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/*!
\page qtquick-quickstart-essentials.html
\title Quick Start Guide - QML Essentials
\brief Essential QML application development examples

\section1 Handling User Input

One of the great advantages of using QML to define a user interface is that it
allows the user interface designer to define how the application should react
to events with simple JavaScript expressions.  In QML, we refer to those events
as \l{Signal and Handler Event System}{signals} and such signals can be handled
with \l{qml-signals-and-handlers}{signal handlers}.

For example, consider the following example:
\qml
import QtQuick 2.0

Rectangle {
    width: 200
    height: 100
    color: "red"

    Text {
        anchors.centerIn: parent
        text: "Hello, World!"
    }

    MouseArea {
        anchors.fill: parent
        onClicked: parent.color = "blue"
    }
}
\endqml

This example can be saved as "ClickableHelloWorld.qml" and run with qmlscene.
Whenever the user clicks anywhere in the window, the rectangle will change
from red to blue.  Note that the \l MouseArea type also emits the clicked
signal for touch events, so this code will also work on a mobile device.

Keyboard user input can be similarly handled with a simple expression:

\qml
import QtQuick 2.0

Rectangle {
    width: 200
    height: 100
    color: "red"

    Text {
        anchors.centerIn: parent
        text: "Hello, World!"
    }

    focus: true
    Keys.onPressed: {
        if (event.key == Qt.Key_Return) {
            color = "blue";
            event.accepted = true;
        }
    }
}
\endqml

By accepting focus, the color can be changed to blue whenever the return key
is pressed.

\section1 Property Bindings

Objects and their properties form the basis of a graphical interface defined
in a QML document.  The QML language allows properties to be bound to each
other in various ways, enabling highly dynamic user interfaces.

In the following example, the geometry of each child \l Rectangle is bound to
that of the parent \l Rectangle.  If the geometry of the parent \l Rectangle
were to change, the geometry of each child \l Rectangle would automatically
update due to the property bindings.

\qml
import QtQuick 2.0

Rectangle {
    width: 400
    height: 200

    Rectangle {
        width: parent.width / 2
        height: parent.height
    }

    Rectangle {
        width: parent.width / 2
        height: parent.height
        x: parent.width / 2
    }
}
\endqml

\section1 Animations

Properties can also be dynamically updated via animations.  The \c QtQuick
import provides various animation types which can be used to animate changes
to a property's value.  In the following example, a property is animated which
then gets displayed in a \l Text area:

\qml
import QtQuick 2.0

Rectangle {
    color: "lightgray"
    width: 200
    height: 200

    property int animatedValue: 0
    SequentialAnimation on animatedValue {
        loops: Animation.Infinite
        PropertyAnimation { to: 150; duration: 1000 }
        PropertyAnimation { to: 0; duration: 1000 }
    }

    Text {
        anchors.centerIn: parent
        text: animatedValue
    }
}
\endqml

The value being displayed will vary from 0 to 150 periodically.

\section1 Defining Custom QML Types for Re-use

One of the most important concepts in QML is that of type re-use.  An
application will probably have multiple visual elements which are all similar
(for example, multiple push buttons), and QML allows these sort of things to
be defined as re-usable, custom types, to minimize code duplication and
maximize readability.

For example, imagine that the developer defines a new \c Button type in the
\c Button.qml file:

\snippet qml/qml-extending-types/components/Button.qml 0

That type may now be re-used multiple times in the application, as follows:

\table
\row
\li \snippet qml/qml-extending-types/components/application.qml 0
\li \image qml-extending-types.png
\endtable


In this way, modular user interface elements can be built up and re-used within
an application.

See \l {QML Object Attributes}
for more details on how to develop your own reusable components.

*/
