path: root/doc/src/declarative/scope.qdoc

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/*!
\page qdeclarativescope.html
\title QML Scope

\tableofcontents

QML property bindings, inline functions and imported JavaScript files all
run in a JavaScript scope.  Scope controls which variables an expression can
access, and which variable takes precedence when two or more names conflict.

As JavaScript's built-in scope mechanism is very simple, QML enhances it to fit
more naturally with the QML language extensions.

\section1 JavaScript Scope

QML's scope extensions do not interfere with JavaScript's natural scoping.
JavaScript programmers can reuse their existing knowledge when programming
functions, property bindings or imported JavaScript files in QML.

In the following example, the \c {addConstant()} method will add 13 to the
parameter passed just as the programmer would expect irrespective of the
value of the QML object's \c a and \c b properties.

\code
QtObject {
    property int a: 3
    property int b: 9

    function addConstant(b) {
        var a = 13;
        return b + a;
    }
}
\endcode

That QML respects JavaScript's normal scoping rules even applies in bindings.
This totally evil, abomination of a binding will assign 12 to the QML object's
\c a property.

\code
QtObject {
    property int a

    a: { var a = 12; a; }
}
\endcode

Every JavaScript expression, function or file in QML has its own unique
variable object.  Local variables declared in one will never conflict
with local variables declared in another.

\section1 Element Names and Imported JavaScript Files

\l {QML Document}s include import statements that define the element names
and JavaScript files visible to the document.  In addition to their use in the
QML declaration itself, element names are used by JavaScript code when accessing
\l {Attached Properties} and enumeration values.

The effect of an import applies to every property binding, and JavaScript
function in the QML document, even those in nested inline components.  The
following example shows a simple QML file that accesses some enumeration
values and calls an imported JavaScript function.

\code
import QtQuick 1.0
import "code.js" as Code

ListView {
    snapMode: ListView.SnapToItem

    delegate: Component {
        Text {
            elide: Text.ElideMiddle
            text: "A really, really long string that will require eliding."
            color: Code.defaultColor()
        }
    }
}
\endcode

\section1 Binding Scope Object

Property bindings are the most common use of JavaScript in QML.  Property
bindings associate the result of a JavaScript expression with a property of an
object.  The object to which the bound property belongs is known as the binding's
scope object.  In this QML simple declaration the \l Item object is the
binding's scope object.

\code
Item {
    anchors.left: parent.left
}
\endcode

Bindings have access to the scope object's properties without qualification.
In the previous example, the binding accesses the \l Item's \c parent property
directly, without needing any form of object prefix.  QML introduces a more
structured, object-oriented approach to JavaScript, and consequently does not
require the use of the JavaScript \c this property.

Care must be used when accessing \l {Attached Properties} from bindings due
to their interaction with the scope object.  Conceptually attached properties
exist on \e all objects, even if they only have an effect on a subset of those.
Consequently unqualified attached property reads will always resolve to an
attached property on the scope object, which is not always what the programmer
intended.

For example, the \l PathView element attaches interpolated value properties to
its delegates depending on their position in the path.  As PathView only
meaningfully attaches these properties to the root element in the delegate, any
sub-element that accesses them must explicitly qualify the root object, as shown
below.

\code
PathView {
    delegate: Component {
        Rectangle {
            id: root
            Image {
                scale: root.PathView.scale
            }
        }
    }
}
\endcode

If the \l Image element omitted the \c root prefix, it would inadvertently access
the unset \c {PathView.scale} attached property on itself.

\section1 Component Scope

Each QML component in a QML document defines a logical scope.  Each document
has at least one root component, but can also have other inline sub-components.
The component scope is the union of the object ids within the component and the
component's root element's properties.

\code
Item {
    property string title

    Text {
        id: titleElement
        text: "<b>" + title + "</b>"
        font.pixelSize: 22
        anchors.top: parent.top
    }

    Text {
        text: titleElement.text
        font.pixelSize: 18
        anchors.bottom: parent.bottom
    }
}
\endcode

The example above shows a simple QML component that displays a rich text title
string at the top, and a smaller copy of the same text at the bottom.  The first
\c Text element directly accesses the component's \c title property when
forming the text to display.  That the root element's properties are directly
accessible makes it trivial to distribute data throughout the component.

The second \c Text element uses an id to access the first's text directly.  IDs
are specified explicitly by the QML programmer so they always take precedence
over other property names (except for those in the \l {JavaScript Scope}).  For
example, in the unlikely event that the binding's \l {Binding Scope Object}{scope
object}  had a \c titleElement property in the previous example, the \c titleElement
id would still take precedence.

\section1 Component Instance Hierarchy

In QML, component instances connect their component scopes together to form a
scope hierarchy.  Component instances can directly access the component scopes of
their ancestors.

The easiest way to demonstrate this is with inline sub-components whose component
scopes are implicitly scoped as children of the outer component.

\code
Item {
    property color defaultColor: "blue"

    ListView {
        delegate: Component {
            Rectangle {
                color: defaultColor
            }
        }
    }
}
\endcode

The component instance hierarchy allows instances of the delegate component
to access the \c defaultColor property of the \c Item element.  Of course,
had the delegate component had a property called \c defaultColor that would
have taken precedence.

The component instance scope hierarchy extends to out-of-line components, too.
In the following example, the \c TitlePage.qml component creates two
\c TitleText instances.  Even though the \c TitleText element is in a separate
file, it still has access to the \c title property when it is used from within
the \c TitlePage.  QML is a dynamically scoped language - depending on where it
is used, the \c title property may resolve differently.

\code
// TitlePage.qml
import QtQuick 1.0
Item {
    property string title

    TitleText {
        size: 22
        anchors.top: parent.top
    }

    TitleText {
        size: 18
        anchors.bottom: parent.bottom
    }
}

// TitleText.qml
import QtQuick 1.0
Text {
    property int size
    text: "<b>" + title + "</b>"
    font.pixelSize: size
}
\endcode

Dynamic scoping is very powerful, but it must be used cautiously to prevent
the behavior of QML code from becoming difficult to predict.  In general it
should only be used in cases where the two components are already tightly
coupled in another way.  When building reusable components, it is preferable
to use property interfaces, like this:

\code
// TitlePage.qml
import QtQuick 1.0
Item {
    id: root
    property string title

    TitleText {
        title: root.title
        size: 22
        anchors.top: parent.top
    }

    TitleText {
        title: root.title
        size: 18
        anchors.bottom: parent.bottom
    }
}

// TitleText.qml
import QtQuick 1.0
Text {
    property string title
    property int size

    text: "<b>" + title + "</b>"
    font.pixelSize: size
}
\endcode

\section1 JavaScript Global Object

In addition to all the properties that a developer would normally expect on
the JavaScript global object, QML adds some custom extensions to make UI or
QML specific tasks a little easier.  These extensions are described in the
\l {QML Global Object} documentation.

QML disallows element, id and property names that conflict with the properties
on the global object to prevent any confusion.  Programmers can be confident
that \c Math.min(10, 9) will always work as expected!

*/