path: root/src/qml/doc/src/cppintegration/data.qdoc

// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\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.

For more information about C++ and the different QML integration methods,
see the
\l {Overview - QML and C++ Integration} {C++ and QML integration overview} page.

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 never delete a QObject instance which
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 Value Types}{QML value type}
when passed from C++ to QML and vice-versa:

\table
    \row
        \li Qt Type
        \li QML Value 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 QDateTime
        \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_ENUM()
        \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 Value 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 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 calls this
function, passing a QVariantList and a QVariantMap, which are automatically
converted to JavaScript array and object values, repectively:

\table
\row
\li QML
\li \snippet qml/qtbinding/variantlistmap/MyItem.qml 0
\row
\li C++
\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++.

Mind that QVariantList and QVariantMap properties of C++ types are stored as
values and cannot be changed in place by QML code. You can only replace the
whole map or list, but not manipulate its contents. The following code does
not work if the property \c l is a QVariantList:

\code
MyListExposingItem {
   l: [1, 2, 3]
   Component.onCompleted: l[0] = 10
}
\endcode

The following code does work:
\code
MyListExposingItem {
   l: [1, 2, 3]
   Component.onCompleted: l = [10, 2, 3]
}
\endcode

\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 expects a JavaScript
\c Date object, and also returns a new \c Date object with the current date and
time. The C++ code below 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
\row
\li QML
\li
\qml
// MyItem.qml
Item {
    function readDate(dt) {
        console.log("The given date is:", dt.toUTCString());
        return new Date();
    }
}
\endqml
\row
\li C++
\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++.

\note Watch out for the difference in month numbering: JavaScript numbers
January as 0 through 11 for December, off by one from Qt's numbering
of January as 1 through 12 for December.

\note When using a string in JavaScript as the value of a \c Date object,
note that a string with no time fields (so a simple date) is
interpreted as the UTC start of the relevant day, in contrast
to \c{new Date(y, m, d)} which uses the local time start of the day.
Most other ways of constructing a \c Date object in JavaScript produce
a local time, unless methods with UTC in their names are used. If your
program is run in a zone behind UTC (nominally west of The Prime
Meridian), use of a date-only string will lead to a \c Date object
whose \c getDate() is one less than the day-number in your string; it
will typically have a large value for \c getHours(). The UTC variants
of these methods, \c getUTCDate() and \c getUTCHours(), will give the
results you expect for such a \c Date objects. See also the next
section.

\section2 QDate and JavaScript Date

The QML engine converts automatically between \l QDate and the
JavaScript \c Date type by representing the date by the UTC start of
its day.  A date is mapped back to QDate via QDateTime, selecting
its \l {QDateTime::}{date()} method, using the local time form of the
date unless the UTC form of it coincides with the start of the next
day, in which case the UTC form is used.

This slighly eccentric arrangement is a work-around for the fact that
JavaScript's construction of a \c Date object from a date-only string
uses the UTC start of the day, but \c{new Date(y, m, d)} uses the
local time start of the indicated date, as discussed in a note at the
end of the previous section.

As a result, where a QDate property or parameter is exposed to QML,
care should be taken in reading its value: the \c
Date.getUTCFullYear(), \c Date.getUTCMonth() and \c Date.getUTCDate()
methods are more likely to deliver the results users expect than the
corresponding methods without UTC in their names.

It is thus commonly more robust to use a \l QDateTime property. This
makes it possible to take control, on the QDateTime side, of whether
the date (and time) is specified in terms of UTC or local time; as
long as the JavaScript code is written to work with the same standard,
it should be possible to avoid trouble.

//! Target adds an anchor, so renaming the section won't break incoming links.
\target QTime to JavaScript Date
\section2 QTime and JavaScript Date

The QML engine provides automatic type conversion from QTime values to
JavaScript \c Date objects. As QTime values do not contain a date component,
one is created for the conversion only. Thus, you should not rely on the date
component of the resulting Date object.

Under the hood, conversion from a JavaScript \c Date object to QTime
is done by converting to a QDateTime object (using local time) and
calling its \l {QDateTime::}{time()} method.

\section2 Sequence Type to JavaScript Array

See \l{QML Sequence Types} for a general description of sequence types. The
\l{Qt QML QML Types}{QtQml module} contains a few sequence types
you may want to use.

You can also create a list-like data structure by constructing a QJSValue using
QJSEngine::newArray(). Such a JavaScript array does not need any conversion
when passing it between QML and C++. See \l{QJSValue#Working With Arrays} for
details on how to manipulate JavaScript arrays from C++.

\section2 QByteArray to JavaScript ArrayBuffer

The QML engine provides automatic type conversion between QByteArray values and
JavaScript \c ArrayBuffer objects.

\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

The usual pattern is to use a gadget class as the type of a property, or to
emit a gadget as a signal argument. In such cases, the gadget instance is
passed by value between C++ and QML (because it's a value type). If QML code
changes a property of a gadget property, the entire gadget is re-created and
passed back to the C++ property setter. In Qt 5, gadget types cannot be
instantiated by direct declaration in QML. In contrast, a QObject instance can
be declared; and QObject instances are always passed by pointer from C++ to QML.

\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_ENUM() 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_PROPERTY(Status status READ status NOTIFY statusChanged)
 public:
     enum Status {
         Ready,
         Loading,
         Error
     };
     Q_ENUM(Status)
     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

To use an enum as a \l {QFlags}{flags} type in QML, see \l Q_FLAG().

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

\code
...
enum class Status {
          Ready,
          Loading,
          Error
}
Q_ENUM(Status)
...
\endcode

Enum classes are registered in QML as scoped and unscoped properties.
The \c Ready value will be registered at \c Message.Status.Ready and \c Message.Ready .

When using enum classes, there can be multiple enums using the same identifiers.
The unscoped registration will be overwriten by the last registered enum. For classes
that contain such name conficts it is possible to disable the unscoped registration by
annotating your class with a special Q_CLASSINFO macro.
Use the name \c RegisterEnumClassesUnscoped with the value \c false to prevent scoped
enums from being merged into the same name space.

\code
class Message : public QObject
    {
        Q_OBJECT
        Q_CLASSINFO("RegisterEnumClassesUnscoped", "false")
        Q_ENUM(ScopedEnum)
        Q_ENUM(OtherValue)

    public:
        enum class ScopedEnum {
              Value1,
              Value2,
              OtherValue
        };
        enum class OtherValue {
              Value1,
              Value2
        };
    };
\endcode

Enums from related types are usually registered in the scope of the relating
type. For example any enum from a different type used in a \l{Q_PROPERTY}
declaration causes all enums from that type to be made available in QML. This
is usually more of a liability than a feature. In order to prevent it from
happening, annotate your class with a special \l{Q_CLASSINFO} macro.
Use the name \c RegisterEnumsFromRelatedTypes with the value \c false to prevent
enums from related types from being registered in this type.

You should explicitly register the enclosing types of any enums you want to use
in QML, using \l{QML_ELEMENT} or \l{QML_NAMED_ELEMENT}, rather than rely on
their enums to be injected into other types.

\code
class OtherType : public QObject
{
    Q_OBJECT
    QML_ELEMENT

public:
    enum SomeEnum { A, B, C };
    Q_ENUM(SomeEnum)

    enum AnotherEnum { D, E, F };
    Q_ENUM(AnotherEnum)
};

class Message : public QObject
{
    Q_OBJECT
    QML_ELEMENT

    // This would usually cause all enums from OtherType to be registered
    // as members of Message ...
    Q_PROPERTY(OtherType::SomeEnum someEnum READ someEnum CONSTANT)

    // ... but this way it doesn't.
    Q_CLASSINFO("RegisterEnumsFromRelatedTypes", "false")

public:
    OtherType::SomeEnum someEnum() const { return OtherType::B; }
};
\endcode

The important difference is the scope for the enums in QML. If an enum from a
related class is automatically registered, the scope is the type it is
imported into. In the above case, without the extra \l{Q_CLASSINFO}, you would
use \c {Message.A}, for example. If C++ type holding the enums is explicitly
registered, and the registration of enums from related types is suppressed, the
QML type for the C++ type holding the enums is the scope for all of its enums.
You would use \c {OtherType.A} instead of \c {Message.A} in QML.

Mind that you can use \l QML_FOREIGN to register a type you cannot modify. You
can also use \l QML_FOREIGN_NAMESPACE to register the enumerators of a C++ type
into a QML namespace of any upper-case name, even if the same C++ type is
also registered as a QML value type.

\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

*/