path: root/doc/src/qml/qmltypes.qdoc

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/*!
\page qml-c++types.html
\title Creating QML Types
\brief exposing Qt C++ types into the QML engine

The \l{The QML Engine}{QML engine} can instantiate any Qt C++ construct
such as \l{The Property System}{properties}, functions, and data models into
the QML context allowing the constructs to be accessible from within QML.

\target register-c++-type
\section1 Register a Type

    In an application or a \l{QML Plugins}{plugin}, the \c qmlRegisterType
    template will register a class to the QML engine.

\code
template<typename T>
int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)
\endcode

    \l qmlRegisterType() registers the C++ type \a T with the QML system, and
    makes it available to the QML context under the name \c qmlName in library
    \c uri version \c versionMajor.versionMinor. The \c qmlName can be the same
    as the C++ type name.

    Suppose that a \c Person class defined in a C++ is to be exposed into the
    QML context. The class must be a subclass of \l{QObject} and have a default
    constructor. The \l{The Property System}{properties} created with the
    Q_PROPERTY macro are visible in the QML context as well.
    \snippet declarative/cppextensions/referenceexamples/adding/person.h 0

    The application registers the class to the runtime with the
    \l{qmlRegisterType()}.

    \snippet declarative/cppextensions/referenceexamples/adding/main.cpp 0

    The Person type is then imported with the \c "People 1.0" module and its
    properties are accessible in a QML file.
\snippet declarative/cppextensions/referenceexamples/adding/example.qml 0

    The \l {Extending QML - Adding Types Example}{Adding Types} example
    demonstrates as usage of the \l qmlRegisterType().

    Alternatively, these functions provide a way for other types of C++ types
    to be visible in the QML context.
    \list
    \o \l qmlRegisterUncreatableType() is suited for attached
    properties and enum types.
    \o \l qmlRegisterTypeNotAvailable() is for
    reserving a namespace and suited for generating useful errors.
    \o \l qmlRegisterInterface() - for registering base or abstract classes for
    \l{qml-c++-coercion}{coercion and inheritance}. This is useful for general
    Qt objects or \l{Qt Objects and Interfaces}{pointers} to objects.
    \o \l qmlRegisterExtendedType() - for \l{qml-c++-extension}{extended types}
    \endlist

    \section2 Qt Objects and Interfaces
    QML can bind to complex objects such as pointers to objects or lists. As QML
    is typesafe, the \l{The QML Engine}{QML engine} ensures that only
    valid types are assigned to these properties.

    The QML engine treats pointers to objects or Qt interfaces the same
    way as regular properties. Thus, the lists or pointers are created as
    properties using the Q_PROPERTY() macro.

    \snippet examples/declarative/cppextensions/referenceexamples/properties/birthdayparty.h 1

    The \c host is an \l{qml-expose-properties}{exposed property} that can bind
    to objects or lists of objects. The property type, in this case \c Person,
    must be \l{register-c++-type}{registered} into the runtime.

    QML also supports assigning Qt interfaces. To assign to a property whose
    type is a Qt interface pointer, the interface must also be registered with
    QML. As they cannot be instantiated directly, registering a Qt interface is
    different from registering a new QML type. The following function is used
    instead:

    \code
    template<typename T>
    int qmlRegisterInterface(const char *typeName)
    \endcode

    This function registers the C++ interface \a T with the QML system as \a
    typeName.

    Following registration, QML can \l{qml-c++-coercion}{coerce} objects that
    implement this interface for assignment to appropriately typed properties.

\target qml-expose-properties
\section1 Exposing Qt C++ Properties

    The \l{The QML Engine}{QML engine} utilizes Qt's
    \l{The Property System}{Property System} and in effect, QML
    \l{Property Binding in QML}{property bindings} also use Qt properties.

    Essentially, a Qt C++ property has a \i write function, \i read function,
    and has a signal function. QML properties are inheritely public, both
    readable and writable, albeit type-safe. QML properties may also have
    signals which are emitted when the property value or binding changes.

    The QML property equivalent of a Qt C++ property is created as a property
    with the \l Q_PROPERTY() macro. There needs to be C++ functions assigned as
    the property's read, write, and signal handler function.

    The \l {register-c++-type}{Register a Type} section mentions that the
    \c Person class has properties that are exposed to the QML context. The QML
    properties are created with the \c Q_PROPERTY macro. The macro associates
    the properties to the read, write, and singal functions in its argument.

\code
Q_PROPERTY(int size READ size WRITE setSize NOTIFY shoeChanged)
\endcode

    A \c Shoe class might have an integer property called \c size. We set the \c
    size() function as the \c READ function and the \c setSize() function to be
    the \c WRITE function. In a QML application, when a property is read, the \c
    size() is called and when the property's binding changes, the \c setSize()
    is called. The READ function, by definition, must return the same type as
    the property.

    We may also connect a \l{signals and slots}{signal} to a property. The \c
    size property may have a \c shoeChanged signal indicated after the \c NOTIFY
    parameter of the macro. The \c shoeChanged becomes a \l{QML Signal and
    Handler Event System}{QML signal} and the runtime will create QML handler
    called \c onShoeChanged. Whenever the size property's binding changes, the
    \c shoeChanged signal is emitted and the \c onShoeChanged handler is
    invoked. In the handler, commands such as \l{JavaScript Expressions in
    QML}{JavaScript expressions} can perform clean-up operations or call other
    functions.

    \bold{Note:} The QML signal handler will always be named
    on<Property-name>Changed, regardless of the name used for the NOTIFY
    signal in C++. We recommend using <property-name>Changed() for the
    NOTIFY signal in C++.

    We may also make the property a \c read-only property by placing
    \c CONSTANT in the parameter. Changing the binding will generate an error.
\code
//A read-only property
Q_PROPERTY(int size READ size CONSTANT)
\endcode

\section2 Default Property

    When imported, QML components will bind their children to their designated
    \l{default-property}{default property}. This is helpful, for example,
    to redirect any declared child components to a property of another
    component.

    The runtime can set a property to be the default property by tagging the
    property with \c DefaultProperty in The Q_CLASSINFO() macro.

    \code
    Q_CLASSINFO("DefaultProperty", "pipe")
    \endcode

    The property tagged as default property, \c pipe, can only be an object
    property, or a list property.

    A default property is optional. A derived class inherits its base class's
    default property, but may override it in its own declaration. The \c pipe
    property can refer to a property declared in the class itself, or a property
    inherited from a base class.

    The \l{Extending QML - Default Property Example}{Default Property} example
    uses \l{default-property}{default properties} to assign the children of
    a component to a specific property.

    \section2 Grouped Properties

    A property group may be functionally defined as a set of related properties.
    For example, the \l{Layouts with Anchors}{anchors} are a group of
    related properties. In practice, property groups resemble a parent object
    where the individual properties are accessed as children.

    A grouped property's member properties are accessed using the
    <group>.<property> notation. For example, shoe.color is the way to access
    the \c color property in the \c shoe property group .

    \snippet examples/declarative/cppextensions/referenceexamples/grouped/example.qml ungrouped

    Alternatively, the group can be accessed as a set.
    \snippet examples/declarative/cppextensions/referenceexamples/grouped/example.qml grouped

    A grouped property block is implemented as a read-only object property. The
    \c shoe property shown is declared like this:

    \snippet examples/declarative/cppextensions/referenceexamples/grouped/person.h 1

    The \c ShoeDescription type declares the properties available to the grouped
    property block - in this case the \c size, \c color, \c brand and \c price properties.

    Grouped property blocks may declared and accessed be recusively.

    \l {Extending QML - Grouped Properties Example} shows the complete code used to
    implement the \c shoe property grouping.

    \section2 Attached Properties

    Attached properties annotate or add properties to another type or component.
    For example, the \l Keys \i{attaching type} contains \i{attached properties}
    that other elements may use to respond to key input. Conceptually, attached
    properties are a \i type exporting a set of additional properties that can
    be set on any other object instance.

    The attaching type is a QObject derived type. The properties on the
    attaching type are those that become available for use as attached
    properties.

    \snippet examples/declarative/cppextensions/referenceexamples/attached/example.qml 1

    The \c BirthdayParty is called the attaching type and the
    \c Boy instance the attachee object instance. The property \c rsvp is the
    attached property.

    Any Qt C++ type can become an attaching type by declaring the \c
    qmlAttachedProperties() a public member function and declaring that the
    class has QML_HAS_ATTACHED_PROPERTIES.

    \code
    static AttachedPropertiesType *qmlAttachedProperties(QObject *object);
    \endcode

    This static pointer returns an attachment object, of type \a
    AttachedPropertiesType, for the attachee \a object instance. It is
    customary, though not strictly required, for the attachment object to be
    parented to \a object to prevent memory leaks.
    The \l {Extending QML - Attached Properties Example}{Birthday}
    class has \c BirthdayPartyAttached attached properties.

    \snippet examples/declarative/cppextensions/referenceexamples/attached/birthdayparty.h static attached

    The QML_DECLARE_TYPEINFO() macro can notify the runtime that the type has
    attached properties with the QML_HAS_ATTACHED_PROPERTIES argument.

    \snippet examples/declarative/cppextensions/referenceexamples/attached/birthdayparty.h declare attached

    The qmlAttachedProperties method will be called at most once for each
    attachee object instance. The QML engine will cache the returned instance
    pointer for subsequent attached property accesses. Consequently the
    attachment object may not be deleted until \a object is destroyed.

    A common usage scenario is for a type to enhance the properties
    available to its children in order to gather instance specific data.

    \snippet examples/declarative/cppextensions/referenceexamples/attached/example.qml begin
    \snippet examples/declarative/cppextensions/referenceexamples/attached/example.qml rsvp
    \snippet examples/declarative/cppextensions/referenceexamples/attached/example.qml end

    However, as a QML type cannot limit the instances to which the attachment
    object must attach, the following is also allowed, even though adding a
    birthday party rsvp in this context will have no effect. Instead, \c
    BirthdayParty could be a separate component with a property \c rsvp.
    \code
    GraduationParty {
        Boy { BirthdayParty.rsvp: "2009-06-01" }
    }
    \endcode

    From C++, including the attaching type implementation, the attachment object
    for an instance can be accessed using the following method:

    \code
    template<typename T>
    QObject *qmlAttachedPropertiesObject<T>(QObject *attachee, bool create = true);
    \endcode

    This returns the attachment object attached to \a attachee by the attaching
    type \a T. If type \a T is not a valid attaching type, this method always
    returns 0. If \a create is true, a valid attachment object will always be
    returned, creating it if it does not already exist. If \a create is false,
    the attachment object will only be returned if it has previously been
    created.

    The \c rsvp properties of each guest in the \c Birthday party is accessible
    through the \c qmlAttachedPropertiesObject function.

    \snippet examples/declarative/cppextensions/referenceexamples/attached/main.cpp query rsvp

    The
    \l {Extending QML - Attached Properties Example}{Attached Properties Example}
    demonstrates the creation of attached properties with a birthday party
    scenario.

\section2 Object and List Properties

    QML can set properties of types that are more complex than basic intrinsics like
    integers and strings.  Properties can also be object pointers, Qt interface
    pointers,  lists of object pointers, and lists of Qt interface pointers.  As QML
    is typesafe it ensures that only valid types are assigned to these properties,
    just like it does for primitive types.

    Properties that are pointers to objects or Qt interfaces are declared with the
    Q_PROPERTY() macro, just like other properties.  The \c host property
    declaration looks like this:

    \snippet examples/declarative/cppextensions/referenceexamples/properties/birthdayparty.h 1

    As long as the property type, in this case \c Person, is registered with QML the
    property can be assigned.

    QML also supports assigning Qt interfaces.  To assign to a property whose type
    is a Qt interface pointer, the interface must also be registered with QML.  As
    they cannot be instantiated directly, registering a Qt interface is different
    from registering a new QML type. The following function is used instead:

    \code
    template<typename T>
    int qmlRegisterInterface(const char *typeName)
    \endcode

    \c qmlRegisterInterface registers the C++ interface \a T with the QML system
    as \a typeName.

    Following registration, QML can coerce objects that implement this interface
    for assignment to appropriately typed properties.


    \snippet examples/declarative/cppextensions/referenceexamples/properties/example.qml 0

    The \c guests property is a \i{list property} of \c Person objects. A list
    of \c Person objects are bound to the \c BirthdayParty's \c host property,
    and assigns three \c Person objects to the guests property.

    Properties that are lists of objects or Qt interfaces are also declared with
    the Q_PROPERTY() macro. However, list properties must have the type
    \l{QQmlListProperty}{QQmlListProperty<T>}.

    \snippet examples/declarative/cppextensions/referenceexamples/properties/birthdayparty.h 2

    As with the other property types, the type of list content, \a T, must be
    \l{register-c++-type}{registered} into the runtime.

    \snippet examples/declarative/cppextensions/referenceexamples/properties/main.cpp register list

    \l {Extending QML - Object and List Property Types Example} shows the
    complete code used to create the \c BirthdayParty type. For more
    information, visit \l{QQmlListProperty}{QQmlListProperty<T>}
    for creating list properties.

\section2 Sequence Types

    Certain C++ sequence types are supported transparently in QML as JavaScript
    Array types.
    In particular, QML currently supports:
    \list
      \o \c {QList<int>}
      \o \c {QList<qreal>}
      \o \c {QList<bool>}
      \o \c {QList<QString>} and \c{QStringList}
      \o \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.

    \bold {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 \o QList<int> \o integer value 0
    \row \o QList<qreal> \o real value 0.0
    \row \o QList<bool> \o boolean value \c {false}
    \row \o QList<QString> and QStringList \o empty QString
    \row \o QList<QUrl> \o 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 Property Signals

    All properties on custom types automatically support property binding.
    However, for binding to work correctly, QML must be able to reliably
    determine when a property has changed so that it knows to reevaluate any
    bindings that depend on the property's value. QML relies on the presence of
    a \l {Qt's Property System}{NOTIFY signal} for this determination.

    Here is the \c host property declaration:

    \snippet examples/declarative/cppextensions/referenceexamples/binding/birthdayparty.h 0

    The NOTIFY attribute is followed by a signal name. It is the responsibility
    of the class implementer to ensure that whenever the property's value
    changes, the NOTIFY signal is emitted. The signature of the NOTIFY signal is
    not important to QML.

    To prevent loops or excessive evaluation, developers should ensure that the
    signal is only emitted whenever the property's value is actually changed. If
    a property, or group of properties, is infrequently used it is permitted to
    use the same NOTIFY signal for several properties. This should be done with
    care to ensure that performance doesn't suffer.

    To keep QML reliable, if a property does not have a NOTIFY signal, it cannot
    be used in a binding expression. However, the property can still be assigned
    a binding as QML does not need to monitor the property for change in that
    scenario.

    Consider a custom type, \c TestElement, that has two properties, \c a and
    \c b. Property \c a does \i not have a NOTIFY signal, and property \c b does
    have a NOTIFY signal.

    \code
    TestElement {
        // This is OK
        a: b
    }
    TestElement {
        // Will NOT work
        b: a
    }
    \endcode

    The presence of a NOTIFY signal does incur a small overhead. There are cases
    where a property's value is set at object construction time, and does not
    subsequently change. The most common case of this is when a type uses \l
    {Grouped Properties}, and the grouped property object is allocated once, and
    only freed when the object is deleted. In these cases, the CONSTANT
    attribute may be added to the property declaration instead of a NOTIFY
    signal.

    \snippet examples/declarative/cppextensions/referenceexamples/binding/person.h 0

    Extreme care must be taken here or applications using your type may misbehave.
    The CONSTANT attribute should only be used for properties whose value is set,
    and finalized, only in the class constructor.  All other properties that want
    to be used in bindings should have a NOTIFY signal instead.

    \l {Extending QML -  Binding Example} shows the BirthdayParty example updated to
    include NOTIFY signals for use in binding.

\section1 Signals Support

    A \l{signals and slots}{signal} in Qt C++ is readily available as a
    \l{QML Signal and Handler Event System}{QML signal}. A signal will have
    a corresponding signal \i{handler}, created automatically. The handler
    name will have \c on prepended at the beginning of the name. The first
    character of the signal is uppercased for the signal handler. The
    signal parameter is also availabe to the QML signal.

    \snippet examples/declarative/cppextensions/referenceexamples/signal/birthdayparty.h 0
    The QML engine will create a handler for the \c partyStarted signal
    called \c onPartyStarted.
    \snippet examples/declarative/cppextensions/referenceexamples/signal/example.qml 0

    Classes may have multiple signals with the same name, but only the final
    signal is accessible as a QML signal. Note that signals with the same name
    but different parameters cannot be distinguished from one another.

    Signal parameters are exposed and can be any one of the QML
    \l{QML Basic Types}{basic types} as well registered object types. Accessing
    unregistered types will not generate an error, but the parameter value will
    not be accessible from the handler.

    To use signals from items not created in QML, access their signals with the
    \l {Connections} element.

    Additionally, if a property is added to a C++ class, all QML elements
    based on that C++ class will have a \i{value-changed} signal handler
    for that property. The name of the signal handler is
    \i{on<Property-name>Changed}, with the first letter of the property
    name being upper case.

    The \l {Extending QML - Signal Support Example}{Signal Support Example}
    shows an example application exposing signals to a QML component.

\section1 Exposing Methods

    The Q_INVOKABLE macro exposes any Qt C++ method as a QML method.

    \snippet examples/declarative/cppextensions/referenceexamples/methods/birthdayparty.h 0

    In a QML file, we can invoke the method as we would a
    \l{JavaScript Expressions in QML}{JavaScript expression}.
    \snippet examples/declarative/cppextensions/referenceexamples/methods/example.qml 0

    \l {Extending QML - Methods Example}{Methods example} uses the Q_INVOKABLE
    method to expose methods and demonstrates some usages of the method in
    an application.

    An alternative to the Q_INVOKABLE macro is to declare the C++ method as a
    \l{signals and slot}{slot}.

    \code
    slots:
        void invite(const QString &name);
    \endcode

\section1 Type Revisions and Versions

    Type revisions and versions allow new properties or methods to exist in the
    new version while remaining compatible with previous versions.

    Consider these two QML files:
    \code
    // main.qml
    import QtQuick 1.0
    Item {
        id: root
        MyComponent {}
    }
    \endcode

    \code
    // MyComponent.qml
    import MyModule 1.0
    CppItem {
        value: root.x
    }
    \endcode
    where \c CppItem maps to the C++ class \c QCppItem.

    If the author of QCppItem adds a \c root property to QCppItem in a new
    version of the module, \c root.x now resolves to a different value because
    \c root is also the \c id of the top level component. The author could
    specify that the new \c root property is available from a specific minor
    version. This permits new properties and features to be added to existing
    elements without breaking existing programs.

    The REVISION tag is used to mark the \c root property as added in revision 1
    of the class. Methods such as Q_INVOKABLE's, signals and slots can also be
    tagged for a revision using the \c Q_REVISION(x) macro:

    \code
    class CppElement : public BaseObject
    {
        Q_OBJECT
        Q_PROPERTY(int root READ root WRITE setRoot NOTIFY rootChanged REVISION 1)

    signals:
        Q_REVISION(1) void rootChanged();
    };
    \endcode

    To register the new class revision to a particular version the following function is used:

    \code
    template<typename T, int metaObjectRevision>
    int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)
    \endcode

    To register \c CppElement version 1 for \c {MyModule 1.1}:

    \code
    qmlRegisterType<QCppElement,1>("MyModule", 1, 1, "CppElement")
    \endcode

    \c root is only available when MyModule 1.1 is imported.

    For the same reason, new elements introduced in later versions should use
    the minor version argument of qmlRegisterType.

    This feature of the language allows for behavioural changes to be made
    without breaking existing applications. Consequently QML module authors
    should always remember to document what changed between minor versions, and
    QML module users should check that their application still runs correctly
    before deploying an updated import statement.

    You may also register the revision of a base class that your module depends upon
    using the qmlRegisterRevision() function:

    \code
    template<typename T, int metaObjectRevision>
    int qmlRegisterRevision(const char *uri, int versionMajor, int versionMinor)
    \endcode

    For example, if \c BaseObject is changed and now has a revision 1, you can specify that
    your module uses the new revision:

    \code
    qmlRegisterRevision<BaseObject,1>("MyModule", 1, 1);
    \endcode

    This is useful when deriving from base classes not declared as part of your
    module, e.g. when extending classes from the QtQuick library.

    The revision feature of QML allows for behavioral changes without breaking
    existing applications. Consequently, QML module authors should always
    remember to document what changed between minor versions, and QML module
    users should check that their application still runs correctly before
    deploying an updated import statement.

\target qml-c++-coercion
\section1 Inheritance and Coercion

    QML supports C++ inheritance hierarchies and can freely coerce between
    known, valid object types. This enables the creation of common base classes
    that allow the assignment of specialized classes to object or list
    properties.

    \snippet examples/declarative/cppextensions/referenceexamples/coercion/example.qml 0

    The QML snippet shown above assigns a \c Boy object to the \c
    BirthdayParty's \c host property, and assigns three other objects to the \c
    guests property. Both the \c host and the \c guests properties binds to the
    \c Person type, but the assignment is valid as both the \c Boy and \c Girl
    objects inherit from \c Person.

    To assign to a property, the property's type must have been
    \l{register-c++-type}{registered} to the \l{The QML Engine}{declarative
    runtime}. If a type that acts purely as a base class that cannot be
    instantiated from QML needs to be registered as well. The
    \l qmlRegisterType() is useful for this occasion.

    \code
        template<typename T>
        int qmlRegisterType()
    \endcode

    This function registers the C++ type \a T with the QML system. The
    parameterless call to the template function qmlRegisterType() does not
    define a mapping between the C++ class and a QML element name, so the type
    is not instantiable from QML, but it is available for type coercion.

    \snippet examples/declarative/cppextensions/referenceexamples/coercion/main.cpp 0
    \snippet examples/declarative/cppextensions/referenceexamples/coercion/main.cpp register boy girl
    The \c Person class is registered withouth the parameters. Both the
    \c Boy and \c Girl class derive from the \c Person class.

    Type \a T must inherit QObject, but there are no restrictions on whether it
    is concrete or the signature of its constructor.

    QML will automatically coerce C++ types when assigning to either an object
    property, or to a list property. Only if coercion fails does an assignment
    error occur.

    The \l{Extending QML - Inheritance and Coercion Example}{Inheritance and Coercion Example}
    shows the complete code used to create the \c Boy and \c Girl types.

\target qml-c++-extension
\section1 Extension Objects

    \snippet examples/declarative/cppextensions/referenceexamples/extended/example.qml 0

    The \c leftMargin property is a new property to an existing C++ type,
    \l QLineEdit, without modifying its source code.

    When integrating existing classes and technology into QML, APIs will
    often need tweaking to fit better into the declarative environment.
    Although the best results are usually obtained by modifying the original
    classes directly, if this is either not possible or is complicated by some
    other concerns, extension objects allow limited extension possibilities
    without direct modifications.

    \i{Extension objects} add additional properties to an existing type.
    Extension objects can only add properties, not signals or methods. An
    extended type definition allows the programmer to supply an additional type,
    known as the \i{extension type}, when registering the class. The
    properties are transparently merged with the original target class when used
    from within QML.

    The \l qmlRegisterExtendedType() is for registering extended types. Note
    that it has two forms.
    \code
    template<typename T, typename ExtendedT>
    int qmlRegisterExtendedType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

    template<typename T, typename ExtendedT>
    int qmlRegisterExtendedType()
    \endcode
    functions should be used instead of the regular \c qmlRegisterType() variations.
    The arguments are identical to the corresponding non-extension registration functions,
    except for the ExtendedT parameter which is the type
    of the extension object.

    An extension class is a regular QObject, with a constructor that takes a
    QObject pointer. However, the extension class creation is delayed until the
    first extended property is accessed. The extension class is created and the
    target object is passed in as the parent. When the property on the original
    is accessed, the corresponding property on the extension object is used
    instead.

    The \l{Extending QML - Extension Objects}{Extension Objects} example
    demonstrates a usage of extension objects.

\section1 Property Value Sources

\snippet examples/declarative/cppextensions/referenceexamples/valuesource/example.qml 0
\snippet examples/declarative/cppextensions/referenceexamples/valuesource/example.qml 1

The QML snippet shown above applies a property value source to the \c announcement property.
A property value source generates a value for a property that changes over time.

Property value sources are most commonly used to do animation.  Rather than
constructing an animation object and manually setting the animation's "target"
property, a property value source can be assigned directly to a property of any
type and automatically set up this association.

The example shown here is rather contrived: the \c announcement property of the
\c BirthdayParty object is a string that is printed every time it is assigned and
the \c HappyBirthdaySong value source generates the lyrics of the song
"Happy Birthday".

\snippet examples/declarative/cppextensions/referenceexamples/valuesource/birthdayparty.h 0

Normally, assigning an object to a string property would not be allowed.  In
the case of a property value source, rather than assigning the object instance
itself, the QML engine sets up an association between the value source and
the property.

Property value sources are special types that derive from the
QQmlPropertyValueSource base class.  This base class contains a single method,
QQmlPropertyValueSource::setTarget(), that the QML engine invokes when
associating the property value source with a property.  The relevant part of
the \c HappyBirthdaySong type declaration looks like this:

\snippet examples/declarative/cppextensions/referenceexamples/valuesource/happybirthdaysong.h 0
\snippet examples/declarative/cppextensions/referenceexamples/valuesource/happybirthdaysong.h 1
\snippet examples/declarative/cppextensions/referenceexamples/valuesource/happybirthdaysong.h 2

In all other respects, property value sources are regular QML types.  They must
be registered with the QML engine using the same macros as other types, and can
contain properties, signals and methods just like other types.

When a property value source object is assigned to a property, QML first tries
to assign it normally, as though it were a regular QML type.  Only if this
assignment fails does the engine call the \l {QQmlPropertyValueSource::}{setTarget()} method.  This allows
the type to also be used in contexts other than just as a value source.

\l {Extending QML -  Property Value Source Example} shows the complete code used
to implement the \c HappyBirthdaySong property value source.

\section1 Optimization and Other Considerations

The \l{qml-engine-optimization}{ QML Engine} article suggests possible
optimization considerations as memory management and QVariant type usages.

*/