采用上下文特性把 C++ 对象嵌入 QML

警告： By using context properties in your QML code, you create a dependency from your QML code to the specific context you have in mind when writing it. This limits re-usability of your code since the context may be different in other places where it might be used. Furthermore, the dependency is not declared. You never import the context or otherwise state what you expect. Therefore, anyone trying to re-use your code will have difficulties finding out whether the place of re-use has a context sufficient for your code.

警告： Context properties are invisible to any tooling that processes QML code ahead of time, before you load it into the QML engine. The Qt Quick 编译器 , qmllint ，和 QML 语言服务器 do not know anything about your context properties and will consider any access to context properties as an unqualified access .

注意： Context properties can generally be replaced either by regular properties on the root object of a component, or by singletons defined either in C++ using QML_SINGLETON or in QML using pragma Singleton .

When loading a QML object into a C++ application, it can be useful to directly embed some C++ data that can be used from within the QML code. This makes it possible, for example, to invoke a C++ method on the embedded object, or use a C++ object instance as a data model for a QML view.

The ability to inject C++ data into a QML object is made possible by the QQmlContext class. This class exposes data to the context of a QML object so that the data can be referred to directly from within the scope of the QML code.

设置简单上下文特性

For example, here is a QML item that refers to a currentDateTime value that does not exist in the current scope:

// MyItem.qml
import QtQuick
Text { text: currentDateTime }

This currentDateTime value can be set directly by the C++ application that loads the QML component, using QQmlContext::setContextProperty ():

QQuickView view;
view.rootContext()->setContextProperty("currentDateTime", QDateTime::currentDateTime());
view.setSource(QUrl::fromLocalFile("MyItem.qml"));
view.show();

注意： Since all expressions evaluated in QML are evaluated in a particular context, if the context is modified, all bindings in that context will be re-evaluated. Thus, context properties should be used with care outside of application initialization, as this may lead to decreased application performance.

把对象设为上下文特性

Context properties can hold either QVariant or QObject * values. This means custom C++ objects can also be injected using this approach, and these objects can be modified and read directly in QML. Here, we modify the above example to embed a QObject instance instead of a QDateTime value, and the QML code invokes a method on the object instance:

C++

class ApplicationData : public QObject
{
    Q_OBJECT
public:
    Q_INVOKABLE QDateTime getCurrentDateTime() const {
        return QDateTime::currentDateTime();
    }
};
int main(int argc, char *argv[]) {
    QGuiApplication app(argc, argv);
    QQuickView view;
    ApplicationData data;
    view.rootContext()->setContextProperty("applicationData", &data);
    view.setSource(QUrl::fromLocalFile("MyItem.qml"));
    view.show();
    return app.exec();
}

QML

// MyItem.qml
import QtQuick
Text { text: applicationData.getCurrentDateTime() }

(Note that date/time values returned from C++ to QML can be formatted through Qt.formatDateTime () and associated functions.)

If the QML item needs to receive signals from the context property, it can connect to them using the Connections 类型。例如，若 ApplicationData has a signal named dataChanged() , this signal can be connected to using an onDataChanged handler within a Connections 对象：

Text {
    text: applicationData.getCurrentDateTime()
    Connections {
        target: applicationData
        onDataChanged: console.log("The application data changed!")
    }
}

Context properties can be useful for using C++ based data models in a QML view. See the following examples:

demonstrating the use of QStringList , QList < QObject *>-based models and QAbstractItemModel in QML views.

另请参阅 QQmlContext 文档编制，了解更多信息。