Michael Hasselmann

Postings by Michael on July 14, 2011

Using C++ enums in QML

When mapping Qt/C++ API's to QML, or, to put it more precisely, making a Qt/C++ API available to QML, road bumps are to be expected. One such bump is the mapping of C++ enums.

If you happen to create enums inside a QObject, then it will be exported to QML via the Q_ENUMS helper:

SomeEnumsWrapper
    : public QObject
{
    Q_OBJECT
    Q_ENUMS(SomeState)
public:
    enum SomeState {
        BeginState,        // Remember that in QML, enum values must start
        IntermediateState, // with a capital letter!
        EndState
    };
};

You will still need to declare this class as an abstract type for QML to be able to use enums from it (put in your main function for example):

qmlRegisterUncreatableType<SomeEnumsWrapper>("com.mydomain.myproject", 1, 0,
                                             "SomeEnums", "This exports SomeState enums to QML");

Now in QML, the enums can be accessed as '''SomeEnums.BeginState'''. Note how the enum is accessed through the exported type name, not an instance.

But what if you've put your enums into a dedicated C++ namespace? Then the same mechanism can be used. Let's start with the namespace:

namespace SomeEnums {
    enum SomeState {
        BeginState,
        IntermediateState,
        EndState
    };
}

We can re-use the idea of wrapping enums in a QObject type, with one tiny change:

SomeEnumsWrapper
    : public QObject
{
    Q_OBJECT
    Q_ENUMS(SomeState)
public:
    enum SomeState {
        BeginState = SomeEnums::BeginState, // Keeps enum values in sync!
        IntermediateState = SomeEnums::IntermediateState,
        EndState = SomeEnums::EndState
    };
};

The process of forwarding all your enums through this mechanism can be tedious, but being able to use enums properly in QML properly will improve the readability and maintainability of your QML code.

For a fully working example check Maliit's Qt Quick support