QDeclarativeExtensionPlugin Class

The QDeclarativeExtensionPlugin class provides an abstract base for custom QML extension plugins. More...

Header: #include <QDeclarativeExtensionPlugin>
Since: Qt 4.7
Inherits: QObject

Public Functions

QDeclarativeExtensionPlugin(QObject * parent = 0)

Reimplemented Public Functions

virtual void initializeEngine(QDeclarativeEngine * engine, const char * uri)
virtual void registerTypes(const char * uri) = 0
  • 29 public functions inherited from QObject

Additional Inherited Members

  • 1 property inherited from QObject
  • 1 public slot inherited from QObject
  • 1 signal inherited from QObject
  • 7 static public members inherited from QObject
  • 8 protected functions inherited from QObject

Detailed Description

The QDeclarativeExtensionPlugin class provides an abstract base for custom QML extension plugins.

QDeclarativeExtensionPlugin is a plugin interface that makes it possible to create QML extensions that can be loaded dynamically into QML applications. These extensions allow custom QML types to be made available to the QML engine.

To write a QML extension plugin:

QML extension plugins can be used to provide either application-specific or library-like plugins. Library plugins should limit themselves to registering types, as any manipulation of the engine's root context may cause conflicts or other issues in the library user's code.

An example

Suppose there is a new TimeModel C++ class that should be made available as a new QML element. It provides the current time through hour and minute properties, like this:

class TimeModel : public QObject
    Q_PROPERTY(int hour READ hour NOTIFY timeChanged)
    Q_PROPERTY(int minute READ minute NOTIFY timeChanged)

To make this class available as a QML type, create a plugin that registers this type with a specific module using qmlRegisterType(). For this example the plugin module will be named com.nokia.TimeExample (as defined in the project file further below).

class QExampleQmlPlugin : public QDeclarativeExtensionPlugin
    void registerTypes(const char *uri)
        Q_ASSERT(uri == QLatin1String("org.qtproject.TimeExample"));
        qmlRegisterType<TimeModel>(uri, 1, 0, "Time");

Q_EXPORT_PLUGIN2(qmlqtimeexampleplugin, QExampleQmlPlugin);

This registers the TimeModel class with the 1.0 version of this plugin library, as a QML type called Time. The Q_ASSERT statement ensures the module is imported correctly by any QML components that use this plugin.

The project file defines the project as a plugin library and specifies it should be built into the com/nokia/TimeExample directory:

CONFIG += qt plugin
QT += declarative

DESTDIR = com/nokia/TimeExample
TARGET = qmlqtimeexampleplugin

Finally, a qmldir file is required in the com/nokia/TimeExample directory that describes the plugin. This directory includes a Clock.qml file that should be bundled with the plugin, so it needs to be specified in the qmldir file:

Once the project is built and installed, the new Time element can be used by any QML component that imports the com.nokia.TimeExample module:

import org.qtproject.TimeExample 1.0 // import types from the plugin

Clock { // this class is defined in QML (org/qtproject/TimeExample/Clock.qml)

    Time { // this class is defined in C++ (plugin.cpp)
        id: time

    hours: time.hour
    minutes: time.minute

The full source code is available in the plugins example.

The Tutorial: Writing QML extensions with C++ also contains a chapter on creating QML plugins.

See also QDeclarativeEngine::importPlugin() and How to Create Qt Plugins.

Member Function Documentation

QDeclarativeExtensionPlugin::QDeclarativeExtensionPlugin(QObject * parent = 0)

Constructs a QML extension plugin with the given parent.

Note that this constructor is invoked automatically by the Q_EXPORT_PLUGIN2() macro, so there is no need for calling it explicitly.

[virtual] void QDeclarativeExtensionPlugin::initializeEngine(QDeclarativeEngine * engine, const char * uri)

Initializes the extension from the uri using the engine. Here an application plugin might, for example, expose some data or objects to QML, as context properties on the engine's root context.

[pure virtual] void QDeclarativeExtensionPlugin::registerTypes(const char * uri)

Registers the QML types in the given uri. Subclasses should implement this to call qmlRegisterType() for all types which are provided by the extension plugin.

The uri is an identifier for the plugin generated by the QML engine based on the name and path of the extension's plugin library.

© 2016 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.