如何从 QML 访问在 C++ class 中声明的 Q_ENUM？

Question

I have a Q_ENUM declared in MyClass like below:

class MyClass {
public:
    enum Enum_Test {
        eTestA,
        eTestB
    }
    Q_ENUM(Enum_Test)

    Q_OBJECT
    Q_PROPERTY(MyClass::Enum_Test enumTest READ GetEnumTest WRITE SetEnumTest )
}

I have MyClass registered on the QML side like below and able to access it.

auto my_class = std::make_shared<MyClass>();
qmlRegisterUncreatableType<MyClass>("MyClass", 1,0, "MyClass","Cannot create type MyClass in QML");
rootContext()->setContextProperty("my_class", my_class.get());

How do I access Q_ENUM Enum_Test from QML?

Answer 1

Your class needs two adjustments.

1. as pointed out by JarMan, it needs a metaObject, which can be obtained by deriving from QObject and adding Q_OBJECT:

    class MyClass: public QObject { Q_Object... }

   Actually, it would also be possible to use Q_GADGET but you already seem to lean towards Q_OBJECT . But, as requested, here we go:

   class MyClass { Q_GADGET

    public: enum Enum_Test { ETestA, ETestB } Q_ENUM(Enum_Test)

   };

   Keep in mind that Q_GADGET cannot have signals , so I left out the property and only have this class as "enum-placeholder".

2. The enum value names need to be capitalized:

    enum Enum_Test { ETestA, ETestB } Q_ENUM(Enum_Test)

Then you can use it in QML as:

     QtObject {
         property int myEnumVal: MyClass.ETestA
     }

Note that support for enums is somewhat limited because the mix with JavaScript. The values will be converted to integers. Also, when used in a JavaScript switch-statement, typo's will not be warned about by QtCreator (assuming version 4.14)

Answer 2

The answer provided by @Amfasis is already good. As I tend to do this pretty often, I wrote a little shortcut for defining enums with all the features Qt offers, including the availability in QML here: https://github.com/carlonluca/lqtutils#lqtutils_enumh .

You only need to include the header, define your enum values like:

L_DECLARE_ENUM(Enum_Test,
    ETestA,
    ETestB
)

and register it where you prefer with:

Enum_Test::qmlRegisterMySharedEnum("some.uri", 1, 0);

Also this uses simple namespaces, which is lighter than using QObjects or gadgets.

Actually, I got used to always declare my enums like this when I'm using Qt, because I can also benefit from QMetaEnum . For instance, I really like to be able to log like this:

qDebug() << "Value:" << Enum_Test::ETestA;

getting:

Value: Enum_Test::ETestA

instead of a simple integer.