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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** 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 either Technology Preview License Agreement or the
** Beta Release License Agreement.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain
** additional rights. These rights are described in the Nokia Qt LGPL
** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
** package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you are unsure which license is appropriate for your use, please
** contact the sales department at qt-sales@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page propertybinding.html
\title Property Binding
Property binding is a declarative way of specifying the value of a property. Binding allows
a property's value to be expressed as an JavaScript expression that defines the value relative
to other property values or data accessible in the application. The property value is
automatically kept up to date if the other properties or data values change.
Property bindings are created implicitly in QML whenever a property is assigned an JavaScript
expression. The following QML uses two property bindings to connect the size of the rectangle
to that of \c otherItem.
\code
Rectangle {
width: otherItem.width
height: otherItem.height
}
\endcode
QML extends a standards compliant JavaScript engine, so any valid JavaScript expression can be
used as a property binding. Bindings can access object properties, make function calls and even
use builtin JavaScript objects like \e {Date} and \e {Math}. Assigning a constant value to a
property can even be thought of as a binding - afterall, a constant is a valid JavaScript
expression! Here are some examples of more complex bindings:
\code
Rectangle {
Script {
function calculateMyHeight() {
return Math.max(otherItem.height, thirdItem.height);
}
}
anchors.centerIn: parent
width: Math.min(otherItem.width, 10)
height: calculateMyHeight()
color: { if (width > 10) "blue"; else "red" }
}
\endcode
Being JavaScript expressions, bindings are evaluated in a scope chain. The \l {QML Scope}
documentation covers the specifics of scoping in QML.
\list
\o When does a binding not get updated?
\o Scope
\o Assigning a constant/other binding clears existing binding
\o Loops
\o Using model data
\endlist
\section1 Binding Element
The implicit binding syntax shown previously is easy to use and works perfectly for most uses
of bindings. In some advanced cases, it is necessary to create bindings explicitly using the
\l Binding element.
For example, to bind a property exposed from C++ (\c system.brightness) to a value
coming from QML (\c slider.value), you could use the Binding element as follows:
\qml
Binding {
target: system
property: "brightness"
value: slider.value
}
\endqml
*/
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