- Qt 5.3
- Qt QML
- C++ Classes
- QQmlExtensionPlugin
- Qt 5.3.2 Reference Documentation

# QQmlExtensionPlugin Class

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

Header: | `#include <QQmlExtensionPlugin>`
qmake: | `QT += qml`
Since: | Qt 5.0

- List of all members, including inherited members

## Public Functions

| QQmlExtensionPlugin(QObject * parent = 0)
QUrl | baseUrl() const

## Reimplemented Public Functions

virtual void | initializeEngine(QQmlEngine * engine, const char * uri)
virtual void | registerTypes(const char * uri) = 0

## Detailed Description

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

QQmlExtensionPlugin 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:

- Subclass QQmlExtensionPlugin, implement registerTypes() method to register types using qmlRegisterType(), and export the class using the Q_PLUGIN_METADATA() macro
- Write an appropriate project file for the plugin
- Create a qmldir file to describe the 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_OBJECT
        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 `TimeExample` (as defined in the project file further below).

    class QExampleQmlPlugin : public QQmlExtensionPlugin
    {
        Q_OBJECT
        Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QQmlExtensionInterface")

    public:
        void registerTypes(const char *uri)
        {
            Q_ASSERT(uri == QLatin1String("TimeExample"));
            qmlRegisterType<TimeModel>(uri, 1, 0, "Time");
        }
    };

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 `imports/TimeExample` directory:

    TEMPLATE = lib
    CONFIG += qt plugin
    QT += qml

    DESTDIR = imports/TimeExample
    TARGET = qmlqtimeexampleplugin
    ...

Finally, a qmldir file is required in the `imports/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:

    module TimeExample
    Clock 1.0 Clock.qml
    plugin qmlqtimeexampleplugin

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

    import TimeExample 1.0 // import types from the plugin

    Clock { // this class is defined in QML (imports/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 Writing QML Extensions with C++ tutorial also contains a chapter on creating QML plugins.

Note that the Qt Quick 1 version is called QDeclarativeExtensionPlugin.

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

## Member Function Documentation

### QQmlExtensionPlugin::QQmlExtensionPlugin(QObject * parent = 0)

Constructs a QML extension plugin with the given parent.

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

### QUrl QQmlExtensionPlugin::baseUrl() const

Returns the URL of the directory from which the extension is loaded.

This is useful when the plugin also needs to load QML files or other assets from the same directory.

This function was introduced in Qt 5.1.

### void QQmlExtensionPlugin::initializeEngine(QQmlEngine * engine, const char * uri)`[virtual]`

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.

### void QQmlExtensionPlugin::registerTypes(const char * uri)`[pure virtual]`

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.