source: trunk/doc/src/declarative/qdeclarativeintro.qdoc@ 980

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/*!
\page qdeclarativeintroduction.html
\title Introduction to the QML language

\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.

This introduction is meant for those with little or no programming
experience. JavaScript is used as a scripting language in QML, so you may want
to learn a bit more about it (see the \l{Javascript Guide}) before diving
deeper into QML. It's also helpful to have a basic understanding of other web
technologies like HTML and CSS, but it's not required.

\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

Here we create two objects, a \l Rectangle object and its child
\l Image object. Objects are specified by their type, followed by a pair of
braces in between which additional data can be defined for the object, such as
its property values and any child objects.

Properties are specified with a \c {property: value} syntax. In the above example, we
can see the \l Image object 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 QtQuick \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 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 \l Text object will have normal opacity, since the
line opacity: 0.5 has been turned into a comment.



\section1 Object identifiers

Each object can be given a special \e id value that allows the object to be identified
and referred to by other objects.

For example, below we have two \l Text objects. The first \l Text object
has an \c id value of "text1". The second \l Text object can now set its own
\c text property value to be the same as that of the first object, by referring to
\c text1.text:

\qml
import QtQuick 1.0

Row {
    Text {
        id: text1
        text: "Hello World"
    }

    Text { text: text1.text }
}
\endqml

An object can be referred to by its \c id from anywhere within the \l {QML Documents}{component}
in which it is declared. Therefore, an \c id value must always be unique within a single component.

The \c id value is a special value for a QML object and should not be thought of as an
ordinary object property; for example, it is not possible to access \c text1.id in the
above example. Once an object is created, its \c id cannot be changed.

Note that an \c id must begin with a lower-case letter or an underscore, and cannot contain
characters other than letters, numbers and underscores.



\section1 Expressions

JavaScript expressions can be used to assign property values. For example:

\code
Item {
    width: 100 * 3
    height: 50 + 22
}
\endcode

These expressions can include references to other objects and properties, in which case
a \l{Property Binding}{binding} is established: when the value of the expression changes,
the property to which the expression is assigned is automatically updated to the
new value. For example:

\code
Item {
    width: 300
    height: 300

    Rectangle {
        width: parent.width - 50
        height: 100
        color: "yellow"
    }
}
\endcode

Here, the \l Rectangle object's \c width property is set relative to the width
of its parent. Whenever the parent's width changes, the width of the \l Rectangle is
automatically updated.



\section1 Properties
\target intro-properties

\section2 Basic property types

QML supports properties of many types (see \l{QML Basic Types}). The basic types include \c int,
\c real, \c bool, \c string and \c color.

\code
Item {
    x: 10.5             // a 'real' property
    state: "details"    // a 'string' property
    focus: true         // a 'bool' property
    ...
}
\endcode

QML properties are what is known as \e type-safe. That is, they only allow you to assign a value that
matches the property type. For example, the \c x property of item is a real, and if you try to assign
a string to it you will get an error.

\badcode
Item {
    x: "hello"  // illegal!
}
\endcode

Note that with the exception of \l {Attached Properties}, properties always begin with a lowercase
letter.


\section2 Property change notifications

When a property changes value, it can send a signal to notify others of this change.

To receive these signals, simply create a \e {signal handler} named with an \c on<Property>Changed
syntax. For example, the \l Rectangle element has \l {Item::}{width} and \l {Rectangle::}{color}
properties.  Below, we have a \l Rectangle object that has defined two signal handlers,
\c onWidthChanged and \c onColorChanged, which will automaticallly be called whenever these
properties are modified:

\qml
Rectangle {
    width: 100; height: 100

    onWidthChanged: console.log("Width has changed to:", width)
    onColorChanged: console.log("Color has changed to:", color)
}
\endqml

Signal handlers are explained further \l {Signal Handlers}{below}.


\section2 List properties

List properties look like this:

\code
Item {
    children: [
        Image {},
        Text {}
    ]
}
\endcode

The list is enclosed in square brackets, with a comma separating the
list elements. In cases where you are only assigning a single item to a
list, you can omit the square brackets:

\code
Image {
    children: Rectangle {}
}
\endcode

Items in the list can be accessed by index. See the \l{list}{list type} documentation
for more details about list properties and their available operations.


\section2 Default properties

Each object type can specify one of its list or object properties as its default property.
If a property has been declared as the default property, the property tag can be omitted.

For example this code:
\code
State {
    changes: [
        PropertyChanges {},
        PropertyChanges {}
    ]
}
\endcode

can be simplified to:

\code
State {
    PropertyChanges {}
    PropertyChanges {}
}
\endcode

because \c changes is the default property of the \c State type.

\section2 Grouped Properties
\target dot properties

In some cases properties form a logical group and use a 'dot' or grouped notation
to show this.

Grouped properties can be written like this:
\qml
Text {
    font.pixelSize: 12
    font.bold: true
}
\endqml

or like this:
\qml
Text {
    font { pixelSize: 12; bold: true }
}
\endqml

In the element documentation grouped properties are shown using the 'dot' notation.

\section2 Attached Properties
\target attached-properties

Some objects attach properties to another object.  Attached Properties
are of the form \e {Type.property} where \e Type is the type of the
element that attaches \e property.

For example, the \l ListView element attaches the \e ListView.isCurrentItem property
to each delegate it creates:

\code
Component {
    id: myDelegate
    Text {
        text: "Hello"
        color: ListView.isCurrentItem ? "red" : "blue"
    }
}
ListView {
    delegate: myDelegate
}
\endcode

Another example of attached properties is the \l Keys element which
attaches properties for handling key presses to
any visual Item, for example:

\code
Item {
    focus: true
    Keys.onSelectPressed: console.log("Selected")
}
\endcode

\section1 Signal Handlers

Signal handlers allow JavaScript code to be executed in response to an event. For
example, the \l MouseArea element has an \l {MouseArea::}{onClicked} handler that can
be used to respond to a mouse click. Below, we use this handler to print a
message whenever the mouse is clicked:

\code
Item {
    width: 100; height: 100

    MouseArea {
        anchors.fill: parent
        onClicked: {
            console.log("mouse button clicked")
        }
    }
}
\endcode

All signal handlers begin with \e "on".

Some signal handlers include an optional parameter. For example
the MouseArea \l{MouseArea::}{onPressed} signal handler has a \c mouse parameter
that contains information about the mouse press. This parameter can be referred to in
the JavaScript code, as below:

\code
MouseArea {
    acceptedButtons: Qt.LeftButton | Qt.RightButton
    onPressed: {
        if (mouse.button == Qt.RightButton)
            console.log("Right mouse button pressed")
    }
}
\endcode


*/