[Development] Improve performance (listview + onVerticalVelocityChanged)

[Martin Leutelt]

Hi,


 From:   Bernhard B <schluchti at gmail.com> 
 To:   <development at qt-project.org> 
 Sent:   1/26/2017 8:09 PM 
 Subject:   Re: [Development] Improve performance (listview + onVerticalVelocityChanged) 





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 0onMovementEnded: {
    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.


I guess you could also bind the visibility of your backToTopButton/tabBar to the ListView properties atYBeginning, flickingVertically and the comparison of
the verticalVelocity against your threshold.


Just as an info: the 'development' list is for questions regarding the development of Qt itself, questions regarding the usage of Qt should go to the 'interest' list.
More people will respond there.






Thanks,
Bernhard


Regards
Martin






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 reached
if(indexAt(contentX, contentY) === 0)
   backToTopButton.visible = false;

I think it should be possible to check that in the 
QVariant QAbstractItemModel::data(const QModelIndex &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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