Access read-only C++ property from QML

Tom asso

I have defined a C++ class SharedConstants, which contains a Q_PROPERTY constant QString value. I want to read this property but not write it from QML.

Here is my C++ class:

/// Constants shared between C++ and QML code
class SharedConstants : public QObject
{
    Q_OBJECT

public:
    
  /// Define read-only QString property called "testString"
  Q_PROPERTY(QString testString READ getTestString)

  /// Just return a dummy string for now...
  QString getTestString() const {
      std::cout << "**** getTestString()\n";
      return QString("Dummy value");
  }
};

Here is my QML:

ApplicationWindow {
    [...]
    property string testString: SharedConstants.testString

    Component.onCompleted: {console.log("onCompleted");
                            console.log("testString: ", testString)}

So I expect testString = “Dummy value” in the QML.

I find that QML can access testString from QML if I set a SharedConstant instance as a context property instead of registering it - i.e. in main.cpp:

SharedConstants constants;
engine.rootContext()->setContextProperty("SharedConstants", &constants);

QML output:
qml: testString: Dummy value

But I will be adding other stuff to SharedConstants (e.g. enum) so I must actually register SharedConstants (as described here) in main.cpp:

// Register SharedConstants so QML can access its properties
qmlRegisterType<SharedConstants>("SharedConstants", 1, 0,
                                 "SharedConstants");

(and I add "import SharedConstants 1.0" to the QML file.)
But now the QString testString is blank/null when accessed from QML :

qml: testString:

Debugging shows that the designated C++ accessor function getTestString() is not invoked either.
What am I doing wrong here?

Thanks!

jeremy_k

@Tom-asso said in Access read-only C++ property from QML:

Here is my QML:

ApplicationWindow {
    [...]
    property string testString: SharedConstants.testString

    Component.onCompleted: {console.log("onCompleted");
                            console.log("testString: ", testString)}

But I will be adding other stuff to SharedConstants (e.g. enum) so I must actually register SharedConstants (as described here) in main.cpp:

// Register SharedConstants so QML can access its properties
qmlRegisterType<SharedConstants>("SharedConstants", 1, 0,
                                 "SharedConstants");

Debugging shows that the designated C++ accessor function getTestString() is not invoked either.
What am I doing wrong here?

qmlRegisterType registers an instantiable type, but the QML code above attempts to use it as a singleton. Either instantiate an instance of the type in QML, or use one of the singleton registration methods such as QML_SINGLETON, qmlRegisterSingletonInstance, or qmlRegisterSingletonType.