1 /****************************************************************************
3 ** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies).
4 ** All rights reserved.
5 ** Contact: Nokia Corporation (qt-info@nokia.com)
7 ** This file is part of the documentation of the Qt Toolkit.
9 ** $QT_BEGIN_LICENSE:FDL$
10 ** No Commercial Usage
11 ** This file contains pre-release code and may not be distributed.
12 ** You may use this file in accordance with the terms and conditions
13 ** contained in the Technology Preview License Agreement accompanying
16 ** GNU Free Documentation License
17 ** Alternatively, this file may be used under the terms of the GNU Free
18 ** Documentation License version 1.3 as published by the Free Software
19 ** Foundation and appearing in the file included in the packaging of this
22 ** If you have questions regarding the use of this file, please contact
23 ** Nokia at qt-info@nokia.com.
26 ****************************************************************************/
29 \page qml-behaviors-and-states.html
30 \title Using QML Behaviors with States
32 \section1 Using Behaviors with States
34 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.
36 Here's an example that shows the problem:
49 anchors.centerIn: parent
64 when: mouser.containsMouse
75 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
76 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.
78 To state the problem more formally, using States and Behaviors together can cause unexpected behavior when:
80 \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
81 \o this Behavior can be interrupted to (re-)enter an explicitly defined state.
84 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
85 to red, and interrupt the animation to return to "GreenState", the base state will include the color in its intermediate, mid-animation form.
87 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.
89 1. Use a transition to animate the change, rather than a Behavior.
102 anchors.centerIn: parent
114 when: mouser.containsMouse
122 transitions: Transition {
129 2. Use a conditional binding to change the property value, rather than a state
142 anchors.centerIn: parent
144 color: mouser.containsMouse ? "green" : "red"
158 3. Use only explicitly defined states, rather than an implicit base state
171 anchors.centerIn: parent
186 when: mouser.containsMouse
195 when: !mouser.containsMouse