QQmlEngineExtensionPlugin Class

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

Header: #include <QQmlEngineExtensionPlugin>
CMake: find_package(Qt6 REQUIRED COMPONENTS Qml)
target_link_libraries(mytarget PRIVATE Qt6::Qml)
qmake: QT += qml
Since: Qt 5.14
Inherits: QObject

Public Functions

QQmlEngineExtensionPlugin(QObject *parent = nullptr)

Reimplemented Public Functions

virtual void initializeEngine(QQmlEngine *engine, const char *uri) override

Macros

Q_IMPORT_QML_PLUGIN(PluginName)

Detailed Description

QQmlEngineExtensionPlugin is a plugin interface that lets you 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:

  1. Subclass QQmlEngineExtensionPlugin and use the Q_PLUGIN_METADATA() macro to register the plugin with the Qt meta object system.
  2. Use the QML_ELEMENT and QML_NAMED_ELEMENT() macros to declare QML types.
  3. Configure your build file.

    CMake:

    qt_add_qml_module(<target>
        URI <my.import.name>
        VERSION 1.0
        QML_FILES <app.qml>
        NO_RESOURCE_TARGET_PATH
    )

    qmake:

    CONFIG += qmltypes
    QML_IMPORT_NAME = <my.import.name>
    QML_IMPORT_MAJOR_VERSION = <version>
  4. If you're using qmake, create a qmldir file to describe the plugin. Note that CMake will, by default, automatically generate the qmldir file.

QML extension plugins are for 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.

Note: When using the CMake qt_add_qml_module API, a plugin will be generated automatically for you. It will take care of type registration. You only need to write a custom plugin if you have special requirements, such as registering custom image providers. In that case, pass NO_GENERATE_PLUGIN_SOURCE to the qt_add_qml_module call to disable the generation of the default plugin.

The linker might erroneously remove the generated type registration function as an optimization. You can prevent that by declaring a synthetic volatile pointer to the function somewhere in your code. If your module is called "my.module", you would add the forward declaration in global scope:

void qml_register_types_my_module();

Then add the following snippet of code in the implementation of any function that's part of the same binary as the registration:

volatile auto registration = &qml_register_types_my_module;
Q_UNUSED(registration);

TimeExample QML Extension Plugin

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)
};

Build Settings for the Plugin

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
    AUTO_RESOURCE_PREFIX
    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

This registers the TimeModel class, with the import TimeExample 1.0, as a QML type called Time. The Defining QML Types from C++ article has more information about registering C++ types for usage in QML.

Plugin Definition in the qmldir

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() and How to Create Qt Plugins.

Member Function Documentation

QQmlEngineExtensionPlugin::QQmlEngineExtensionPlugin(QObject *parent = nullptr)

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.

[override virtual] void QQmlEngineExtensionPlugin::initializeEngine(QQmlEngine *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.

Macro Documentation

[since 6.2] Q_IMPORT_QML_PLUGIN(PluginName)

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 Q_IMPORT_PLUGIN.

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