Animation and Transitions in Qt Quick

Animation and Transitions Types

Transition - Animates transitions during state changes
SequentialAnimation - Runs animations sequentially
ParallelAnimation - Runs animations in parallel
Behavior - Specifies a default animation for property changes
PropertyAction - Sets immediate property changes during animation
PauseAnimation - Introduces a pause in an animation
SmoothedAnimation - Allows a property to smoothly track a value
SpringAnimation - Allows a property to track a value in a spring-like motion
ScriptAction - Runs scripts during an animation

Types that animate properties based on data types

AnchorAnimation	Animates changes in anchor values
ColorAnimation	Animates changes in color values
NumberAnimation	Animates changes in qreal-type values
ParentAnimation	Animates changes in parent values
PathAnimation	Animates an item along a path
PropertyAnimation	Animates changes in property values
RotationAnimation	Animates changes in rotation values
Vector3dAnimation	Animates changes in QVector3d values

Animations are created by applying animation types to property values. Animation types will interpolate property values to create smooth transitions. As well, state transitions may assign animations to state changes.

To create an animation, use an appropriate animation type for the type of the property that is to be animated, and apply the animation depending on the type of behavior that is required.

Triggering Animations

There are several ways of setting animation to an object.

Direct Property Animation

Animations are created by applying animation objects to property values to gradually change the properties over time. These property animations apply smooth movements by interpolating values between property value changes. Property animations provide timing controls and allows different interpolations through easing curves.

 Rectangle {
     id: flashingblob
     width: 75; height: 75
     color: "blue"
     opacity: 1.0

     MouseArea {
         anchors.fill: parent
         onClicked: {
             animateColor.start()
             animateOpacity.start()
         }
     }

     PropertyAnimation {id: animateColor; target: flashingblob; properties: "color"; to: "green"; duration: 100}

     NumberAnimation {
         id: animateOpacity
         target: flashingblob
         properties: "opacity"
         from: 0.99
         to: 1.0
         loops: Animation.Infinite
         easing {type: Easing.OutBack; overshoot: 500}
    }
 }

Specialized property animation types have more efficient implementations than the PropertyAnimation type. They are for setting animations to different QML types such as int, color, and rotations. Similarly, the ParentAnimation can animate parent changes.

See the Controlling Animations section for more information about the different animation properties.

Using Predefined Targets and Properties

In the previous example, the PropertyAnimation and NumberAnimation objects needed to specify particular target and properties values to specify the objects and properties that should be animated. This can be avoided by using the <Animation> on <Property> syntax, which specifies the animation is to be applied as a property value source.

Below are two PropertyAnimation objects that are specified using this syntax:

 import QtQuick 2.0

 Rectangle {
     id: rect
     width: 100; height: 100
     color: "red"

     PropertyAnimation on x { to: 100 }
     PropertyAnimation on y { to: 100 }
 }

The animation starts as soon as the rectangle is loaded, and will automatically be applied to its x and y values. Since the <Animation> on <Property> syntax has been used, it is not necessary to set the target value of the PropertyAnimation objects to rect, and neither is it necessary to set the property values to x and y.

This can also be used by grouped animations to ensure that all animations within a group are applied to the same property. For example, the previous example could instead use SequentialAnimation to animate the rectangle's color first to yellow, then to blue:

 import QtQuick 2.0

 Rectangle {
     width: 100; height: 100
     color: "red"

     SequentialAnimation on color {
         ColorAnimation { to: "yellow"; duration: 1000 }
         ColorAnimation { to: "blue"; duration: 1000 }
     }
 }

Since the SequentialAnimation object has been specified on the color property using the <Animation> on <Property> syntax, its child ColorAnimation objects are also automatically applied to this property and do not need to specify target or property animation values.

Transitions During State Changes

Qt Quick States are property configurations where a property may have different values to reflect different states. State changes introduce abrupt property changes; animations smooth transitions to produce visually appealing state changes.

The Transition type can contain animation types to interpolate property changes caused by state changes. To assign the transition to an object, bind it to the transitions property.

A button might have two states, the pressed state when the user clicks on the button and a released state when the user releases the button. We can assign different property configurations for each state. A transition would animate the change from the pressed state to the released state. Likewise, there would be an animation during the change from the released state to the pressed state.

 Rectangle {
     width: 75; height: 75
     id: button
     state: "RELEASED"

     MouseArea {
         anchors.fill: parent
         onPressed: button.state = "PRESSED"
         onReleased: button.state = "RELEASED"
     }

     states: [
         State {
             name: "PRESSED"
             PropertyChanges { target: button; color: "lightblue"}
         },
         State {
             name: "RELEASED"
             PropertyChanges { target: button; color: "lightsteelblue"}
         }
     ]

     transitions: [
         Transition {
             from: "PRESSED"
             to: "RELEASED"
             ColorAnimation { target: button; duration: 100}
         },
         Transition {
             from: "RELEASED"
             to: "PRESSED"
             ColorAnimation { target: button; duration: 100}
         }
     ]
 }

Binding the to and from properties to the state's name will assign that particular transition to the state change. For simple or symmetric transitions, setting the to to property to the wild card symbol, "*", denotes that the transition applies to any state change.

     transitions:
         Transition {
             to: "*"
             ColorAnimation { target: button; duration: 100}
         }

Default Animation as Behaviors

Default property animations are set using behavior animations. Animations declared in Behavior types apply to the property and animates any property value changes. However, Behavior types have an enabled property to purposely enable or disable the behavior animations.

A ball component might have a behavior animation assigned to its x, y, and color properties. The behavior animation could be set up to simulate an elastic effect. In effect, this behavior animation would apply the elastic effect to the properties whenever the ball moves.

 Rectangle {
     width: 75; height: 75; radius: width
     id: ball
     color: "salmon"

     component BounceAnimation : NumberAnimation {
        easing {
           type: Easing.OutElastic
           amplitude: 1.0
           period: 0.5
        }
     }

     Behavior on x {
         BounceAnimation {}
     }
     Behavior on y {
         BounceAnimation {}
     }
     Behavior on color {
         ColorAnimation { target: ball; duration: 100 }
     }
 }

There are several methods of assigning behavior animations to properties. The Behavior on <property> declaration is a convenient way of assigning a behavior animation onto a property.

See the Qt Quick Examples - Animation for a demonstration of behavioral animations.

Playing Animations in Parallel or in Sequence

Animations can run in parallel or in sequence. Parallel animations will play a group of animations at the same time while sequential animations play a group of animations in order: one after the other. Grouping animations in SequentialAnimation and ParallelAnimation will play the animations in sequence or in parallel.

A banner component may have several icons or slogans to display, one after the other. The opacity property could transform to 1.0 denoting an opaque object. Using the SequentialAnimation type, the opacity animations will play after the preceding animation finishes. The ParallelAnimation type will play the animations at the same time.

 Rectangle {
     id: banner
     width: 150; height: 100; border.color: "black"

     Column {
         anchors.centerIn: parent
         Text {
             id: code
             text: "Code less."
             opacity: 0.01
         }
         Text {
             id: create
             text: "Create more."
             opacity: 0.01
         }
         Text {
             id: deploy
             text: "Deploy everywhere."
             opacity: 0.01
         }
     }

     MouseArea {
         anchors.fill: parent
         onPressed: playbanner.start()
     }

     SequentialAnimation {
         id: playbanner
         running: false
         NumberAnimation { target: code; property: "opacity"; to: 1.0; duration: 200}
         NumberAnimation { target: create; property: "opacity"; to: 1.0; duration: 200}
         NumberAnimation { target: deploy; property: "opacity"; to: 1.0; duration: 200}
     }
 }

Once individual animations are placed into a SequentialAnimation or ParallelAnimation, they can no longer be started and stopped independently. The sequential or parallel animation must be started and stopped as a group.

The SequentialAnimation type is also useful for playing transition animations because animations are played in parallel inside transitions.

Controlling Animations

There are different methods to control animations.

Animation Playback

All animation types inherit from the Animation type. It is not possible to create Animation objects; instead, this type provides the essential properties and methods for animation types. Animation types have start(), stop(), resume(), pause(), restart(), and complete() -- all of these methods control the execution of animations.

Easing

Easing curves define how the animation will interpolate between the start value and the end value. Different easing curves might go beyond the defined range of interpolation. The easing curves simplify the creation of animation effects such as bounce effects, acceleration, deceleration, and cyclical animations.

A QML object may have different easing curve for each property animation. There are also different parameters to control the curve, some of which are exclusive to a particular curve. For more information about the easing curves, visit the easing documentation.

The easing example visually demonstrates each of the different easing types.

Other Animation Types

In addition, QML provides several other types useful for animation:

PauseAnimation: enables pauses during animations
ScriptAction: allows JavaScript to be executed during an animation, and can be used together with StateChangeScript to reused existing scripts
PropertyAction: changes a property immediately during an animation, without animating the property change

These are specialized animation types that animate different property types

SmoothedAnimation: a specialized NumberAnimation that provides smooth changes in animation when the target value changes
SpringAnimation: provides a spring-like animation with specialized attributes such as mass, damping and epsilon
ParentAnimation: used for animating a parent change (see ParentChange)
AnchorAnimation: used for animating an anchor change (see AnchorChanges)

Sharing animation instances between Transitions or Behaviors is not supported, and may lead to undefined behavior. In the following example, changes to the Rectangle's position will most likely not be correctly animated.

 Rectangle {
     // NOT SUPPORTED: this will not work correctly as both Behaviors
     // try to control a single animation instance
     NumberAnimation { id: anim; duration: 300; easing.type: Easing.InBack }
     Behavior on x { animation: anim }
     Behavior on y { animation: anim }
 }

The easiest fix is to repeat the NumberAnimation for both Behaviors. If the repeated animation is rather complex, you might also consider creating a custom animation component and assigning an instance to each Behavior, for example:

 Rectangle {
     component MyNumberAnimation : NumberAnimation {
         duration: 300; easing.type: Easing.InBack
     }
     Behavior on x { MyNumberAnimation {} }
     Behavior on y { MyNumberAnimation {} }
 }

See also Qt Quick Examples - Animation.

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