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.
Thank you ever so much,
Jaakko
-----Original Message-----
From: ext Adriano Rezende [mailto:[email protected]]
Sent: 03 February, 2011 15:25
To: Korpela Jaakko (Nokia-MS-Qt/Tampere)
Cc: [email protected]
Subject: Re: [Qt-qml] ListView and visible items
On Thu, Feb 3, 2011 at 4:56 AM, <[email protected]> wrote:
> Hi,
>
> I am looking at ways to dig out the visible items in ListView. I have a
> requirement to trigger certain behavior every time a new item becomes
> visible while scrolling. ListView is internally aware of the items that are
> currently visible as ListView.indexAt returns -1 if the item at a certain
> coordinate is not visible. But what are the ways to find out what items are
> visible visible. I have tried a couple of approaches with my particular
> requirement.
>
> First, make use of Component.onCompleted in ListView delegates. The only
> trouble with this is that the first list item in ListView is never deleted,
> so you cannot rely on this tell you when that first item becomes visible
> while scrolling.
>
> The second approach involves using ListView.indexAt and pulling the indexes
> of the two items at the edges at a certain interval and see if there are any
> changes. This actually works, but I am not really happy with this approach.
>
> Any suggestions?
You can add to your delegate a boolean property binded to the ListView
visible bounding rect. So any change you want to apply on the item you
can bind to this property.
For example:
Item {
width: 500
height: 500
Component {
id: listDelegate
Rectangle {
width: parent.width
height: 60
opacity: 0.7
border.width: 1
color: isVisible ? "red" : "blue"
property bool isVisible : ((y >= ListView.view.contentY &&
y <= ListView.view.contentBottom) ||
(y + height >= ListView.view.contentY &&
y + height <=
ListView.view.contentBottom))
}
}
Rectangle {
anchors.fill: listView
color: "green"
}
ListView {
id: listView
cacheBuffer: 1000
anchors.fill: parent
anchors.topMargin: 80
anchors.bottomMargin: 80
model: 100
delegate: listDelegate
property real contentBottom: contentY + height
}
}
Maybe an attached property could be useful (like ListView.isInside ..
dummy name).
Br,
Adriano
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