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/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** 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.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qmlsyntax.html
\title QML Syntax
\ingroup QML Reference
\contentspage QML Reference
\tableofcontents
QML is a declarative language designed to describe the user interface of a
program: both what it looks like, and how it behaves. In QML, a user
interface is specified as a tree of objects with properties.
JavaScript is used as a scripting language in QML, so you may want
to learn a bit more about it (\l{Javascript Guide}) before diving
deeper into QML.
\section1 Basic QML Syntax
QML looks like this:
\code
import QtQuick 1.0
Rectangle {
width: 200
height: 200
color: "blue"
Image {
source: "pics/logo.png"
anchors.centerIn: parent
}
}
\endcode
Objects are specified by their type, followed by a pair of braces. Object
types always begin with a capital letter. In the above example, there are
two objects, a \l Rectangle, and an \l Image. Between the braces, we can specify
information about the object, such as its properties.
Properties are specified as \c {propertyname: value}. In the above example, we
can see the Image has a property named \c source, which has been assigned the
value \c "pics/logo.png". The property and its value are separated by a colon.
Properties can be specified one-per-line:
\code
Rectangle {
width: 100
height: 100
}
\endcode
or you can put multiple properties on a single line:
\code
Rectangle { width: 100; height: 100 }
\endcode
When multiple property/value pairs are specified on a single line, they
must be separated by a semicolon.
The \c import statement imports the \c Qt \l{QML Modules}{module}, which contains all of the
standard \l {QML Elements}. Without this import statement, the \l Rectangle
and \l Image elements would not be available.
\section1 Expressions
In addition to assigning values to properties, you can also assign
expressions written in JavaScript.
\code
Rotation {
angle: 360 * 3
}
\endcode
These expressions can include references to other objects and properties, in which case
a \e binding is established: when the value of the expression changes, the property the
expression has been assigned to is automatically updated to that value.
\code
Item {
Text {
id: text1
text: "Hello World"
}
Text {
id: text2
text: text1.text
}
}
\endcode
In the example above, the \c text2 object will display the same text as \c text1. If \c text1 is changed,
\c text2 is automatically changed to the same value.
Note that to refer to other objects, we use their \e id values. (See below for more
information on the \e id property.)
\section1 QML Comments
Commenting in QML is similar to JavaScript.
\list
\o Single line comments start with // and finish at the end of the line.
\o Multiline comments start with /* and finish with *\/
\endlist
\snippet doc/src/snippets/declarative/comments.qml 0
Comments are ignored by the engine. They are useful for explaining what you
are doing; for referring back to at a later date, or for others reading
your QML files.
Comments can also be used to prevent the execution of code, which is
sometimes useful for tracking down problems.
\code
Text {
text: "Hello world!"
//opacity: 0.5
}
\endcode
In the above example, the Text object will have normal opacity, since the
line opacity: 0.5 has been turned into a comment.
*/
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