source: trunk/doc/src/declarative/propertybinding.qdoc

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/*!
\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 - after all, a constant is a valid JavaScript
expression!  Here are some examples of more complex bindings:

\code
Rectangle {
    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

While syntactically bindings can be of arbitrary complexity, if a binding starts to become
overly complex - such as involving multiple lines, or imperative loops - it may be better
to refactor the component entirely, or at least factor the binding out into a separate
function.

\section1 Changing Bindings

The \l PropertyChanges element can be used within a state change to modify the bindings on 
properties.  

This example modifies the \l Rectangle's width property binding to be \c {otherItem.height} 
when in the "square" state.  When it returns to its default state, width's original property
binding will have been restored.

\code
Rectangle {
    id: rectangle
    width: otherItem.width
    height: otherItem.height

    states: State {
        name: "square"
        PropertyChanges {
            target: rectangle
            width: otherItem.height
        }
    }
}
\endcode


\section1 Effects of Property Assignment in JavaScript

Assigning a property value from JavaScript does \e not create a property binding.
For example:

\code
Rectangle {

    Component.onCompleted: {
        width = otherItem.width;
    }
}
\endcode

Instead of creating a property binding, this simply sets the \c width of the \l Rectangle
to the value of \c other.width at the time the JavaScript code is invoked.  See 
\l {Property Assignment vs Property Binding} for more details.

Also note that assigning a value to a property that is currently bound will remove the binding.
A property can only have one value at a time, and if any code explicitly sets
this value, the binding is removed.  The \l Rectangle in the example below will have
a width of 13, regardless of the \c otherItem's width.

\code
Rectangle {
    width: otherItem.width

    Component.onCompleted: {
        width = 13;
    }
}
\endcode

There is no way to create a property binding directly from imperative JavaScript code,
although it is possible to set up a \l Binding object (shown below).


\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


*/