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ViewTransition

Specifies items under transition in a view. More...

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Attached Properties

• destination : point
• index : int
• item : item
• targetIndexes : list
• targetItems : list

Detailed Description

With ListView and GridView, it is possible to specify transitions that should be applied whenever the items in the view change as a result of modifications to the view's model. They both have the following properties that can be set to the appropriate transitions to be run for various operations:

• populate - the transition to apply to the items created initially for the view, or when the model changes
• add - the transition to apply to items that are added to the view after it has been created
• remove - the transition to apply to items that are removed from the view
• move - the transition to apply to items that are moved within the view (i.e. as a result of a move operation in the model)
• displaced - the generic transition to be applied to any items that are displaced by an add, move or remove operation
• addDisplaced, removeDisplaced and moveDisplaced - the transitions to be applied when items are displaced by add, move, or remove operations, respectively (these override the generic displaced transition if specified)

For the Row, Column, Grid and Flow positioner types, which operate with collections of child items rather than data models, the following properties are used instead:

• populate - the transition to apply to items that have been added to the positioner at the time of its creation
• add - the transition to apply to items that are added to or reparented to the positioner, or items that have become visible
• move - the transition to apply to items that have moved within the positioner, including when they are displaced due to the addition or removal of other items, or when items are otherwise rearranged within the positioner, or when items are repositioned due to the resizing of other items in the positioner

View transitions have access to a ViewTransition attached property that provides details of the items that are under transition and the operation that triggered the transition. Since view transitions are run once per item, these details can be used to customize each transition for each individual item.

The ViewTransition attached property provides the following properties specific to the item to which the transition is applied:

• ViewTransition.item - the item that is under transition
• ViewTransition.index - the index of this item
• ViewTransition.destination - the (x,y) point to which this item is moving for the relevant view operation

In addition, ViewTransition provides properties specific to the items which are the target of the operation that triggered the transition:

• ViewTransition.targetIndexes - the indexes of the target items
• ViewTransition.targetItems - the target items themselves

(Note that for the Row, Column, Grid and Flow positioner types, the move transition only provides these two additional details when the transition is triggered by the addition of items to a positioner.)

View transitions can be written without referring to any of the attributes listed above. These attributes merely provide extra details that are useful for customising view transitions.

Following is an introduction to view transitions and the ways in which the ViewTransition attached property can be used to augment view transitions.

View Transitions: a Simple Example

Here is a basic example of the use of view transitions. The view below specifies transitions for the add and displaced properties, which will be run when items are added to the view:

ListView {
    width: 240; height: 320
    model: ListModel {}

    delegate: Rectangle {
        width: 100; height: 30
        border.width: 1
        color: "lightsteelblue"
        Text {
            anchors.centerIn: parent
            text: name
        }
    }

    add: Transition {
        NumberAnimation { property: "opacity"; from: 0; to: 1.0; duration: 400 }
        NumberAnimation { property: "scale"; from: 0; to: 1.0; duration: 400 }
    }

    displaced: Transition {
        NumberAnimation { properties: "x,y"; duration: 400; easing.type: Easing.OutBounce }
    }

    focus: true
    Keys.onSpacePressed: model.insert(0, { "name": "Item " + model.count })
}

When the space key is pressed, adding an item to the model, the new item will fade in and increase in scale over 400 milliseconds as it is added to the view. Also, any item that is displaced by the addition of a new item will animate to its new position in the view over 400 milliseconds, as specified by the displaced transition.

If five items were inserted in succession at index 0, the effect would be this:

Notice that the NumberAnimation objects above do not need to specify a target to animate the appropriate item. Also, the NumberAnimation in the addTransition does not need to specify the to value to move the item to its correct position in the view. This is because the view implicitly sets the target and to values with the correct item and final item position values if these properties are not explicitly defined.

At its simplest, a view transition may just animate an item to its new position following a view operation, just as the displaced transition does above, or animate some item properties, as in the add transition above. Additionally, a view transition may make use of the ViewTransition attached property to customize animation behavior for different items. Following are some examples of how this can be achieved.

Using the ViewTransition Attached Property

As stated, the various ViewTransition properties provide details specific to the individual item being transitioned as well as the operation that triggered the transition. In the animation above, five items are inserted in succession at index 0. When the fifth and final insertion takes place, adding "Item 4" to the view, the add transition is run once (for the inserted item) and the displaced transition is run four times (once for each of the four existing items in the view).

At this point, if we examined the displaced transition that was run for the bottom displaced item ("Item 0"), the ViewTransition property values provided to this transition would be as follows:

The ViewTransition.targetIndexes and ViewTransition.targetItems lists provide the items and indexes of all delegate instances that are the targets of the relevant operation. For an add operation, these are all the items that are added into the view; for a remove, these are all the items removed from the view, and so on. (Note these lists will only contain references to items that have been created within the view or its cached items; targets that are not within the visible area of the view or within the item cache will not be accessible.)

So, while the ViewTransition.item, ViewTransition.index and ViewTransition.destination values vary for each individual transition that is run, the ViewTransition.targetIndexes and ViewTransition.targetItems values are the same for every add and displaced transition that is triggered by a particular add operation.

Delaying Animations Based on Index

Since each view transition is run once for each item affected by the transition, the ViewTransition properties can be used within a transition to define custom behavior for each item's transition. For example, the ListView in the previous example could use this information to create a ripple-type effect on the movement of the displaced items.

This can be achieved by modifying the displaced transition so that it delays the animation of each displaced item based on the difference between its index (provided by ViewTransition.index) and the first removed index (provided by ViewTransition.targetIndexes):

    displaced: Transition {
        id: dispTrans
        SequentialAnimation {
            PauseAnimation {
                duration: (dispTrans.ViewTransition.index -
                        dispTrans.ViewTransition.targetIndexes[0]) * 100
            }
            NumberAnimation { properties: "x,y"; duration: 400; easing.type: Easing.OutBounce }
        }
    }

Each displaced item delays its animation by an additional 100 milliseconds, producing a subtle ripple-type effect when items are displaced by the add, like this:

Animating Items to Intermediate Positions

The ViewTransition.item property gives a reference to the item to which the transition is being applied. This can be used to access any of the item's attributes, custom property values, and so on.

Below is a modification of the displaced transition from the previous example. It adds a ParallelAnimation with nested NumberAnimation objects that reference ViewTransition.item to access each item's x and y values at the start of their transitions. This allows each item to animate to an intermediate position relative to its starting point for the transition, before animating to its final position in the view:

    displaced: Transition {
        id: dispTrans
        SequentialAnimation {
            PauseAnimation {
                duration: (dispTrans.ViewTransition.index -
                        dispTrans.ViewTransition.targetIndexes[0]) * 100
            }
            ParallelAnimation {
                NumberAnimation {
                    property: "x"; to: dispTrans.ViewTransition.item.x + 20
                    easing.type: Easing.OutQuad
                }
                NumberAnimation {
                    property: "y"; to: dispTrans.ViewTransition.item.y + 50
                    easing.type: Easing.OutQuad
                }
            }
            NumberAnimation { properties: "x,y"; duration: 500; easing.type: Easing.OutBounce }
        }
    }

Now, a displaced item will first move to a position of (20, 50) relative to its starting position, and then to its final, correct position in the view:

Since the final NumberAnimation does not specify a to value, the view implicitly sets this value to the item's final position in the view, and so this last animation will move this item to the correct place. If the transition requires the final position of the item for some calculation, this is accessible through ViewTransition.destination.

Instead of using multiple NumberAnimations, you could use a PathAnimation to animate an item over a curved path. For example, the add transition in the previous example could be augmented with a PathAnimation as follows: to animate newly added items along a path:

    add: Transition {
        id: addTrans
        NumberAnimation { property: "opacity"; from: 0; to: 1.0; duration: 400 }
        NumberAnimation { property: "scale"; from: 0; to: 1.0; duration: 400 }

        PathAnimation {
            duration: 1000
            path: Path {
                startX: addTrans.ViewTransition.destination.x + 200
                startY: addTrans.ViewTransition.destination.y + 200
                PathCurve { relativeX: -100; relativeY: -50 }
                PathCurve { relativeX: 50; relativeY: -150 }
                PathCurve {
                    x: addTrans.ViewTransition.destination.x
                    y: addTrans.ViewTransition.destination.y
                }
            }
        }
    }

This animates newly added items along a path. Notice that each path is specified relative to each item's final destination point, so that items inserted at different indexes start their paths from different positions:

Handling Interrupted Animations

A view transition may be interrupted at any time if a different view transition needs to be applied while the original transition is in progress. For example, say Item A is inserted at index 0 and undergoes an "add" transition; then, Item B is inserted at index 0 in quick succession before Item A's transition has finished. Since Item B is inserted before Item A, it will displace Item A, causing the view to interrupt Item A's "add" transition mid-way and start a "displaced" transition on Item A instead.

For simple animations that simply animate an item's movement to its final destination, this interruption is unlikely to require additional consideration. However, if a transition changes other properties, this interruption may cause unwanted side effects. Consider the first example on this page, repeated below for convenience:

ListView {
    width: 240; height: 320
    model: ListModel {}

    delegate: Rectangle {
        width: 100; height: 30
        border.width: 1
        color: "lightsteelblue"
        Text {
            anchors.centerIn: parent
            text: name
        }
    }

    add: Transition {
        NumberAnimation { property: "opacity"; from: 0; to: 1.0; duration: 400 }
        NumberAnimation { property: "scale"; from: 0; to: 1.0; duration: 400 }
    }

    displaced: Transition {
        NumberAnimation { properties: "x,y"; duration: 400; easing.type: Easing.OutBounce }
    }

    focus: true
    Keys.onSpacePressed: model.insert(0, { "name": "Item " + model.count })
}

If multiple items are added in rapid succession, without waiting for a previous transition to finish, this is the result:

Each newly added item undergoes an add transition, but before the transition can finish, another item is added, displacing the previously added item. Because of this, the add transition on the previously added item is interrupted and a displaced transition is started on the item instead. Due to the interruption, the opacity and scale animations have not completed, thus producing items with opacity and scale that are below 1.0.

To fix this, the displaced transition should additionally ensure the item properties are set to the end values specified in the add transition, effectively resetting these values whenever an item is displaced. In this case, it means setting the item opacity and scale to 1.0:

    displaced: Transition {
        NumberAnimation { properties: "x,y"; duration: 400; easing.type: Easing.OutBounce }

        // ensure opacity and scale values return to 1.0
        NumberAnimation { property: "opacity"; to: 1.0 }
        NumberAnimation { property: "scale"; to: 1.0 }
    }

Now, when an item's add transition is interrupted, its opacity and scale are animated to 1.0 upon displacement, avoiding the erroneous visual effects from before:

The same principle applies to any combination of view transitions. An added item may be moved before its add transition finishes, or a moved item may be removed before its moved transition finishes, and so on; so, the rule of thumb is that every transition should handle the same set of properties.

Restrictions Regarding ScriptAction

When a view transition is initialized, any property bindings that refer to the ViewTransition attached property are evaluated in preparation for the transition. Due to the nature of the internal construction of a view transition, the attributes of the ViewTransition attached property are only valid for the relevant item when the transition is initialized, and may not be valid when the transition is actually run.

Therefore, a ScriptAction within a view transition should not refer to the ViewTransition attached property, as it may not refer to the expected values at the time that the ScriptAction is actually invoked. Consider the following example:

ListView {
    width: 240; height: 320
    model: ListModel {
        Component.onCompleted: {
            for (var i=0; i<8; i++)
                append({"name": i})
        }
    }

    delegate: Rectangle {
        width: 100; height: 30
        border.width: 1
        color: "lightsteelblue"
        Text {
            anchors.centerIn: parent
            text: name
        }
        objectName: name
    }

    move: Transition {
        id: moveTrans
        SequentialAnimation {
            ColorAnimation { property: "color"; to: "yellow"; duration: 400 }
            NumberAnimation { properties: "x,y"; duration: 800; easing.type: Easing.OutBack }
            ScriptAction { script: moveTrans.ViewTransition.item.color = "lightsteelblue" }
        }
    }

    displaced: Transition {
        NumberAnimation { properties: "x,y"; duration: 400; easing.type: Easing.OutBounce }
    }

    focus: true
    Keys.onSpacePressed: model.move(5, 1, 3)
}

When the space key is pressed, three items are moved from index 5 to index 1. For each moved item, the moveTransition sequence presumably animates the item's color to "yellow", then animates it to its final position, then changes the item color back to "lightsteelblue" using a ScriptAction. However, when run, the transition does not produce the intended result:

Only the last moved item is returned to the "lightsteelblue" color; the others remain yellow. This is because the ScriptAction is not run until after the transition has already been initialized, by which time the ViewTransition.item value has changed to refer to a different item; the item that the script had intended to refer to is not the one held by ViewTransition.item at the time the ScriptAction is actually invoked.

In this instance, to avoid this issue, the view could set the property using a PropertyAction instead:

    move: Transition {
        id: moveTrans
        SequentialAnimation {
            ColorAnimation { property: "color"; to: "yellow"; duration: 400 }
            NumberAnimation { properties: "x,y"; duration: 800; easing.type: Easing.OutBack }
            //ScriptAction { script: moveTrans.ViewTransition.item.color = "lightsteelblue" } BAD!

            PropertyAction { property: "color"; value: "lightsteelblue" }
        }
    }

When the transition is initialized, the PropertyAction target will be set to the respective ViewTransition.item for the transition and will later run with the correct item target as expected.

Attached Property Documentation