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.

QQmlEngineExtensionPlugin 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 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. Write a project file for the plugin. Add:
   • CONFIG += qmltypes to instruct the build system to generate QML types.
   • QML_IMPORT_NAME = <my.import.name> to specify the import name.
   • QML_IMPORT_MAJOR_VERSION = <version> to specify the import major version.
4. 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.

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, we 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)
 };

Project settings for the plugin

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 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 (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