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:
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.
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
特性。
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 QQmlExtensionInterface_iid) 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
时间
。
Q_ASSERT
() macro can ensure the type namespace is imported correctly by any QML components that use this plugin. The
从 C++ 定义 QML 类型
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 文件
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
文件：
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
模块标识符
(
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
时间
component is accessible by any QML component that imports the
TimeExample
模块
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 .
