ubuntu-phone team mailing list archive

[Michael Zanetti <michael.zanetti@xxxxxxxxxxxxx>]

On 18.04.2016 13:57, Andrea Bernabei wrote:
> 
> 
> On Mon, Apr 18, 2016 at 12:43 PM, Alberto Mardegan
> <alberto.mardegan@xxxxxxxxxxxxx <mailto:alberto.mardegan@xxxxxxxxxxxxx>>
> wrote:
> 
>     On 18/04/2016 13:57, Omer Akram wrote:
>     > While working on a pet project[1] for the Phone, an issue that I faced
>     > was stuttering scrolling in listviews. QML tips page[2] suggested to use
>     > Flickable+Column+Repeater combination to have smooth scrolling, that
>     > worked and is currently what I have implemented in my app but I always
>     > thought that was not the perfect solution.
>     >
>     > With some knowledge of how Android ListView works and its mechanism of
>     > recycling ListView items, I wrote a hack[3] to achieve something similar
>     > in QML.
>     [...]
> 
>     You are trying to reinvent the QML ListView :-)
>     The QML ListView recycles the delegates, and this behaviour is partially
>     configurable via the cacheBuffer property:
> 
>       http://doc.qt.io/qt-5/qml-qtquick-listview.html#cacheBuffer-prop
> 
>     Now, this means that when scrolling a listview, occasionally the
>     delegates need to be either created (the first time they are used) or
>     reused (which causes a reassignment of their properties based on the
>     model data).
>     Both these operations can be expensive, that's why someone gave the tip
>     of using a Column+Repeater when the number of delegates is low and
>     doesn't change, so that the whole list is created only once and
>     scrolling will be smooth.
> 
>     If now you found that this tip doesn't work well for you (typically
>     because of high memory consumption if the model has many items), it
>     means that you should go back to using a ListView. :-)
> 
> 
> Does ListView reuse delegates?
> As far as I know, it creates new ones on the fly, every time.
> 
> I guess Omar's solution was for cases where:
> - you have fixed delegate height
> - you have a fixed and known number of delegates
> - you want to avoid instantiating all of them at once
> 
> in that case reusing the delegates could work better than recreating
> them like ListView does.
> 
> If I'm wrong and ListView already reuses delegates instances, then my
> points above are of course void.

AFAIK I think too that the ListView does not reuse them but instead
always destroys/creates them when they leave/enter the cacheBuffer zone.


>   
> 
>     Ciao,
>       Alberto
> 
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