diff options
Diffstat (limited to 'doc/src/declarative/propertybinding.qdoc')
-rw-r--r-- | doc/src/declarative/propertybinding.qdoc | 110 |
1 files changed, 110 insertions, 0 deletions
diff --git a/doc/src/declarative/propertybinding.qdoc b/doc/src/declarative/propertybinding.qdoc new file mode 100644 index 0000000..ad4f13e --- /dev/null +++ b/doc/src/declarative/propertybinding.qdoc @@ -0,0 +1,110 @@ +/**************************************************************************** +** +** 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 +*/ + |