Hiding an item outside viewport of a ListView - qt

Question

I've been learning QtQuick for about a week and I'm facing a weird behaviour on what I'm trying to achieve. I would like to make a vertical ListView with a Keyboard navigation so that when I press UP or DOWN, the items move up or down and if an item goes in or out of the "viewport", its opacity property will change smoothly to 0 or 1.
Here is my current QML code:
import QtQuick 2.4
Rectangle {
width:200
height:400
ListView {
property int activePosition:1
property int itemDisplayed:3
width:parent.width-50
height:parent.height-50
anchors.centerIn:parent
model:10
snapMode:ListView.SnapToItem
focus:true
cacheBuffer:2000
Component.onCompleted: {
console.log(count+' != '+contentItem.children.length+' ???')
}
Keys.onPressed: {
var i = 0
console.log('pos='+activePosition)
console.log(count+' != '+contentItem.children.length+' ???')
if (event.key === Qt.Key_Up) {
if (activePosition == 1 && currentIndex > 0) {
i = currentIndex+itemDisplayed-1
if (i < contentItem.children.length - 2/* why -2 instead of -1 ??? */) {
console.log('out='+i)
contentItem.children[i].state = 'out'
}
}
activePosition = activePosition > 1 ? activePosition - 1 : activePosition
}
if (event.key === Qt.Key_Down) {
if (activePosition == itemDisplayed && currentIndex < contentItem.children.length - 2) {
i = currentIndex-itemDisplayed+1
if (i >= 0) {
console.log('out='+i)
contentItem.children[i].state = 'out'
}
}
activePosition = activePosition < itemDisplayed ? activePosition + 1 : activePosition
}
}
delegate: Rectangle {
id:rect
state:index < ListView.view.itemDisplayed ? 'in' : 'out'
opacity:1.0
width:ListView.view.width
height:ListView.view.height/ListView.view.itemDisplayed
border.color:'white'
border.width:1
color:activeFocus ? 'red': 'gray'
onActiveFocusChanged: {
if (activeFocus) {
state = 'in'
console.log('in='+index)
}
}
states: [
State { name:'in'; PropertyChanges { target:rect; opacity:1.0 } },
State { name:'out'; PropertyChanges { target:rect; opacity:0.0 } }
]
transitions: [
Transition {
to:'in'
NumberAnimation { property:'opacity'; duration:250 }
},
Transition {
to:'out'
NumberAnimation { property:'opacity'; duration:250 }
}
]
Text {
text:index
anchors.centerIn:parent
}
}
}
}
First question : model=10, why model.count is not equal to contentItem.children.length? onCompleted gives 5 vs 11 and during navigation 10 vs 11
Second question: If I press UP or DOWN, it works fine until I reach index=4, why?
As I'm a beginner on QtQuick so maybe it's not the right approach. I tried to use the visible property but every item has visible = true even if they are outside. I tried also indexAt() with no success.
Any help would be great :-)

Answer 1

Now I know better about the ListView behavior. My previous code can be fixed by removing the Keys.onPressed event which is no longer useful, and by using the itemAt() method directly into the onActiveFocusChanged handler.