Creating C++ Plugins for QML | Qt QML 5.5

- Qt 5.5
- Qt QML
- Creating C++ Plugins for QML
- Qt 5.5.1 Reference Documentation

# Creating C++ Plugins for QML

## Creating a Plugin

The QML engine loads C++ plugins for QML. Such plugins are usually provided in a QML extension module, and can provide types for use by clients in QML documents which import the module. A module requires at least one type registered in order to be considered valid.

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:

1. Subclass QQmlExtensionPlugin
   - Use the Q_PLUGIN_METADATA() macro to register the plugin with the Qt meta object system
   - Override the registerTypes() method and call qmlRegisterType() to register the types to be exported by the plugin
2. Write a project file for the plugin
3. Create a qmldir file to describe the plugin

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.

## Plugin Example

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.

    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 type available, we create a plugin class named `QExampleQmlPlugin` which is a subclass of QQmlExtensionPlugin. It overrides the registerTypes() method in order to register the `TimeModel` type using qmlRegisterType(). It also 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 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 version `1.0` of this plugin library, as a QML type called `Time`. The Q_ASSERT() macro can ensure the type namespace is imported correctly by any QML components that use this plugin. The Defining QML Types from C++ article has more information about registering C++ types into the runtime.

Additionally, the project file (`.pro`) defines the project as a plugin library, specifies it should be built into the `imports/TimeExample` directory, and registers the plugin target name and various other details:

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

    DESTDIR = imports/TimeExample
    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 (as shown above in the `.pro` file) so both of these need to be specified 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 so 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, and 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 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.

## Reference

- Writing QML Extensions with C++ - contains a chapter on creating QML plugins.
- Defining QML Types from C++ - information about registering C++ types into the runtime.
- How to Create Qt Plugins - information about Qt plugins