QML和C ++：如何将对导出的C ++对象的引用传递给导出的C ++类？

Question

I exported a C++ class and object like so:

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QtQuick2ApplicationViewer viewer;
    viewer.setMainQmlFile(QStringLiteral("qml/QQuickViewExample/main.qml"));
    viewer.showExpanded();

    //object export
    ClassA classA(QString stuff);
    QQmlContext* context = viewer.engine()->rootContext();
    context->setContextProperty("_classA", &classA);

    //class export
    qmlRegisterType<ClassB>("CustomComponents", 1, 0, "ClassB");


    return app.exec();
}

I will create an arbitrary number of QML objects with ClassB embedded inside. I want every ClassB to have a reference or pointer to the 1 ClassA that exists in this program. How can I, in QML , grab a reference of ClassA when I am initializing ClassB in my QML objects, so that ClassB can use ClassA ?

Answer 1

You have to change the order of actions:

1. register B
2. set A instance on the root context

3. load the source, where you instantiate (as many times as you need):

   B { aPtr: _classA }

The rationale is:

• for smth to appear in QML sources and be used to instantiate classes, those classes have to be registered first (a C++ QML plugin could do that too)
• if items are relying on existence of a context property, they should be created only after the context property has been set

Given that, steps (1) and (2) above can actually be swapped.

There are ways to create, parent and wire everything by hand from C++, I believe, but that would be an example of extremely dark art (think creating bindings manually from JS expressions, creating a context, then instantiating items into it...); never seen that in the wild :)