[Development] Improve performance (listview + onVerticalVelocityChanged)

[Bernhard B]

I think I solved my problem. In case someone is interested, that's my
solution:

//hide "back to top" button when movement ended and we are

//at index 0

onMovementEnded: {
    if(indexAt(contentX, contentY) === 0){

      backToTopButton.visible = false;

  }

}

onFlickStarted: {

    //when user scrolls fast enough up, show the tab bar

    //and the "back to top" button

    if(verticalVelocity < -flickTabBarTreshold) {

        backToTopButton.visible = true;

        tabBar.show();

    }


    //when user scrolls fast enough down hide "back to top" button

    //and the tab bar

    if(verticalVelocity > flickTabBarTreshold){

        backToTopButton.visible = false;

        tabBar.hide();

    }

}

I don't know if this is the best solution, but it seems to work.

Thanks,
Bernhard

2017-01-26 15:38 GMT+01:00 Bernhard B <schluchti at gmail.com>:

> Hi,
>
> I am currently trying to optimize my Listview for performance. I already
> removed most of the bottlenecks by following the great recommendations at:
> http://doc.qt.io/qt-5/qtquick-performance.html
>
> However, there is one problem I don't how to solve. It's about this code
> part:
>
> ListView{
>     clip: true
>     property real flickTabBarTreshold;
>
>     Component.onCompleted: {
>         flickTabBarTreshold = (2/3) * maximumFlickVelocity;
>     }
>
>     onVerticalVelocityChanged: {
>
>
>     //when user scrolls fast enough up, show the tab bar     //and the "back to top" button
>
>     if(verticalVelocity < -flickTabBarTreshold) {
>         backToTopButton.visible = true;
>
>         tabBar.show();
>
>     }
>
>
>
>     //when user scrolls fast enough down hide "back to top" button    //and the tab bar
>     if(verticalVelocity > flickTabBarTreshold){
>
>         backToTopButton.visible = false;
>
>         tabBar.hide();
>
>     }
>
>     //always disable "back to top button" when top reached    if(indexAt(contentX, contentY) === 0)        backToTopButton.visible = false;
>
>     }
> }
>
> This part is one of the biggest performance bottlenecks at the moment (as
> it gets called a lot of times) and has a significant impact on the
> scrolling behavior (lagging).
>
> My idea was to put that code somehow in my C++ Listmodel and emit only a
> signal when something actually changed.
>
> For the last part
>
> //always disable "back to top button" when top reachedif(indexAt(contentX, contentY) === 0)   backToTopButton.visible = false;
>
>
> I think it should be possible to check that in the
> QVariant <http://doc.qt.io/qt-5/qvariant.html> QAbstractItemModel::data(const
> QModelIndex <http://doc.qt.io/qt-5/qmodelindex.html> &*index*, int *role*
> = Qt::DisplayRole) const
>
> method. When index.row() === 0, I a signal will be emitted. In QML I am
> connecting on that signal and execute  backToTopButton.visible = false;
>
> I haven't tested it yet, but I think it should work.
>
> For the other two conditions however, I am a little bit clueless.
>
> Does anyone of you guys maybe have an idea on how to improve that?
>
> Any help is really appreciated.
>
>
> Thanks,
>
> Bernhard
>
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