[Interest] Setting QQuickItem properties inside QQuickItem::UpdatePaintNode

[manish sharma]

HI Gunnar, Thanks for the suggestion, I will use updatePolish(). Also the
fix on codereview works.


On Thu, Jun 5, 2014 at 10:43 AM, Gunnar Sletta <gunnar.sletta at jolla.com>
wrote:

>
> On 05 Jun 2014, at 06:45, manish sharma <83.manish at gmail.com> wrote:
>
> > Hi,
> >
> > Qt documentation says "The main thread is blocked while this function is
> executed so it is safe to read values from the QQuickItem instance and
> other objects in the main thread."
> >
> > Can we only read QQuickItem properties? We can not set it?
>
> You can, but anything tied to those properties will have their signals
> emitted on the render thread which may result in bad stuff. For instance,
> if something is bound to that property in QML the signal emission will be
> done using a queued connection so the binding will not be evaluated until
> later on the GUI thread. That effectively means you get incorrect anchors
> and positioners and similar. Not ideal. On the other hand, if you start
> timers, as a result of something tied to the setter or its notify signal,
> those timers will have affinity to the render thread and be fired there.
> That can result in pretty much anything.
>
> So I would strongly discourage it :) If you need to delay something and do
> it "just before rendering", you should use QQuickItem::polish() and
> QQuickItem::updatePolish().
>
> > I have few QQuickItems and I am trying to set QQuickItem properties
> (height/width) from one of the QQuickItem's updatePaintNode function. So
> far it worked but when I try QQuickWindow::grabWindow() it asserts both in
> windows (QML_FORCE_THREADED_RENDERER=1) and Linux.
> >
> > ASSERT failure in QQuickItem::update(): "Function can only be called
> from GUI thread or during QQuickItem::updatePaintNode()", file
> E:\Qt\qt5.2\qt5\qtdeclarative\src\quick\scenegraph\qsgthreadedrenderloop.cpp,
> line 988
> >
> > I noticed QSGThreadedRenderLoop::m_locked is false. Is it a bug in
> QSGThreadedRenderLoop OR we can not set QQuickItem properties inside
> updatePaintNode which might cause QQuickItem::update()?
>
> It is a bug, does https://codereview.qt-project.org/86823 help?
>
> cheers,
> Gunnar
>
> >
> > Regards,
> > Manish
> > _______________________________________________
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> > Interest at qt-project.org
> > http://lists.qt-project.org/mailman/listinfo/interest
>
>
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