response_type & ~0x80 instead of getting a XCB_GE_GENERIC):

if (xcbEventType == XCB_KEY_PRESS)
{
    xcb_key_press_event_t* keyPressEvent = reinterpret_cast<xcb_key_press_event_t*>(ev);
    // Here keyPressEvent->detail has its value changing according to key press
}

Christian Ehrlicher

Since XCB_BUTTON_PRESS is directly evaluated in handleXcbEvent() I would guess you must be doing something wrong.

Gluttony

Indeed, but I'm not sure I'm doing something wrong, since, all I've done is following Qt's instruction on how to replace QApplication::x11EventFilter with a QAbstractNativeEventFilter class implementing its nativeEventFilter and catching eventType == "xcb_generic_event_t", after that, what comes in nativeEventFilter parameters is definitively not something I have the hand on, and then, in my opinion, the real problem here is, for some reason, I got a XCB_GE_GENERIC event instead of XCB_BUTTON_PRESS event, and I would like to know either how to be able, with current behaviour to manage XCB_BUTTON_PRESS event as a XCB_GE_GENERIC event or to have directly XCB_BUTTON_PRESS event (changing a parameter on Qt for example ?) in nativeEventFilter.

Gluttony

Hi again,

I came back on this subject after a while since updated to Qt5.13 but unfortunately with no amelioration on behaviour, and I then created a small application to reproduce the issue if it can help (https://we.tl/t-SUhYHVWOTj), and if it can help too, here are which event are logged on mouse click:

virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_ALLOC_NAMED_COLOR (uint16_t: 85)"]
virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_CLIENT_MESSAGE (uint16_t: 33)"]
virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_PROPERTY_NOTIFY (uint16_t: 28)"]
virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_GE_GENERIC (uint16_t: 35)"]
virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_PROPERTY_NOTIFY (uint16_t: 28)"]

And on mouse release:

virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_GE_GENERIC (uint16_t: 35)"]
virtual bool XcbEventFilter::nativeEventFilter(const QByteArray&, void*, long int*) ; Event type is ["XCB_ALLOC_NAMED_COLOR (uint16_t: 85)"]

I somebody can try (notice I put a timer that writes endl to cout every half second in order to properly separate click from release), is it the same result ?

Or should I follow the handleXcbEvent ? That for what I see is only in QXcbConnection for which I found no documentation. Or follow another way ?

Thanks

DalePennington

@Gluttony
I am also doing a QT4 to QT5 port and ran into the same issue. But I have also found the answer.

The problem lies in that QT5 will use the XInput extension if it is present (and I think it almost always is). This means your button presses will come via the xcb_input_button_press_event_t (defined in xcb/xinput.h) as opposed to xcb_button_press_event_t. The good news is that most of the fields in the the xcb_button_press_event_t are in the xcb_input_button_press_event_t (I did not find state and same_screen), with the same name and meaning. The bad news is that they are not at the same offsets, so you cannot just do a structure map. I ended up creating a helper function that I passed the fields I was interested in.

Now the even is delivered via the XCB_GE_GENERIC event. This is one of the common ways for extensions to pass in events. To get to it properly you need to do the following :

Check if XInput is installed and query xcb for its major opcode. QXcbBasicConnection::initializeXInput2() has a good example of this.
Check to see if the XCB_GE_GENERIC event is from XInput. This is done by comparing the extension field with the major opcode you got from (1)
Now the event_type field will contain the extension specific opcodes. For XInput, they are defined in xcb/xinput.h. You then recast and go like you do for normal events.

As an example

void HandleButtonPress(uint8_t button)
{
    ...DoStuff...
}

// Main thread 

DoXInputInitialize(); // gets bHasXi2 and iXi2Opcode

switch( eventType )
{
  case XCB_BUTTON_PRESS:
    auto *pButtonEvent = 
      reinterpret_cast<xcb_button_press_event_t *>(pEvent);
    HandleButtonPress(pButtonEvent->detail);
    return true;
       /// More Stuff
  case XCB_GE_GENERIC:
    auto *pGeEvent = reinterpret_cast<xcb_ge_generic_event_t *>(pEvent);
    if( bHasXi2 && iXi2Opcode == pGeEvent->extension )
    {
      // This is from XInput
      switch( pGeEvent->event_type )
      {
        case XCB_INPUT_BUTTON_PRESS:
          auto *pXiButtonEvent = 
            reinterpret_cast<xcb_input_button_press_event_t *>(pGeEvent);
          HandleButtonPress(pXiButtonEvent->detail);
          return true;
      }
    }
}

I hope this answers your issues.

Dale Pennington