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

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/*!
\page qtbinding.html
\target qtbinding
\title Using QML in C++ Applications

\tableofcontents

The QML API is split into three main classes - QmlEngine, QmlComponent and QmlContext.  
QmlEngine provides the environment in which QML is run, QmlComponent encapsulates 
\l {QML Documents}, and QmlContext allows applications to expose data to QML component instances.

QML also includes a convenience API, QmlView, for applications that simply want to embed QML
components into a new QGraphicsView.  QmlView covers up many of the details discussed below.  
While QmlView is mainly intended for rapid prototyping it can have uses in production applications.

If you are looking at retrofitting an existing Qt application with QML,
read \l{Integrating QML with existing Qt UI code}.
\section1 Basic Usage

Every application requires at least one QmlEngine.  A QmlEngine allows the configuration of
global settings that apply to all the QML component instances - such as the QNetworkAccessManager
that is used for network communications, and the path used for persistent storage.  
Multiple QmlEngine's are only needed if the application requires these settings to differ 
between QML component instances.

\l {QML Documents} are loaded using the QmlComponent class.  Each QmlComponent instance 
represents a single QML document.  A QmlComponent can be passed a document URL, or raw text
representing the content of the document.  The document URL can be a local filesystem URL, or
any network URL supported by QNetworkAccessManager.

QML component instances can then be created by calling the QmlComponent::create() method.  Here's
an example of loading a QML document, and creating an object from it.

\code
QmlEngine *engine = new QmlEngine(parent);
QmlComponent component(engine, QUrl("main.qml"));
QObject *myObject = component.create();
\endcode

\section1 Exposing Data

QML components are instantiated in a QmlContext.  A context allows the application to expose data
to the QML component instance.  A single QmlContext can be used to instantiate all the objects
used by an application, or several QmlContext can be created for more fine grained control over
the data exposed to each instance.  If a context is not passed to the QmlComponent::create()
method, the QmlEngine's \l {QmlEngine::rootContext()}{root context} is used.  Data exposed through
the root context is available to all object instances.

\section1 Simple Data

To expose data to a QML component instance, applications set \l {QmlContext::setContextProperty()}
{context properties} which are then accessible by name from QML  \l {Property Binding}s and 
\l {JavaScript Blocks}.  The following example shows how to expose a background color to a QML
file.

\table
\row
\o
\code
// main.cpp
QmlContext *windowContext = new QmlContext(engine->rootContext());
windowContext->setContextProperty("backgroundColor", 
                                  QColor(Qt::lightsteelblue));

QmlComponent component(&engine, "main.qml");
QObject *window = component.create(windowContext);
\endcode
\o
\code
// main.qml
import Qt 4.6

Rectangle {
    color: backgroundColor

    Text {
        anchors.centerIn: parent
        text: "Hello Light Steel Blue World!"
    }
}
\endcode
\endtable

Context properties work just like normal properties in QML bindings - if the \c backgroundColor
context property in the previous example was changed to red, the component object instances would
all be automatically updated.  Note that it is the responsibility of the creator to delete any
QmlContext it constructs.  If the \c windowContext in the example above is no longer needed when
the \c window component instantiation is destroyed, the \c windowContext must be destroyed
explicitly.  The simplest way to ensure this is to set \c window as \c windowContext's parent.

QmlContexts form a tree - each QmlContext except for the root context has a parent.  Child 
QmlContexts effectively inherit the context properties present in their parents.  This gives
applications more freedom in partitioning the data exposed to different QML object instances.  
If a QmlContext sets a context property that is also set in one of its parents, the new context
property shadows that in the parent.  In The following example, the \c background context property
in \c {Context 1} shadows the \c background context property in the root context.

\image qml-context-tree.png

\section2 Structured Data

Context properties can also be used to expose structured and writable data to QML objects.  In 
addition to all the types already supported by QVariant, QObject derived types can be assigned to
context properties.  QObject context properties allow the data exposed to be more structured, and
allow QML to set values.

The following example creates a \c CustomPalette object, and sets it as the \c palette context
property.

\code
class CustomPalette : public QObject
{
Q_OBJECT
Q_PROPERTY(QColor background READ background WRITE setBackground NOTIFY backgroundChanged)
Q_PROPERTY(QColor text READ text WRITE setText NOTIFY text)
public:
    CustomPalette() : m_background(Qt::white), m_text(Qt::black) {}

    QColor background() const { return m_background; }
    void setBackground(const QColor &c) { 
        if (c != m_background) {
            m_background = c;
            emit backgroundChanged();
        }
    }

    QColor text() const { return m_text; }
    void setText(const QColor &c) { 
        if (c != m_text) {
            m_text = c;
            emit textChanged();
        }
    }
private:
    QColor m_background;
    QColor m_text;
};

int main(int argc, char **argv) 
{
    // ...

    QmlContext *windowContext = new QmlContext(engine->rootContext());
    windowContext->setContextProperty("palette", new CustomPalette);

    QmlComponent component(&engine, "main.qml");
    QObject *window = component.create(windowContext);
}
\endcode

The QML that follows references the palette object, and its properties, to set the appropriate
background and text colors.  When the window is clicked, the palette's text color is changed, and
the window text will update accordingly.

\code
// main.qml
import Qt 4.6

Rectangle {
    width: 240
    height: 320
    color: palette.background

    Text {
        anchors.centerIn: parent
        color: palette.text
        text: "Hello Colorful World!"
    }

    MouseRegion {
        anchors.fill: parent
        onClicked: {
            palette.text = "blue";
        }
    }
}
\endcode

To detect when a C++ property value - in this case the \c CustomPalette's \c text property - 
changes, the property must have a corresponding NOTIFY signal.  The NOTIFY signal specifies a signal
that is emitted whenever the property changes value.  Implementers should take care to only emit the
signal if the value \e changes to prevent loops from occuring.  Accessing a property from a 
binding that does not have a NOTIFY signal will cause QML to issue a warning at runtime.

\section2 Dynamic Structured Data

If an application is too dynamic to structure data as compile-time QObject types, dynamically
structured data can be constructed at runtime using the QmlPropertyMap class.


\section1 Calling C++ methods from QML

It is possible to call methods of QObject derived types by either exposing the
methods as public slots, or by marking the methods Q_INVOKABLE.

The C++ methods can also have parameters and return values.  QML has support for
the following types:

\list
\o bool
\o unsigned int, int
\o float, double, qreal
\o QString
\o QUrl
\o QColor
\o QDate, QTime, QDateTime
\o QPoint, QPointF
\o QSize, QSizeF
\o QRect, QRectF
\o QVariant
\endlist

This example toggles the "LED Blinker" when the MouseRegion is clicked:

\table
\row
\o
\code
// main.cpp
class LEDBlinker : public QObject
{
    Q_OBJECT
public:
    LEDBlinker();

    Q_INVOKABLE bool isRunning();

public slots:
    void start();
    void stop();
};

int main(int argc, char **argv)
{
    // ...

    QmlContext *context = engine->rootContext();
    context->setContextProperty("ledBlinker", new LEDBlinker);

    // ...
}
\endcode
\o
\code
// main.qml
import Qt 4.6

Rectangle {
    MouseRegion {
        anchors.fill: parent
        onClicked: {
            if (ledBlinker.isRunning())
                ledBlinker.stop()
            else
                ledBlicker.start();
        }
    }
}
\endcode
\endtable

Note that in this particular example a better way to achieve the same result
is to have a "running" property.  This leads to much nicer QML code:

\table
\row
\o
\code
// main.qml
import Qt 4.6

Rectangle {
    MouseRegion {
        anchors.fill: parent
        onClicked: ledBlinker.running = !ledBlinker.running
    }
}
\endcode
\endtable


Of course, it is also possible to call \l {Adding new methods}{functions declared in QML from C++}.


\section1 Network Components

If the URL passed to QmlComponent is a network resource, or if the QML document references a
network resource, the QmlComponent has to fetch the network data before it is able to create
objects.  In this case, the QmlComponent will have a \l {QmlComponent::Loading}{Loading}
\l {QmlComponent::status()}{status}.  An application will have to wait until the component
is \l {QmlComponent::Ready}{Ready} before calling \l {QmlComponent::create()}.

The following example shows how to load a QML file from a network resource.  After creating
the QmlComponent, it tests whether the component is loading.  If it is, it connects to the
QmlComponent::statusChanged() signal and otherwise calls the \c {continueLoading()} method
directly.  This test is necessary, even for URLs that are known to be remote, just in case
the component has been cached and is ready immediately.

\code
MyApplication::MyApplication()
{
    // ...
    component = new QmlComponent(engine, QUrl("http://www.example.com/main.qml"));
    if (component->isLoading())
        QObject::connect(component, SIGNAL(statusChanged(QmlComponent::Status)),
                         this, SLOT(continueLoading()));
    else
        continueLoading();
}

void MyApplication::continueLoading()
{
    if (component->isError()) {
        qWarning() << component->errors();
    } else {
        QObject *myObject = component->create();
    }
}
\endcode
*/