Which code decides the target widget is for a touch point

mdb977

I'm currently debugging a strange touch-event bug in an application that mixes widgets and QML GUI elements.

In QGuiApplicationPrivate::processTouchEvent, the touch points delivered within the TouchEvent contain a target widget.

So somewhere between the QPA event handler implemented in qwindowsysteminterface.cpp and QGuiApplicationPrivate::processTouchEvent, the system decides the target widget for a touch point.

Can you please point me to the corresponding Qt code for this assignment?

I want to understand this better.

Thanks

Axel Spoerl

@mdb977
It would be good to understand more about the bug you are investigating.
processTouchEvent is probably a good start. The touch event already knows, to which QWindow the event has been delivered. That is not up to Qt to determine. The decision is made on the underlying window system, e.g. X11 or Windows. QGuiApplication doesn't know about widgets or quick windows. It delivers events on a QWindow level.

So what goes wrong in your application?

mdb977

@Axel-Spoerl

Thanks for your message.

You are talking about the target of the touch event.
I'm, however, talking about the target of the touchpoints.

This target is sometimes corrected for touch-press in translateRawTouchEvent in qapplication.cpp.

https://codebrowser.dev/qt6/qtbase/src/widgets/kernel/qapplication.cpp.html#3856

I want to know who is setting the target for the touchpoints.

Regarding my problem: In a particular case, I have a pair of touch events (press and corresponding release). The press event is synthesized into a mouse click as it should in processTouchEvent. However, the release is delivered to my quick widget, which accepts the release (so the release is not synthesized).

I have a hacky patch solving my problem, but I want to understand it better.

I also have a small standalone Qt example showing the problem, but I wonder if I should open a ticket in bugreports.qt.io for a bug I do not fully understand.

Axel Spoerl

@mdb977 said in Which code decides the target widget is for a touch point:

Could it be, that your problem is related to this?
I am asking, because a colleague of mine implemented a great fix recently, where we swallowed touch events. In parallel, I'll look into our code for setting the target of touch points.

mdb977

@Axel-Spoerl

Thanks for the link.
At a glance, it is unrelated.

mdb977

@Axel-Spoerl

I'm still interested in the code for setting the target of touchpoints.
Meanwhile, I managed to reproduce the issue with a handful of lines of code on a desktop Qt application using Qt 6.6.1. So, I think this is a Qt Bug.

Regards

Axel Spoerl

@mdb977
File a bug report, I am getting curious.