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/*!
\page qtqml-cppintegration-data.html
\title Data Type Conversion Between QML and C++
\brief Description of how data types are exchanged between QML and C++

When data values are exchanged between QML and C++, they are converted by the
QML engine to have the correct data types as appropriate for use in QML or
C++. This requires the exchanged data to be of a type that is recognizable by
the engine.

The QML engine provides built-in support for a large number of Qt C++ data
types. Additionally, custom C++ types may be registered with the QML type
system to make them available to the engine.

This page discusses the data types supported by the QML engine and how
they are converted between QML and C++.


\section1 Data Ownership

When data is transferred from C++ to QML, the ownership of the data always
remains with C++. The exception to this rule is when a QObject is returned from
an explicit C++ method call: in this case, the QML engine assumes ownership of
the object, unless the ownership of the object has explicitly been set to
remain with C++ by invoking QQmlEngine::setObjectOwnership() with
QQmlEngine::CppOwnership specified.

Additionally, the QML engine respects the normal QObject parent ownership
semantics of Qt C++ objects, and will not ever take ownership of a QObject
instance which already has a parent.


\section1 Basic Qt Data Types

By default, QML recognizes the following Qt data types, which are
automatically converted to a corresponding \l {QML Basic Types}{QML basic type}
when passed from C++ to QML and vice-versa:

\table
    \row
        \li Qt Type
        \li QML Basic Type
    \row
        \li bool
        \li \l bool
    \row
        \li unsigned int, int
        \li \l int
    \row
        \li double
        \li \l double
    \row
        \li float, qreal
        \li \l real
    \row
        \li QString
        \li \l string
    \row
        \li QUrl
        \li \l url
    \row
        \li QColor
        \li \l color
    \row
        \li QFont
        \li \l font
    \row
        \li QDate
        \li \l date
    \row
        \li QPoint, QPointF
        \li \l point
    \row
        \li QSize, QSizeF
        \li \l size
    \row
        \li QRect, QRectF
        \li \l rect
    \row
        \li QMatrix4x4
        \li \l matrix4x4
    \row
        \li QQuaternion
        \li \l quaternion
    \row
        \li QVector2D, QVector3D, QVector4D
        \li \l vector2d, \l vector3d, \l vector4d
    \row
        \li Enums declared with Q_ENUMS()
        \li \l enumeration
\endtable

\note Classes provided by the \l {Qt GUI} module, such as QColor, QFont,
QQuaternion and QMatrix4x4, are only available from QML when the \l {Qt Quick}
module is included.

As a convenience, many of these types can be specified in QML by string values,
or by a related method provided by the \l {QtQml::Qt} object. For example, the \l
{Image::sourceSize} property is of type \l size (which automatically translates
to the QSize type) and can be specified by a string value formatted as
"width\c{x}height", or by the Qt.size() function:

\qml
Item {
    Image { sourceSize: "100x200" }
    Image { sourceSize: Qt.size(100, 200) }
}
\endqml

See documentation for each individual type under \l {QML Basic Types} for more
information.


\section1 QObject-derived Types

Any QObject-derived class may be used as a type for the exchange of data between
QML and C++, providing the class has been registered with the QML type system.

The engine allows the registration of both instantiable and non-instantiable
types. Once a class is registered as a QML type, it can be used as a data type
for exchanging data between QML and C++. See
\l{qtqml-cppintegration-definetypes.html#registering-c++-types-with-the-qml-type-system}{Registering C++ types with the QML type system} for further details on type registration.


\section1 Conversion Between Qt and JavaScript Types

The QML engine has built-in support for converting a number of Qt types to
related JavaScript types, and vice-versa, when transferring data between QML
and C++. This makes it possible to use these types and receive them in C++ or
JavaScript without needing to implement custom types that provide access to
the data values and their attributes.

(Note that the JavaScript environment in QML modifies native JavaScript object
prototypes, including those of \c String, \c Date and \c Number, to provide
additional features. See the \l {qtqml-javascript-hostenvironment.html}
{JavaScript Host Environment} for further details.)


\section2 QVariantList and QVariantMap to JavaScript Array and Object

The QML engine provides automatic type conversion between QVariantList and
JavaScript arrays, and between QVariantMap and JavaScript objects.

For example, the function defined in QML below left expects two arguments, an
array and an object, and prints their contents using the standard JavaScript
syntax for array and object item access. The C++ code below right calls this
function, passing a QVariantList and a QVariantMap, which are automatically
converted to JavaScript array and object values, repectively:

\table
\header
\row
\li \snippet qml/qtbinding/variantlistmap/MyItem.qml 0
\li \snippet qml/qtbinding/variantlistmap/main.cpp 0
\endtable

This produces output like:

\code
Array item: 10
Array item: #00ff00
Array item: bottles
Object item: language = QML
Object item: released = Tue Sep 21 2010 00:00:00 GMT+1000 (EST)
\endcode

Similarly, if a C++ type uses a QVariantList or QVariantMap type for a property
type or method parameter, the value can be created as a JavaScript array or
object in QML, and is automatically converted to a QVariantList or QVariantMap
when it is passed to C++.


\section2 QDateTime to JavaScript Date

The QML engine provides automatic type conversion between QDateTime values and
JavaScript \c Date objects.

For example, the function defined in QML below left expects a JavaScript
\c Date object, and also returns a new \c Date object with the current date and
time. The C++ code below right calls this function, passing a QDateTime value
that is automatically converted by the engine into a \c Date object when it is
passed to the \c readDate() function. In turn, the readDate() function returns
a \c Date object that is automatically converted into a QDateTime value when it
is received in C++:

\table
\header
\row

\li
\qml
// MyItem.qml
Item {
    function readDate(dt) {
        console.log("The given date is:", dt.toUTCString());
        return new Date();
    }
}
\endqml

\li
\code
// C++
QQuickView view(QUrl::fromLocalFile("MyItem.qml"));

QDateTime dateTime = QDateTime::currentDateTime();
QDateTime retValue;

QMetaObject::invokeMethod(view.rootObject(), "readDate",
        Q_RETURN_ARG(QVariant, retValue),
        Q_ARG(QVariant, QVariant::fromValue(dateTime)));

qDebug() << "Value returned from readDate():" << retValue;
\endcode

\endtable

Similarly, if a C++ type uses a QDateTime for a property type or method
parameter, the value can be created as a JavaScript \c Date object in QML, and
is automatically converted to a QDateTime value when it is passed to C++.


\section2 Sequence Type to JavaScript Array

Certain C++ sequence types are supported transparently in QML as JavaScript
\c Array types.

In particular, QML currently supports:
\list
  \li \c {QList<int>}
  \li \c {QList<qreal>}
  \li \c {QList<bool>}
  \li \c {QList<QString>} and \c{QStringList}
  \li \c {QList<QUrl>}
\endlist

These sequence types are implemented directly in terms of the underlying C++
sequence. There are two ways in which such sequences can be exposed to QML:
as a Q_PROPERTY of the given sequence type; or as the return type of a
Q_INVOKABLE method. There are some differences in the way these are
implemented, which are important to note.

If the sequence is exposed as a Q_PROPERTY, accessing any value in the
sequence by index will cause the sequence data to be read from the QObject's
property, then a read to occur. Similarly, modifying any value in the
sequence will cause the sequence data to be read, and then the modification
will be performed and the modified sequence will be written back to the
QObject's property.

If the sequence is returned from a Q_INVOKABLE function, access and mutation
is much cheaper, as no QObject property read or write occurs; instead, the
C++ sequence data is accessed and modified directly.

Other sequence types are not supported transparently, and instead an
instance of any other sequence type will be passed between QML and C++ as an
opaque QVariantList.

\b {Important Note:} There are some minor differences between the
semantics of such sequence Array types and default JavaScript Array types
which result from the use of a C++ storage type in the implementation. In
particular, deleting an element from an Array will result in a
default-constructed value replacing that element, rather than an Undefined
value. Similarly, setting the length property of the Array to a value larger
than its current value will result in the Array being padded out to the
specified length with default-constructed elements rather than Undefined
elements.  Finally, the Qt container classes support signed (rather than
unsigned) integer indexes; thus, attempting to access any index greater
than INT_MAX will fail.

The default-constructed values for each sequence type are as follows:
\table
\row \li QList<int> \li integer value 0
\row \li QList<qreal> \li real value 0.0
\row \li QList<bool> \li boolean value \c {false}
\row \li QList<QString> and QStringList \li empty QString
\row \li QList<QUrl> \li empty QUrl
\endtable

If you wish to remove elements from a sequence rather than simply replace
them with default constructed values, do not use the indexed delete operator
("delete sequence[i]") but instead use the \c {splice} function
("sequence.splice(startIndex, deleteCount)").

\section2 Value types

Some value types in Qt such as QPoint are represented in JavaScript as objects
that have the same properties and functions like in the C++ API. The same
representation is possible with custom C++ value types. To enable a custom
value type with the QML engine, the class declaration needs to be annotated
with \c{Q_GADGET}. Properties that are intended to be visible in the JavaScript
representation need to be declared with \c Q_PROPERTY. Similarly functions need
to be marked with \c Q_INVOKABLE. This is the same with QObject based C++ APIs.
For example, the \c Actor class below is annotated as gadget and has
properties:

\code
 class Actor
 {
     Q_GADGET
     Q_PROPERTY(QString name READ name WRITE setName)
 public:
     QString name() const { return m_name; }
     void setName(const QString &name) { m_name = name; }

 private:
     QString m_name;
 }

 Q_DECLARE_METATYPE(Actor)
\endcode

\section1 Enumeration Types

To use a custom enumeration as a data type, its class must be registered and
the enumeration must also be declared with Q_ENUMS() to register it with Qt's
meta object system. For example, the \c Message class below has a \c Status
enum:

\code
 class Message : public QObject
 {
     Q_OBJECT
     Q_ENUMS(Status)
     Q_PROPERTY(Status status READ status NOTIFY statusChanged)
 public:
     enum Status {
         Ready,
         Loading,
         Error
     };
     Status status() const;
 signals:
     void statusChanged();
 };
\endcode

Providing the \c Message class has been
\l{qtqml-cppintegration-definetypes.html#registering-c++-types-with-the-qml-type-system}{registered} with the QML type system, its \c Status enum can be used from QML:

\qml
Message {
     onStatusChanged: {
         if (status == Message.Ready)
             console.log("Message is loaded!")
     }
 }
\endqml

\note The names of enum values must begin with a capital letter in order to
be accessible from QML.


\section2 Enumeration Types as Signal and Method Parameters

C++ signals and methods with enumeration-type parameters can be used from QML
provided that the enumeration and the signal or method are both declared
within the same class, or that the enumeration value is one of those declared
in the \l {Qt}{Qt Namespace}.

Additionally, if a C++ signal with an enum parameter should be connectable to a
QML function using the \l{qtqml-syntax-signals.html#connecting-signals-to-methods-and-signals}
{connect()} function, the enum type must be registered
using qRegisterMetaType().

For QML signals, enum values may be passed as signal parameters using the \c int
type:

\qml
 Message {
     signal someOtherSignal(int statusValue)

     Component.onCompleted: {
         someOtherSignal(Message.Loading)
     }
 }
\endqml

*/
