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diff --git a/doc/src/declarative/behaviors-and-states.qdoc b/doc/src/declarative/behaviors-and-states.qdoc new file mode 100644 index 0000000..0815b39 --- /dev/null +++ b/doc/src/declarative/behaviors-and-states.qdoc @@ -0,0 +1,206 @@ +/**************************************************************************** +** +** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies). +** All rights reserved. +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:FDL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Free Documentation License +** 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. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! +\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 + +*/ |