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:
\list 1
\li Subclass QQmlExtensionPlugin
    \list
        \li Use the Q_PLUGIN_METADATA() macro to register the plugin with
            the Qt meta object system
        \li Override the \l{QQmlExtensionPlugin::}{registerTypes()} method
            and call qmlRegisterType() to register the types to be exported
            by the plugin
    \endlist
\li Write a project file for the plugin
\li Create a \l{Module Definition qmldir Files}{qmldir file} to
    describe the plugin
\endlist

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.

\section1 Plugin Example

Suppose there is a new \c TimeModel C++ class that should be made available
as a new QML type. It provides the current time through \c hour and \c minute
properties.

\snippet qmlextensionplugins/plugin.cpp 0
\dots

To make this type available, we create a plugin class named \c QExampleQmlPlugin
which is a subclass of \l QQmlExtensionPlugin. It overrides the
\l{QQmlExtensionPlugin::}{registerTypes()} method in order to register the \c 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.

\snippet qmlextensionplugins/plugin.cpp plugin

This registers the \c TimeModel class with version \c{1.0} of this plugin library, as
a QML type called \c Time. The Q_ASSERT() macro can ensure the type namespace is
imported correctly by any QML components that use this plugin. The
\l{Defining QML Types from C++} article has more information about registering C++
types into the runtime.

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

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

DESTDIR = imports/TimeExample
TARGET = qmlqtimeexampleplugin
SOURCES += qexampleqmlplugin.cpp
\endcode

Finally, a \l{Module Definition qmldir Files}{qmldir file} is required
in the \c imports/TimeExample directory to describe the plugin and the types that it
exports. The plugin includes a \c Clock.qml file along with the \c qmlqtimeexampleplugin
that is built by the project (as shown above in the \c .pro file) so both of these
need to be specified in the \c qmldir file:

\quotefile qmlextensionplugins/imports/TimeExample/qmldir

To make things easier for this example, the TimeExample source directory is in
\c{imports/TimeExample}, and we build
\l{Source, Build, and Install Directories}{in-source}. However, the structure
of the source directory is not so important, as the \c 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
\l{Contents of a Module Definition qmldir File}{module identifier}
(\c 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 \c Time component is
accessible by any QML component that imports the \c TimeExample
module

\snippet qmlextensionplugins/plugins.qml 0

The full source code is available in the \l {qmlextensionplugins}{plugins example}.
