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/****************************************************************************
**
** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qml-behaviors-and-states.html
\title Using QML Behaviors with States
\section1 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:
\qml
import QtQuick 1.0
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"
}
}
}
}
\endqml
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:
\list
\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
\o this Behavior can be interrupted to (re-)enter an explicitly defined state.
\endlist
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.
\qml
import QtQuick 1.0
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 {}
}
}
}
\endqml
2. Use a conditional binding to change the property value, rather than a state
\qml
import QtQuick 1.0
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
}
}
}
\endqml
3. Use only explicitly defined states, rather than an implicit base state
\qml
import QtQuick 1.0
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"
}
}]
}
}
\endqml
*/
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