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/*!
\page qtqml-cppintegration-interactqmlfromcpp.html
\title Interacting with QML Objects from C++
\brief Description of how to load and access QML objects from C++ code

All QML object types are QObject-derived types, whether they are internally
implemented by the engine or \l
{qtqml-cppintegration-definetypes.html}{defined by third-party
sources}. This means the QML engine can use the Qt \l{Meta Object System} to
dynamically instantiate any QML object type and inspect the created objects.

This is useful for creating QML objects from C++ code, whether to display a QML
object that can be visually rendered, or to integrate non-visual QML object data
into a C++ application. Once a QML object is created, it can be inspected from
C++ in order to read and write to properties, invoke methods and receive signal
notifications.


\section1 Loading QML Objects from C++

A QML document can be loaded with QQmlComponent or QQuickView. QQmlComponent
loads a QML document as a C++ object that can then be modified from C++ code.
QQuickView also does this, but as QQuickView is a QWindow-derived class, the
loaded object will also be rendered into a visual display; QQuickView is
generally used to integrate a displayable QML object into an application's
user interface.

For example, suppose there is a \c MyItem.qml file that looks like this:

\snippet qml/qtbinding/loading/MyItem.qml start
\snippet qml/qtbinding/loading/MyItem.qml end

This QML document can be loaded with QQmlComponent or QQuickView with the
following
C++ code. Using a QQmlComponent requires calling QQmlComponent::create() to
create
a new instance of the component, while a QQuickView automatically creates an
instance of the
component, which is accessible via QQuickView::rootObject():

\table
\row
\li
\snippet qml/qtbinding/loading/main.cpp QQmlComponent-a
\dots 0
\snippet qml/qtbinding/loading/main.cpp QQmlComponent-b
\li
\snippet qml/qtbinding/loading/main.cpp QQuickView
\endtable

This \c object is the instance of the \c MyItem.qml component that has been
created. You can now modify the item's properties using QObject::setProperty()
or QQmlProperty:

\snippet qml/qtbinding/loading/main.cpp properties

Alternatively, you can cast the object to its actual type and call methods with
compile-time safety. In this case the base object of \c MyItem.qml is an
\l Item, which is defined by the QQuickItem class:

\snippet qml/qtbinding/loading/main.cpp cast

You can also connect to any signals or call methods defined in the component
using QMetaObject::invokeMethod() and QObject::connect(). See \l {Invoking QML Methods}
and \l {Connecting to QML Signals} below for further details.


\section1 Accessing Loaded QML Objects by Object Name

QML components are essentially object trees with children that have siblings and
their own children. Child objects of QML components can be located using the
QObject::objectName property with QObject::findChild(). For example, if the root
item in \c MyItem.qml had a child \l Rectangle item:

\snippet qml/qtbinding/loading/MyItem.qml start
\codeline
\snippet qml/qtbinding/loading/MyItem.qml child
\snippet qml/qtbinding/loading/MyItem.qml end

The child could be located like this:

\snippet qml/qtbinding/loading/main.cpp findChild

Note that an object may have multiple children with the same \c objectName.
For example, \l ListView creates multiple instances of its delegate, so if its
delegate is declared with a particular objectName, the \l ListView will have
multiple children with the same \c objectName. In this case,
QObject::findChildren() can be used to find all children with a matching
\c objectName.

\warning While it is possible to use C++ to access and manipulate QML objects
deep into the object tree, we recommend that you do not take this approach
outside of application testing and prototyping. One strength of QML and C++
integration is the ability to implement the QML user interface separately
from the C++ logic and dataset backend, and this strategy breaks if the C++
side reaches deep into the QML components to manipulate them directly. This
would make it difficult to, for example, swap a QML view component for
another view, if the new component was missing a required \c objectName. It
is better for the C++ implementation to know as little as possible about the
QML user interface implementation and the composition of the QML object tree.


\section1 Accessing Members of a QML Object Type from C++

\section2 Properties

Any properties declared in a QML object are automatically accessible from C++.
Given a QML item like this:

\snippet qml/qtbinding/properties-qml/MyItem.qml 0

The value of the \c someNumber property can be set and read using QQmlProperty,
or QObject::setProperty() and QObject::property():

\snippet qml/qtbinding/properties-qml/main.cpp 0

You should always use QObject::setProperty(), QQmlProperty or
QMetaProperty::write() to change a QML property value, to ensure the QML
engine is made aware of the property change. For example, say you have a
custom element \c PushButton with a \c buttonText property that internally
reflects the value of a \c m_buttonText member variable. Modifying the member
variable directly like this is not a good idea:

\badcode
// BAD!
QQmlComponent component(engine, "MyButton.qml");
PushButton *button = qobject_cast<PushButton*>(component.create());
button->m_buttonText = "Click me";
\endcode

Since the value is changed directly, this bypasses Qt's \l{The Meta-Object
System}{meta-object system} and the QML engine is not made aware of the
property change. This means property bindings to \c buttonText would not be
updated, and any \c onButtonTextChanged handlers would not be called.

\section2 Invoking QML Methods

All QML methods are exposed to the meta-object system and can be called from C++
using QMetaObject::invokeMethod(). Method parameters and return values passed
from QML are always translated into QVariant values in C++.

Here is a C++ application that calls a QML method using
QMetaObject::invokeMethod():

\table
\row
\li \snippet qml/qtbinding/functions-qml/MyItem.qml 0
\li \snippet qml/qtbinding/functions-qml/main.cpp 0
\endtable

Notice the Q_RETURN_ARG() and Q_ARG() arguments for QMetaObject::invokeMethod()
must be specified as QVariant types, as this is the generic data type used for
QML method parameters and return values.


\section2 Connecting to QML Signals

All QML signals are automatically available to C++, and can be connected to
using QObject::connect() like any ordinary Qt C++ signal. In return, any C++
signal can be received by a QML object using
\l {qtqml-syntax-signals.html}{signal handlers}.

Here is a QML component with a signal named \c qmlSignal that is emitted with
a string-type parameter. This signal is connected to a C++ object's slot using
QObject::connect(), so that the \c cppSlot() method is called whenever the
\c qmlSignal is emitted:

\table
\row
\li
\snippet qml/qtbinding/signals-qml/MyItem.qml 0
\li
\snippet qml/qtbinding/signals-qml/myclass.h 0
\codeline
\snippet qml/qtbinding/signals-qml/main.cpp 0
\endtable

When a QML object type is used as a signal parameter, the parameter should
use \l var as the type, and the value should be received in C++ using the
QVariant type:

\table
\row
\li

\qml
 // MyItem.qml
 import QtQuick 2.0

 Item {
     id: item
     width: 100; height: 100

     signal qmlSignal(var anObject)

     MouseArea {
         anchors.fill: parent
         onClicked: item.qmlSignal(item)
     }
 }
\endqml

\li
\code
 class MyClass : public QObject
 {
     Q_OBJECT
 public slots:
     void cppSlot(const QVariant &v) {
        qDebug() << "Called the C++ slot with value:" << v;

        QQuickItem *item = qobject_cast<QQuickItem*>(v.value<QObject*>());
        qDebug() << "Item dimensions:" << item->width() << item->height();
     }
 };

 int main(int argc, char *argv[]) {
     QApplication app(argc, argv);

     QQuickView view(QUrl::fromLocalFile("MyItem.qml"));
     QObject *item = view.rootObject();

     MyClass myClass;
     QObject::connect(item, SIGNAL(qmlSignal(QVariant)),
                      &myClass, SLOT(cppSlot(QVariant)));

     view.show();
     return app.exec();
 }
\endcode
\endtable

*/
