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/*!
\page qtqml-syntax-objectattributes.html
\title QML Object Attributes
\brief Description of QML object type attributes

Every QML object type has a defined set of attributes. Each instance of an
object type is created with the set of attributes that have been defined for
that object type.  There are several different kinds of attributes which
can be specified, which are described below.

\section1 Attributes in Object Declarations

An \l{qtqml-syntax-basics.html#object-declarations}{object declaration} in a
QML document defines a new type.  It also declares an object hierarchy that
will be instantiated should an instance of that newly defined type be created.

The set of QML object-type attribute types is as follows:

\list
\li the \e id attribute
\li property attributes
\li signal attributes
\li signal handler attributes
\li method attributes
\li attached properties and attached signal handler attributes
\endlist

These attributes are discussed in detail below.

\section2 The \e id Attribute

Every QML object type has exactly one \e id attribute. This attribute is
provided by the language itself, and cannot be redefined or overridden by any
QML object type.

A value may be assigned to the \e id attribute of an object instance to allow
that object to be identified and referred to by other objects.  This \c id must
begin with a lower-case letter or an underscore, and cannot contain characters
other than letters, numbers and underscores.

Below is a \l TextInput object and a \l Text object. The \l TextInput object's
\c id value is set to "myTextInput". The \l Text object sets its \c text
property to have the same value as the \c text property of the \l TextInput,
by referring to \c myTextInput.text. Now, both items will display the same
text:

\qml
import QtQuick 2.0

Column {
    width: 200; height: 200

    TextInput { id: myTextInput; text: "Hello World" }

    Text { text: myTextInput.text }
}
\endqml

An object can be referred to by its \c id from anywhere within the
\e {component scope} in which it is declared. Therefore, an \c id value must
always be unique within its component scope. See
\l{qtqml-documents-scope.html}{Scope and Naming Resolution} for more
information.

Once an object instance is created, the value of its \e id attribute cannot
be changed.  While it may look like an ordinary property, the \c id attribute
is \b{not} an ordinary \c property attribute, and special semantics apply
to it; for example, it is not possible to access \c myTextInput.id in the above
example.


\section2 Property Attributes

A property is an attribute of an object that can be assigned a static value
or bound to a dynamic expression. A property's value can be read by other
objects. Generally it can also be modified by another object, unless a
particular QML type has explicitly disallowed this for a specific property.

\section3 Defining Property Attributes

A property may be defined for a type in C++ by registering a
Q_PROPERTY of a class which is then registered with the QML type system.
Alternatively, a custom property of an object type may be defined in
an object declaration in a QML document with the following syntax:

\code
    [default] property <propertyType> <propertyName>
\endcode

In this way an object declaration may \l {Defining Object Types from QML}
{expose a particular value} to outside objects or maintain some internal
state more easily.

Property names must begin with a lower case letter and can only contain
letters, numbers and underscores. \l {JavaScript Reserved Words}
{JavaScript reserved words} are not valid property names.  The \c default
keyword is optional, and modifies the semantics of the property being declared.
See the upcoming section on \l {Default Properties}{default properties} for
more information about the \c default property modifier.

Declaring a custom property implicitly creates a value-change
\l{Signal attributes}{signal} for that property, as well as an associated
\l{Signal handler attributes}{signal handler} called
\e on<PropertyName>Changed, where \e <PropertyName> is the name of the
property, with the first letter capitalized.

For example, the following object declaration defines a new type which
derives from the Rectangle base type.  It has two new properties,
with a \l{Signal handler attributes}{signal handler} implemented for one of
those new properties:

\qml
Rectangle {
    property color previousColor
    property color nextColor
    onNextColorChanged: console.log("The next color will be: " + nextColor.toString())
}
\endqml

\section4 Valid Types in Custom Property Definitions

Any of the \l {QML Basic Types} aside from the \l enumeration type can be used
as custom property types. For example, these are all valid property declarations:

\qml
Item {
    property int someNumber
    property string someString
    property url someUrl
}
\endqml

(Enumeration values are simply whole number values and can be referred to with
the \l int type instead.)

Some basic types are provided by the \c QtQuick module and thus cannot be used
as property types unless the module is imported. See the \l {QML Basic Types}
documentation for more details.

Note the \l var basic type is a generic placeholder type that can hold any
type of value, including lists and objects:

\code
property var someNumber: 1.5
property var someString: "abc"
property var someBool: true
property var someList: [1, 2, "three", "four"]
property var someObject: Rectangle { width: 100; height: 100; color: "red" }
\endcode

Additionally, any \l{QML Object Types}{QML object type} can be used as a
property type. For example:

\code
property Item someItem
property Rectangle someRectangle
\endcode

This applies to \l {Defining Object Types from QML}{custom QML types} as well.
If a QML type was defined in a file named \c ColorfulButton.qml (in a directory
which was then imported by the client), then a property of type
\c ColorfulButton would also be valid.


\section3 Assigning Values to Property Attributes

The value of a property of an object instance may specified in two separate ways:
\list
  \li a value assignment on initialization
  \li an imperative value assignment
\endlist

In either case, the value may be either a \e static value or a \e {binding expression}
value.

\section4 Value Assignment on Initialization

The syntax for assigning a value to a property on initialization is:

\code
    <propertyName> : <value>
\endcode

An initialization value assignment may be combined with a property definition
in an object declaration, if desired.  In that case, the syntax of the property
definition becomes:

\code
    [default] property <propertyType> <propertyName> : <value>
\endcode

An example of property value initialization follows:

\qml
import QtQuick 2.0

Rectangle {
    color: "red"
    property color nextColor: "blue" // combined property declaration and initialization
}
\endqml

\section4 Imperative Value Assignment

An imperative value assignment is where a property value (either static value
or binding expression) is assigned to a property from imperative JavaScript
code.  The syntax of an imperative value assignment is just the JavaScript
assignment operator, as shown below:

\code
    [<objectId>.]<propertyName> = value
\endcode

An example of imperative value assignment follows:

\qml
import QtQuick 2.0

Rectangle {
    id: rect
    Component.onCompleted: {
        rect.color = "red"
    }
}
\endqml

\section3 Static Values and Binding Expression Values

As previously noted, there are two kinds of values which may be assigned to a
property: \e static values, and \e {binding expression} values.  The latter are
also known as \l{Property Binding}{property bindings}.

\table
    \header
    \li Kind
    \li Semantics

    \row
    \li Static Value
    \li A constant value which does not depend on other properties.

    \row
    \li Binding Expression
    \li A JavaScript expression which describes a property's relationship with
        other properties.  The variables in this expression are called the
        property's \e dependencies.

        The QML engine enforces the relationship between a property and its
        dependencies.  When any of the dependencies change in value, the QML
        engine automatically re-evaluates the binding expression and assigns
        the new result to the property.
\endtable

Here is an example that shows both kinds of values being assigned to properties:

\qml
import QtQuick 2.0

Rectangle {
    // both of these are static value assignments on initialization
    width: 400
    height: 200

    Rectangle {
        // both of these are binding expression value assignments on initialization
        width: parent.width / 2
        height: parent.height
    }
}
\endqml

\note To assign a binding expression imperatively, the binding expression
must be contained in a function that is passed into \l{Qt::binding()}{Qt.binding()},
and then the value returned by Qt.binding() must be assigned to the property.
In contrast, Qt.binding() must not be used when assigning a binding expression
upon initialization. See \l{Property Binding} for more information.


\section3 Type Safety

Properties are type safe. A property can only be assigned a value that matches
the property type.

For example, if a property is a real, and if you try to assign a string to it,
you will get an error:

\code
property int volume: "four"  // generates an error; the property's object will not be loaded
\endcode

Likewise if a property is assigned a value of the wrong type during run time,
the new value will not be assigned, and an error will be generated.

Some property types do not have a natural
value representation, and for those property types the QML engine
automatically performs string-to-typed-value conversion.  So, for example,
even though properties of the \c color type store colors and not strings,
you are able to assign the string \c "red" to a color property, without an
error being reported.

See \l {QML Basic Types} for a list of the types of properties that are
supported by default.  Additionally, any available \l {QML Object Types}
{QML object type} may also be used as a property type.

\section3 Special Property Types

\section4 Object List Property Attributes

A \l list type property can be assigned a list of QML object-type values.
The syntax for defining an object list value is a comma-separated list
surrounded by square brackets:

\code
    [ <item 1>, <item 2>, ... ]
\endcode

For example, the \l Item type has a \l {Item::states}{states} property that is
used to hold a list of \l State type objects. The code below initializes the
value of this property to a list of three \l State objects:

\qml
import QtQuick 2.0

Item {
    states: [
        State { name: "loading" },
        State { name: "running" },
        State { name: "stopped" }
    ]
}
\endqml

If the list contains a single item, the square brackets may be omitted:

\qml
import QtQuick 2.0

Item {
    states: State { name: "running" }
}
\endqml

A \l list type property may be specified in an object declaration with the
following syntax:

\code
    [default] property list<<objectType>> propertyName
\endcode

and, like other property declarations, a property initialization may be
combined with the property declaration with the following syntax:

\code
    [default] property list<<objectType>> propertyName: <value>
\endcode

An example of list property declaration follows:

\qml
import QtQuick 2.0

Rectangle {
    // declaration without initialization
    property list<Rectangle> siblingRects

    // declaration with initialization
    property list<Rectangle> childRects: [
        Rectangle { color: "red" },
        Rectangle { color: "blue"}
    ]
}
\endqml

If you wish to declare a property to store a list of values which are not
necessarily QML object-type values, you should declare a \l var property
instead.


\section4 Grouped Properties

In some cases properties contain a logical group of sub-property attributes.
These sub-property attributes can be assigned to using either the dot notation
or group notation.

For example, the \l Text type has a \l{Text::font.family}{font} group property. Below,
the first \l Text object initializes its \c font values using dot notation,
while the second uses group notation:

\code
Text {
    //dot notation
    font.pixelSize: 12
    font.b: true
}

Text {
    //group notation
    font { pixelSize: 12; b: true }
}
\endcode

Grouped property types are basic types which have subproperties.  Some of these
basic types are provided by the QML language, while others may only be used if
the Qt Quick module is imported.  See the documentation about
\l{QML Basic Types} for more information.


\section3 Property Aliases

Property aliases are properties which hold a reference to another property.
Unlike an ordinary property definition, which allocates a new, unique storage
space for the property, a property alias connects the newly declared property
(called the aliasing property) as a direct reference to an existing property
(the aliased property).

A property alias declaration looks like an ordinary property definition, except
that it requires the \c alias keyword instead of a property type, and the
right-hand-side of the property declaration must be a valid alias reference:

\code
[default] property alias <name>: <alias reference>
\endcode

Unlike an ordinary property, an alias can only refer to an object, or the
property of an object, that is within the scope of the \l{QML Object Types}
{type} within which the alias is declared. It cannot contain arbitrary
JavaScript expressions and it cannot refer to objects declared outside of
the scope of its type. Also note the \e {alias reference} is not optional,
unlike the optional default value for an ordinary property; the alias reference
must be provided when the alias is first declared.

For example, below is a \c Button type with a \c buttonText aliased property
which is connected to the \c text object of the \l Text child:

\qml
// Button.qml
import QtQuick 2.0

Rectangle {
    property alias buttonText: textItem.text

    width: 100; height: 30; color: "yellow"

    Text { id: textItem }
}
\endqml

The following code would create a \c Button with a defined text string for the
child \l Text object:

\qml
Button { buttonText: "Click Me" }
\endqml

Here, modifying \c buttonText directly modifies the textItem.text value; it
does not change some other value that then updates textItem.text. If
\c buttonText was not an alias, changing its value would not actually change
the displayed text at all, as property bindings are not bi-directional: the
\c buttonText value would have changed if textItem.text was changed, but not
the other way around.


\section4 Considerations for Property Aliases

Aliases are only activated once a component has been fully initialized. An
error is generated when an uninitialized alias is referenced. Likewise,
aliasing an aliasing property will also result in an error.

\snippet qml/properties.qml alias complete

When importing a \l{QML Object Types}{QML object type} with a property alias in
the root object, however, the property appear as a regular Qt property and
consequently can be used in alias references.

It is possible for an aliasing property to have the same name as an existing
property, effectively overwriting the existing property. For example,
the following QML type has a \c color alias property, named the same as the
built-in \l {Rectangle::color} property:

\snippet qml/properties.qml alias overwrite

Any object that use this type and refer to its \c color property will be
referring to the alias rather than the ordinary \l {Rectangle::color} property.
Internally, however, the rectangle can correctly set its \c color
property and refer to the actual defined property rather than the alias.


\section3 Default Properties

An object definition can have a single \e default property. A default property
is the property to which a value is assigned if an object is declared within
another object's definition without declaring it as a value for a particular
property.

Declaring a property with the optional \c default keyword marks it as the
default property. For example, say there is a file MyLabel.qml with a default
property \c someText:

\qml
// MyLabel.qml
import QtQuick 2.0

Text {
    default property var someText

    text: "Hello, " + someText.text
}
\endqml

The \c someText value could be assigned to in a \c MyLabel object definition,
like this:

\qml
MyLabel {
    Text { text: "world!" }
}
\endqml

This has exactly the same effect as the following:

\qml
MyLabel {
    someText: Text { text: "world!" }
}
\endqml

However, since the \c someText property has been marked as the default
property, it is not necessary to explicitly assign the \l Text object
to this property.

You will notice that child objects can be added to any \l {Item}-based type
without explicitly adding them to the \l {Item::children}{children} property.
This is because the default property of \l Item is its \c data property, and
any items added to this list for an \l Item are automatically added to its
list of \l {Item::children}{children}.

Default properties can be useful for reassigning the children of an item. See
the \l{TabWidget Example}, which uses a default property to
automatically reassign children of the TabWidget as children of an inner
ListView.  See also \l {Extending QML}.

\section3 Read-Only Properties

An object declaration may define a read-only property using the \c readonly
keyword, with the following syntax:

\code
    readonly property <propertyType> <propertyName> : <initialValue>
\endcode

Read-only properties must be assigned a value on initialization. After a
read-only property is initialized, it no longer possible to give it a value,
whether from imperative code or otherwise.

For example, the code in the \c Component.onCompleted block below is invalid:

\qml
Item {
    readonly property int someNumber: 10

    Component.onCompleted: someNumber = 20  // doesn't work, causes an error
}
\endqml

\note A read-only property cannot also be a \l{Default Properties}{default}
property.


\section3 Property Modifier Objects

Properties can have
\l{qtqml-cppintegration-definetypes.html#property-modifier-types}
{property value modifier objects} associated with them.
The syntax for declaring an instance of a property modifier type associated
with a particular property is as follows:

\code
<PropertyModifierTypeName> on <propertyName> {
    // attributes of the object instance
}
\endcode

It is important to note that the above syntax is in fact an
\l{qtqml-syntax-basics.html#object-declarations}{object declaration} which
will instantiate an object which acts on a pre-existing property.

Certain property modifier types may only be applicable to specific property
types, however this is not enforced by the language.  For example, the
\c NumberAnimation type provided by \c QtQuick will only animate
numeric-type (such as \c int or \c real) properties.  Attempting to use a
\c NumberAnimation with non-numeric property will not result in an error,
however the non-numeric property will not be animated.  The behavior of a
property modifier type when associated with a particular property type is
defined by its implementation.


\section2 Signal Attributes

A signal is a notification from an object that some event has occurred: for
example, a property has changed, an animation has started or stopped, or
when an image has been downloaded. The \l MouseArea type, for example, has
a \l {MouseArea::}{clicked} signal that is emitted when the user clicks
within the mouse area.

An object can be notified through a \l{Signal handler attributes}
{signal handler} whenever it a particular signal is emitted. A signal handler
is declared with the syntax \e on<Signal> where \e <Signal> is the name of the
signal, with the first letter capitalized. The signal handler must be declared
within the definition of the object that emits the signal, and the handler
should contain the block of JavaScript code to be executed when the signal
handler is invoked.

For example, the \e onClicked signal handler below is declared within the
\l MouseArea object definition, and is invoked when the \l MouseArea is
clicked, causing a console message to be printed:

\qml
import QtQuick 2.0

Item {
    width: 100; height: 100

    MouseArea {
        anchors.fill: parent
        onClicked: {
            console.log("Click!")
        }
    }
}
\endqml

\section3 Defining Signal Attributes

A signal may be defined for a type in C++ by registering a Q_SIGNAL of a class
which is then registered with the QML type system.  Alternatively, a custom
signal for an object type may be defined in an object declaration in a QML
document with the following syntax:

\code
    signal <signalName>[([<type> <parameter name>[, ...]])]
\endcode

Attempting to declare two signals or methods with the same name in the same
type block is an error. However, a new signal may reuse the name of an existing
signal on the type. (This should be done with caution, as the existing signal
may be hidden and become inaccessible.)

Here are three examples of signal declarations:

\qml
import QtQuick 2.0

Item {
    signal clicked
    signal hovered()
    signal actionPerformed(string action, var actionResult)
}
\endqml

If the signal has no parameters, the "()" brackets are optional. If parameters
are used, the parameter types must be declared, as for the \c string and \c var
arguments for the \c actionPerformed signal above. The allowed parameter types
are the same as those listed under \l {Defining Property Attributes} on this page.

To emit a signal, invoke it as a method. Any relevant
\l{Signal handler attributes}{signal handlers} will be invoked when the signal
is emitted, and handlers can use the defined signal argument names to access
the respective arguments.

\section3 Property Change Signals

QML types also provide built-in \e {property change signals} that are emitted
whenever a property value changes, as previously described in the section on
\l{Property attributes}{property attributes}.  See the upcoming section on
\l{Property change signal handlers}{property change signal handlers} for more
information about why these signals are useful, and how to use them.


\section2 Signal Handler Attributes

Signal handlers are a special sort of \l{Method attributes}{method attribute},
where the method implementation is invoked by the QML engine whenever the
associated signal is emitted.  Adding a signal to an object definition in QML
will automatically add an associated signal handler to the object definition,
which has, by default, an empty implementation.  Clients can provide an
implementation, to implement program logic.

Consider the following \c SquareButton type, whose definition is provided in
the \c SquareButton.qml file as shown below, with signals \c activated and
\c deactivated:

\qml
// SquareButton.qml
Rectangle {
    id: root

    signal activated(real xPosition, real yPosition)
    signal deactivated

    width: 100; height: 100

    MouseArea {
        anchors.fill: parent
        onPressed: root.activated(mouse.x, mouse.y)
        onReleased: root.deactivated()
    }
}
\endqml

These signals could be received by any \c SquareButton objects in another QML
file in the same directory, where implementations for the signal handlers are
provided by the client:

\qml
// myapplication.qml
SquareButton {
    onActivated: console.log("Activated at " + xPosition + "," + yPosition)
    onDeactivated: console.log("Deactivated!")
}
\endqml

See the \l {Signal and Handler Event System} for more details on use of
signals.

\section3 Property Change Signal Handlers

Signal handlers for property change signal take the syntax form
\e on<Property>Changed where \e <Property> is the name of the property,
with the first letter capitalized. For example, although the \l TextInput type
documentation does not document a \c textChanged signal, this signal is
implicitly available through the fact that \l TextInput has a
\l {TextInput::text}{text} property and so it is possible to write an
\c onTextChanged signal handler to be called whenever this property changes:

\qml
import QtQuick 2.0

TextInput {
    text: "Change this!"

    onTextChanged: console.log("Text has changed to:", text)
}
\endqml


\section2 Method Attributes

A method of an object type is a function which may be called to perform some
processing or trigger further events.  A method can be connected to a signal so
that it is automatically invoked whenever the signal is emitted.  See
\l {Signal and Handler Event System} for more details.

\section3 Defining Method Attributes

A method may be defined for a type in C++ by tagging a function of a class
which is then registered with the QML type system with Q_INVOKABLE or by
registering it as a Q_SLOT of the class.  Alternatively, a custom method can
be added to an object declaration in a QML document with the following syntax:

\code
    function <functionName>([<parameterName>[, ...]]) { <body> }
\endcode

Methods can be added to a QML type in order to define standalone, reusable
blocks of JavaScript code.  These methods can be invoked either internally or
by external objects.

Unlike signals, method parameter types do not have to be declared as they
default to the \c var type.

Attempting to declare two methods or signals with the same name in the same
type block is an error. However, a new method may reuse the name of an existing
method on the type. (This should be done with caution, as the existing method
may be hidden and become inaccessible.)

Below is a \l Rectangle with a \c calculateHeight() method that is called when
assigning the \c height value:

\qml
import QtQuick 2.0
Rectangle {
    id: rect

    function calculateHeight() {
        return rect.width / 2;
    }

    width: 100
    height: calculateHeight()
}
\endqml

If the method has parameters, they are accessible by name within the method.
Below, when the \l MouseArea is clicked it invokes the \c moveTo() method which
can then refer to the received \c newX and \c newY parameters to reposition the
text:

\qml
import QtQuick 2.0

Item {
    width: 200; height: 200

    MouseArea {
        anchors.fill: parent
        onClicked: label.moveTo(mouse.x, mouse.y)
    }

    Text {
        id: label

        function moveTo(newX, newY) {
            label.x = newX;
            label.y = newY;
        }

        text: "Move me!"
    }
}
\endqml


\section2 Attached Properties and Attached Signal Handlers

\e {Attached properties} and \e {attached signal handlers} are mechanisms that
enable objects to be annotated with extra properties or signal handlers that
are otherwise unavailable to the object. In particular, they allow objects to
access properties or signals that are specifically relevant to the individual
object.

A QML type implementation may choose to \l {Providing Attached Objects for
Data Annotations}{create an \e {attaching type} in C++} with
particular properties and signals. Instances of this type can then be created
and \e attached to specific objects at run time, allowing those objects to
access the properties and signals of the attaching type. These are accessed by
prefixing the properties and respective signal handlers with the name of the
attaching type.

References to attached properties and handlers take the following syntax form:

\code
<AttachingType>.<propertyName>
<AttachingType>.on<SignalName>
\endcode

For example, the \l ListView type has an attached property
\l {ListView::isCurrentItem}{ListView.isCurrentItem} that is available to each delegate object in a
ListView. This can be used by each individual delegate object to determine
whether it is the currently selected item in the view:

\qml
import QtQuick 2.0

ListView {
    width: 240; height: 320
    model: 3
    delegate: Rectangle {
        width: 100; height: 30
        color: ListView.isCurrentItem ? "red" : "yellow"
    }
}
\endqml

In this case, the name of the \e {attaching type} is \c ListView and the
property in question is \c isCurrentItem, hence the attached property is
referred to as \c ListView.isCurrentItem.

An attached signal handler is referred to in the same way. For example, the
\l{Component::completed}{Component.onCompleted} attached signal handler is
commonly used to execute some JavaScript code when a component's creation
process has been completed. In the example below, once the \l ListModel has
been fully created, its \c Component.onCompleted signal handler will
automatically be invoked to populate the model:

\qml
import QtQuick 2.0

ListView {
    width: 240; height: 320
    model: ListModel {
        id: listModel
        Component.onCompleted: {
            for (var i = 0; i < 10; i++)
                listModel.append({"Name": "Item " + i})
        }
    }
    delegate: Text { text: index }
}
\endqml

Since the name of the \e {attaching type} is \c Component and that type has a
\l{Component::completed}{completed} signal, the attached signal handler is
referred to as \c Component.onCompleted.


\section3 A Note About Accessing Attached Properties and Signal Handlers

A common error is to assume that attached properties and signal handlers are
directly accessible from the children of the object to which these attributes
have been attached. This is not the case. The instance of the
\e {attaching type} is only attached to specific objects, not to the object
and all of its children.

For example, below is a modified version of the earlier example involving
attached properties. This time, the delegate is an \l Item and the colored
\l Rectangle is a child of that item:

\qml
import QtQuick 2.0

ListView {
    width: 240; height: 320
    model: 3
    delegate: Item {
        width: 100; height: 30

        Rectangle {
            width: 100; height: 30
            color: ListView.isCurrentItem ? "red" : "yellow"    // WRONG! This won't work.
        }
    }
}
\endqml

This does not work as expected because \c ListView.isCurrentItem is attached
\e only to the root delegate object, and not its children. Since the
\l Rectangle is a child of the delegate, rather than being the delegate itself,
it cannot access the \c isCurrentItem attached property as
\c ListView.isCurrentItem. So instead, the rectangle should access
\c isCurrentItem through the root delegate:

\qml
ListView {
    //....
    delegate: Item {
        id: delegateItem
        width: 100; height: 30

        Rectangle {
            width: 100; height: 30
            color: delegateItem.ListView.isCurrentItem ? "red" : "yellow"   // correct
        }
    }
}
\endqml

Now \c delegateItem.ListView.isCurrentItem correctly refers to the
\c isCurrentItem attached property of the delegate.

*/