qtlistviewqmlqtquick2qt5.3

Animate snap in ListView


I would like to animate the "snap" effect when a ListView snaps to a specific item.

I enable "snapping" with the snapMode: ListView.SnapOneItem property. Currently it just de-accelerates to the current item and stops, but it would be nice if I could get it to make a "bounce" effect when it stops.

Any ideas about how I can do this?

Flickable has a rebound property, but this seems to not work for the snapping on elements inside a ListView.


Solution

  • Since you used SnapOneItem, you can insert a bounce effect once the movement is finished, i.e. once movementEnded signal is emitted. IMO applying an animation on each item would be overkill in this case. Better would be to animate the contentY property of the ListView.

    The following is a possible approach which produces a bounce (don't know if this is the exact effect you are searching for):

    ListView {
        id: list
        anchors.fill: parent
        spacing: 10
        snapMode: ListView.SnapOneItem
        model: 100
    
        delegate: Rectangle {
            width: parent.width
            height: 50
            Text {                // added text to distinguish items
                text: index
                font.pixelSize: 20
                anchors.centerIn: parent
            }
            color: index % 2 ? "lightGray" : "darkGrey"
        }
    
        onMovementEnded: {
            bounce.start()        // starts the animation when the movement ends
        }
    
        onCurrentIndexChanged: { 
            if(!list.moving)      // animation started only if the list is not moving
                bounce.start()
        }
    
        Timer {
            repeat: true
            interval: 2000     // enough time to let the animation play
            running: true
            onTriggered: {
                list.incrementCurrentIndex()
            }
        }
    
        SequentialAnimation {
            id: bounce
            running: false
    
            NumberAnimation {
                target: list
                property: "contentY"
                to: list.contentY - 10
                easing {
                    type: Easing.InOutBack
                    amplitude: 2.0
                    period: 0.5
                }
                duration: 250
            }
    
            NumberAnimation {
                target: list
                property: "contentY"
                to: list.contentY
                duration: 800
                easing {
                    type: Easing.OutBounce
                    amplitude: 3.0
                    period: 0.7
                }
            }
        }
    }   
    

    When you release the items from a drag or movement the bounce is produced. The amplitude and period properties can be adjusted to obtain a stronger or lighter effect (the same applies for the value of to properties).

    EDIT

    As you've seen, if the list is moved via incrementCurrentIndex() no real movement occurs, i.e. the movementEnded signal is not emitted. In such a case you can exploit the change of value that occurs for the currentIndex. I've modified the example to combine this approach with the previous one and to show the usage I've inserted a Timer to call incrementCurrentIndex() function.

    The !list.moving check is added to avoid double animations when the list is moved and to guarantee consistency in the example, since the Timer can generate inconsistent jumps while the list is dragged. Clearly, other more specific constraints can be added, depending on your requirements.