[Qt-qml] ListView (optimization flags)

[martin.jones at nokia.com]

We've done some preliminary experimentation with this, but it did not generally lead to an improvement because the case of item creation is optimized by delaying evaluation of bindings until component completion. Changing a number of bindings (i.e. roles) on existing items can result in bindings being evaluated multiple times which can be costly, especially for text layout.  There are probably delegates which would see improvement from this and it is something we will revisit in the future.

Reuse would also not work if the delegates store any state - there is no way to restore a delegate to its initial state.

Br,
Martin.


> -----Original Message-----
> From: qt-qml-bounces+martin.jones=nokia.com at qt.nokia.com [mailto:qt-qml-
> bounces+martin.jones=nokia.com at qt.nokia.com] On Behalf Of ext Adriano
> Rezende
> Sent: Saturday, 5 February 2011 1:04 AM
> To: qt-qml at qt.nokia.com
> Subject: [Qt-qml] ListView (optimization flags)
> 
> Hi,
> 
> While comparing the QML ListView performance with an old ListView of
> mine (http://qt.gitorious.org/~arezende/qt-labs/arezende-
> clone/blobs/master/shared/listview.cpp),
> it's clear that in 'low end' devices (X6/N97/5800) the QML ListView is
> not smooth as it could be.
> 
> I didn't take any benchmark, but looking at the code I've saw a
> possible culprit; In the current implementation the delegate items are
> constantly being destroyed and recreated while scrolling the list, and
> the cacheBuffer property just postpone the problem and is not scalable
> for huge lists.
> 
> In my implementation I just reuse the items, switching them like a
> carousel. The items are being kept alive and erased before reuse (when
> needed).
> 
> Probably the QML ListView is implemented that way to ease the
> developer work. But in most of the use cases the delegates does not
> create items dynamically inside them, so they could be reused even
> without any clean process. For the rare cases where a clean is needed,
> an "indexAboutToChange" signal could be provided.
> 
> It would be nice if an optimization flag were provided like the following:
> 
> ListView {
>     recycle: true
>     delegate: listDelegate
> }
> 
> Component {
>     id: listDelegate
> 
>     Item {
>         onIndexAboutToChange: {
>             // clean dynamically created items if needed
>         }
>     }
> }
> 
> This recycle property would be more powerful than cacheBuffer property
> and would also be scalable.
> 
> I didn't look deeply at the QML ListView, so my assumptions could be
> wrong. Someone with a deeper knowledge of the ListView source code
> would like to discuss this?
> 
> Br,
> Adriano
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