Using QML Behaviors with States

Using Behaviors with States

In some cases you may choose to use a Behavior to animate a property change caused by a state change. While this works well for some situations, in other situations it may lead to unexpected behavior.

Here's an example that shows the problem:

 import QtQuick 2.0

 Rectangle {
     width: 400
     height: 400

     Rectangle {
         id: coloredRect
         width: 100
         height: 100
         anchors.centerIn: parent

         color: "red"
         Behavior on color {
             ColorAnimation {}
         }

         MouseArea {
             id: mouser
             anchors.fill: parent
             hoverEnabled: true
         }

         states: State {
             name: "GreenState"
             when: mouser.containsMouse

             PropertyChanges {
                 target: coloredRect
                 color: "green"
             }
         }
     }
 }

Testing the example by quickly and repeatedly moving the mouse in to and out of the colored rectangle shows that the colored rectangle will settle into a green color over time, never returning to full red. This is not what we wanted! The problem occurs because we have used a Behavior to animate the change in color, and our state change is trigged by the mouse entering or exiting the MouseArea, which is easily interrupted.

To state the problem more formally, using States and Behaviors together can cause unexpected behavior when:

• a Behavior is used to animate a property change, specifically when moving from an explicitly defined state back to the implicit base state; and
• this Behavior can be interrupted to (re-)enter an explicitly defined state.

The problem occurs because of the way the base state is defined for QML: as the "snapshot" state of the application just prior to entering an explicitly defined state. In this case, if we are in the process of animating from green back to red, and interrupt the animation to return to "GreenState", the base state will include the color in its intermediate, mid-animation form.

While future versions of QML should be able to handle this situation more gracefully, there are currently several ways to rework your application to avoid this problem.

1. Use a transition to animate the change, rather than a Behavior.

 import QtQuick 2.0

 Rectangle {
     width: 400
     height: 400

     Rectangle {
         id: coloredRect
         width: 100
         height: 100
         anchors.centerIn: parent

         color: "red"

         MouseArea {
             id: mouser
             anchors.fill: parent
             hoverEnabled: true
         }

         states: State {
             name: "GreenState"
             when: mouser.containsMouse

             PropertyChanges {
                 target: coloredRect
                 color: "green"
             }
         }

         transitions: Transition {
             ColorAnimation {}
         }
     }
 }

2. Use a conditional binding to change the property value, rather than a state

 import QtQuick 2.0

 Rectangle {
     width: 400
     height: 400

     Rectangle {
         id: coloredRect
         width: 100
         height: 100
         anchors.centerIn: parent

         color: mouser.containsMouse ? "green" : "red"
         Behavior on color {
             ColorAnimation {}
         }

         MouseArea {
             id: mouser
             anchors.fill: parent
             hoverEnabled: true
         }
     }
 }

3. Use only explicitly defined states, rather than an implicit base state

 import QtQuick 2.0

 Rectangle {
     width: 400
     height: 400

     Rectangle {
         id: coloredRect
         width: 100
         height: 100
         anchors.centerIn: parent

         Behavior on color {
             ColorAnimation {}
         }

         MouseArea {
             id: mouser
             anchors.fill: parent
             hoverEnabled: true
         }

         states: [
         State {
             name: "GreenState"
             when: mouser.containsMouse

             PropertyChanges {
                 target: coloredRect
                 color: "green"
             }
         },
         State {
             name: "RedState"
             when: !mouser.containsMouse

             PropertyChanges {
                 target: coloredRect
                 color: "red"
             }
         }]
     }
 }