Defining QML Types from C++

Any QObject-derived C++ class can be registered as the definition of a QML object type. Once a class is registered with the QML type system, the class can be declared and instantiated like any other object type from QML code. Once created, a class instance can be manipulated from QML; as Exposing Attributes of C++ Types to QML explains, the properties, methods and signals of any QObject-derived class are accessible from QML code.

To register a QObject-derived class as an instantiable QML object type, call qmlRegisterType() to register the class as QML type into a particular type namespace. Clients can then import that namespace in order to use the type.

For example, suppose there is a Message class with author and creationDate properties:

class Message : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged)
    Q_PROPERTY(QDateTime creationDate READ creationDate WRITE setCreationDate NOTIFY creationDateChanged)
public:
    // ...
};

This type can be registered by calling qmlRegisterType() with an appropriate type namespace and version number. For example, to make the type available in the com.mycompany.messaging namespace with version 1.0:

qmlRegisterType<Message>("com.mycompany.messaging", 1, 0, "Message");

The type can be used in an object declaration from QML, and its properties can be read and written to, as per the example below:

import com.mycompany.messaging 1.0

Message {
    author: "Amelie"
    creationDate: new Date()
}

Sometimes a QObject-derived class may need to be registered with the QML type system but not as an instantiable type. For example, this is the case if a C++ class:

• is an interface type that should not be instantiable

• is a base class type that does not need to be exposed to QML

• declares some enum that should be accessible from QML, but otherwise should not be instantiable

• is a type that should be provided to QML through a singleton instance, and should not be instantiable from QML

The Qt QML module provides several methods for registering non-instantiable types:

• qmlRegisterType() (with no parameters) registers a C++ type that is not instantiable and cannot be referred to from QML. This enables the engine to coerce any inherited types that are instantiable from QML.

• qmlRegisterInterface() registers an existing Qt interface type. The type is not instantiable from QML, and you cannot declare QML properties with it. Using C++ properties of this type from QML will do the expected interface casts, though.

• qmlRegisterUncreatableType() registers a named C++ type that is not instantiable but should be identifiable as a type to the QML type system. This is useful if a type's enums or attached properties should be accessible from QML but the type itself should not be instantiable.

• qmlRegisterSingletonType() registers a singleton type that can be imported from QML, as discussed below.

Note that all C++ types registered with the QML type system must be QObject-derived, even if they are non-instantiable.

Registering Singleton Objects with a Singleton Type▲

import MyThemeModule 1.0 as Theme

Rectangle {
    color: Theme.color // binding.
}

Many of the type registration functions require versions to be specified for the registered type. 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:

// main.qml
import QtQuick 1.0

Item {
    id: root
    MyType {}
}

// MyType.qml
import MyTypes 1.0

CppType {
    value: root.x
}

where CppType maps to the C++ class CppType.

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

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

class CppType : public BaseType
{
    Q_OBJECT
    Q_PROPERTY(int root READ root WRITE setRoot NOTIFY rootChanged REVISION 1)

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

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

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

To register CppType version 1 for MyTypes 1.1:

qmlRegisterType<CppType,1>("MyTypes", 1, 1, "CppType")

root is only available when MyTypes version 1.1 is imported.

For the same reason, new types 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 type depends upon using the qmlRegisterRevision() function:

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

template<typename T, int metaObjectRevision>
int qmlRegisterUncreatableType(const char *uri, int versionMajor, int versionMinor, const char *qmlName, const QString& reason)

template<typename T, typename E, int metaObjectRevision>
int qmlRegisterExtendedUncreatableType(const char *uri, int versionMajor, int versionMinor, const char *qmlName, const QString& reason)

For example, if BaseType is changed and now has a revision 1, you can specify that your type uses the new revision:

qmlRegisterRevision<BaseType,1>("MyTypes", 1, 1);

This is useful when deriving from base classes provided by other authors, e.g. when extending classes from the Qt Quick module.

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.

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 extension type, when registering the class. The properties are transparently merged with the original target class when used from within QML. For example:

QLineEdit {
    leftMargin: 20
}

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

The qmlRegisterExtendedType() function is for registering extended types. Note that it has two forms.

template<typename T, typename ExtendedT>
int qmlRegisterExtendedType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

template<typename T, typename ExtendedT>
int qmlRegisterExtendedType()

This functions should be used instead of the regular 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 Extension Objects Example demonstrates a usage of extension objects.

In the QML language syntax, there is a notion of attached properties and attached signal handlers, which are additional attributes that are attached to an object. Essentially, such attributes are implemented and provided by an attaching type, and these attributes may be attached to an object of another type. This contrasts with ordinary object properties which are provided by the object type itself (or the object's inherited type).

For example, the Item below uses attached properties and attached handlers:

import QtQuick 2.0

Item {
    width: 100; height: 100

    focus: true
    Keys.enabled: false
    Keys.onReturnPressed: console.log("Return key was pressed")
}

Here, the Item object is able to access and set the values of Keys.enabled and Keys.onReturnPressed. This allows the Item object to access these extra attributes as an extension to its own existing attributes.

Steps for Implementing Attached Objects▲

Implementing Attached Objects: An Example▲

class Message : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString author READ author WRITE setAuthor NOTIFY authorChanged)
    Q_PROPERTY(QDateTime creationDate READ creationDate WRITE setCreationDate NOTIFY creationDateChanged)
public:
    // ...
};

class MessageBoardAttachedType : public QObject
{
    Q_OBJECT
    Q_PROPERTY(bool expired READ expired WRITE setExpired NOTIFY expiredChanged)
public:
    MessageBoardAttachedType(QObject *parent);
    bool expired() const;
    void setExpired(bool expired);
signals:
    void published();
    void expiredChanged();
};

class MessageBoard : public QObject
{
    Q_OBJECT
public:
    static MessageBoardAttachedType *qmlAttachedProperties(QObject *object)
    {
        return new MessageBoardAttachedType(object);
    }
};
QML_DECLARE_TYPEINFO(MessageBoard, QML_HAS_ATTACHED_PROPERTIES)

Message {
    author: "Amelie"
    creationDate: new Date()

    MessageBoard.expired: creationDate < new Date("January 01, 2015 10:45:00")
    MessageBoard.onPublished: console.log("Message by", author, "has been
published!")
}

Message *msg = someMessageInstance();
MessageBoardAttachedType *attached =
        qobject_cast<MessageBoardAttachedType*>(qmlAttachedPropertiesObject<MessageBoard>(msg));

qDebug() << "Value of MessageBoard.expired:" << attached->expired();

A property modifier type is a special kind of QML object type. A property modifier type instance affects a property (of a QML object instance) which it is applied to. There are two different kinds of property modifier types:

• property value write interceptors

• property value sources

A property value write interceptor can be used to filter or modify values as they are written to properties. Currently, the only supported property value write interceptor is the Behavior type provided by the QtQuick import.

A property value source can be used to automatically update the value of a property over time. Clients can define their own property value source types. The various property animation types provided by the QtQuick import are examples of property value sources.

Property modifier type instances can be created and applied to a property of a QML object through the "<ModifierType> on <propertyName>" syntax, as the following example shows:

import QtQuick 2.0

Item {
    width: 400
    height: 50

    Rectangle {
        width: 50
        height: 50
        color: "red"

        NumberAnimation on x {
            from: 0
            to: 350
            loops: Animation.Infinite
            duration: 2000
        }
    }
}

Clients can register their own property value source types, but currently not property value write interceptors.

Property Value Sources▲