Extension Objects.
此范例构建于：
Shows how to use QML_EXTENDED to provide an extension object 到 QLineEdit without modifying or subclassing it.
Firstly, the LineEditExtension class is registered with the QML system as an extension of QLineEdit . We declare a foreign type to do this as we cannot modify Qt's internal QLineEdit 类。
struct QLineEditForeign { Q_GADGET QML_FOREIGN(QLineEdit) QML_NAMED_ELEMENT(QLineEdit) QML_EXTENDED(LineEditExtension) };
Note the usage of QML_NAMED_ELEMENT () 而不是 QML_ELEMENT . QML_ELEMENT uses the name of the containing type by default, "LineEditExtension" in this case. As the class being an extension class is an implementation detail, we choose the more natural name "LineEdit" instead
The QML engine then instantiates a QLineEdit :
QQmlEngine engine;
QQmlComponent component(&engine, QUrl("qrc:example.qml"));
auto *edit = qobject_cast<QLineEdit *>(component.create());
In QML, a property is set on the line edit that only exists in the LineEditExtension class:
QLineEdit { leftMargin: 20 }
The extension type performs calls on the QLineEdit that otherwise will not be accessible to the QML engine.