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.