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29 \page qml-behaviors-and-states.html
30 \inqmlmodule QtQuick 2
31 \title Using QML Behaviors with States
33 \section1 Using Behaviors with States
35 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.
37 Here's an example that shows the problem:
50 anchors.centerIn: parent
65 when: mouser.containsMouse
76 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
77 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.
79 To state the problem more formally, using States and Behaviors together can cause unexpected behavior when:
81 \o a Behavior is used to animate a property change, specifically when moving from an explicitly defined state back to the implicit base state; and
82 \o this Behavior can be interrupted to (re-)enter an explicitly defined state.
85 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
86 to red, and interrupt the animation to return to "GreenState", the base state will include the color in its intermediate, mid-animation form.
88 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.
90 1. Use a transition to animate the change, rather than a Behavior.
103 anchors.centerIn: parent
115 when: mouser.containsMouse
123 transitions: Transition {
130 2. Use a conditional binding to change the property value, rather than a state
143 anchors.centerIn: parent
145 color: mouser.containsMouse ? "green" : "red"
159 3. Use only explicitly defined states, rather than an implicit base state
172 anchors.centerIn: parent
187 when: mouser.containsMouse
196 when: !mouser.containsMouse