[Qt-qml] ListView and visible items

[petrus.lundqvist at nokia.com]

Keep in mind that QML can optimize expressions like this so that they do
not get executed as JS at all.

Peppe

On 3.2.2011 22:58, "ext Juha Turunen" <turunen at iki.fi> wrote:

>On Thu, Feb 3, 2011 at 5:39 PM,  <jaakko.korpela at nokia.com> wrote:
>> Ah yes,
>>
>> It seems so simple now. I actually did not know (or had forgotten) that
>>QML generates onSomePropertyChanged on property someProperty. And so in
>>my particular predicament you write something like this:
>>
>>    Component {
>>        id: listDelegate
>>
>>        Item {
>>            ...
>>            property bool isVisible : (((index * height) >=
>>ListView.view.contentY &&
>>                                        (index * height) <=
>>ListView.view.contentBottom) ||
>>                                       ((index * height) + height >=
>>ListView.view.contentY &&
>>                                        (index * height) + height <=
>>ListView.view.contentBottom))
>>
>>            onIsVisibleChanged: { if (isVisible && ListView.view.moving)
>>triggerSomeAction(); }
>>        }
>>    }
>>
>> Note that you can't match list item's y property against ListView's
>>contentY, as both are in their local coordinates.
>
>Keep in mind that the javascript monster (sorry :D) will get evaluated
>on every frame of the scrolling animation as many times as there are
>instantiated delegates (having cacheBuffer naturally makes things
>worse). I don't know what platform you're targeting, but at least on
>Symbian you have to be careful in order to get good performance. I
>think it might be feasible to do these types of things also in c++
>side. All the properties you can use in javascript are also available
>in the c++ side by using the metaobject system and pointers of qml
>elements can be passed as parameters to c++ methods. I guess the
>delegates should get an insideListViewBounds attached property or
>something (don't know though what the overhead is for cases where it's
>not needed).
>
>If you take into consideration what Adriano said about the coordinates
>you should be able to simplify the logic a bit. Also wouldn't it be
>simpler to organize the logic so that you're checking for
>non-visibility rather than visibility? I mean check that the delegate
>y + height is less than contentY or delegate y is more than the
>content bottom and then do a ! in the onChanged handler.
>
>Juha
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