QQmlEngineExtensionPlugin Class

Suppose there is a new TimeModel C++ class that should be made available as a new QML type. It provides the current time through hour and minute properties. It declares a QML type called Time via QML_NAMED_ELEMENT().

class TimeModel : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int hour READ hour NOTIFY timeChanged)
    Q_PROPERTY(int minute READ minute NOTIFY timeChanged)
    QML_NAMED_ELEMENT(Time)
    ...

To make this type available, create a plugin class named QExampleQmlPlugin, which is a subclass of QQmlEngineExtensionPlugin. It uses the Q_PLUGIN_METADATA() macro in the class definition to register the plugin with the Qt meta object system using a unique identifier for the plugin.

class QExampleQmlPlugin : public QQmlEngineExtensionPlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID QQmlEngineExtensionInterface_iid)
};

The build file defines the project as a plugin library, specifies it should be built into the imports/TimeExample directory, and registers the plugin target name.

Using CMake:▲

set(qml_files
    imports/TimeExample/Clock.qml
)

set(images
    imports/TimeExample/center.png
    imports/TimeExample/clock.png
    imports/TimeExample/hour.png
    imports/TimeExample/minute.png
)
qt_add_qml_module(qmlqtimeexample
    OUTPUT_DIRECTORY imports/TimeExample
    VERSION 1.0
    URI "TimeExample"
    SOURCES timemodel.cpp timemodel.h
    QML_FILES ${qml_files}
    RESOURCES ${images}
)

Using qmake:▲

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

QML_IMPORT_NAME = TimeExample
QML_IMPORT_MAJOR_VERSION = 1

DESTDIR = imports/$$QML_IMPORT_NAME
TARGET  = qmlqtimeexampleplugin

SOURCES += qexampleqmlplugin.cpp

Finally, a qmldir file is required in the imports/TimeExample directory to describe the plugin and the types that it exports. The plugin includes a Clock.qml file along with the qmlqtimeexampleplugin that is built by the project.

CMake will, by default, automatically generate this file. For more information, see Auto-generating qmldir and typeinfo files.

When using qmake, specify the following in the qmldir file:

module TimeExample
Clock 1.0 Clock.qml
plugin qmlqtimeexampleplugin

To make things easier for this example, the TimeExample source directory is in imports/TimeExample, and we build in-source. However, the structure of the source directory is not important, as the qmldir file can specify paths to installed QML files.

What is important is the name of the directory that the qmldir is installed into. When the user imports our module, the QML engine uses the module identifier (TimeExample) to find the plugin, so the directory in which it is installed must match the module identifier.

Once the project is built and installed, the new Time component is accessible by any QML component that imports the TimeExample module.

import TimeExample // 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.

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

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.

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.

Ensures the plugin whose metadata-declaring class is named PluginName is linked into static builds.

This macro was introduced in Qt 6.2.

See Also▲