Q_ENUMS are “undefined” in QML?

问题:

Enums are not working out for me.

• I have registered them with Q_ENUMS()
• I did not forget the Q_OBJECT macro
• the type is registered using qmlRegisterType()
• the module is imported in QML

In short, everything is "by-the-book" but for some reason I continue getting undefined for each and every enum in QML. Am I missing something?

class UI : public QQuickItem {
    Q_OBJECT
    Q_ENUMS(ObjectType)
public:
enum ObjectType {
        _Root = 0,
        _Block
    };
...
};

...

qmlRegisterType<UI>("Nodes", 1, 0, "UI");

...

import Nodes 1.0
...
console.log(UI._Root) // undefined

EDIT: Also note that the registered enums are indeed available to use for the metasystem, for some reason they do not work in QML.

UPDATE: I just found this bug: https://bugreports.qt.io/browse/QTBUG-33248 But unlike that bug my root component is a bare UI not a custom element with UI as its root.

Turns out that it is actually possible to use enum values form QML in console.log(), the following code is actually working.

class A : public QObject {
    Q_OBJECT
    Q_ENUMS(EA)
public:
    enum EA {
        EA_NULL = 0,
        EA_ONE
    };
};

class B : public A {
    Q_OBJECT
    Q_ENUMS(EB)
public:
    enum EB {
        EA_TWO = 2,
        EA_THREE
    };
};

#include "main.moc"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    qmlRegisterType<A>("test", 1, 0, "A");
    qmlRegisterType<B>("test", 1, 0, "B");

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

    return app.exec();
}

and...

Component.onCompleted: {
        console.log(A.EA_NULL)
        console.log(A.EA_ONE)

        console.log(B.EA_NULL)
        console.log(B.EA_ONE)
        console.log(B.EA_TWO)
        console.log(B.EA_THREE)
    }

Output is:

0
1
0
1
2
3

So I guess there is another problem besides "you are not using it correctly"... It might have to do with the bug I mentioned above, and the fact that when I instantiate the UI element, I actually instantiate a QML component which is a tree of objects with the UI as the root. While this doesn't prove to be any problem for working with pointers from C++ with the full QML objects, it does seem to mess enums for some reason.

回答1:

Your problem is not the exposure of the enum, but the fact that you have a leading underscore. Once you remove that, it will work.

You need to start the enum value with uppercase letter. Some rule is necessary to distinguish enums from attached properties from enums. Leading uppercase will refer to enums, and the rest for attached properties (or undefined if not set).

Admittedly, there is also a warning in Qt itself because if you try to assign that enum value to an int or var property, you are currently not getting a warning, and having discussed that issue a little bit with the current maintainer, it seems to be a bug which will be fixed later on.

See the working code below with the correspondigly proposed solution:

main.cpp

#include <QQuickView>
#include <QQuickItem>

#include <QGuiApplication>

#include <QUrl>

class UI : public QQuickItem {
    Q_OBJECT
    Q_ENUMS(ObjectType)
public:
enum ObjectType {
        Root = 0,
        _Block
    };
};

#include "main.moc"

int main(int argc, char **argv)
{
    QGuiApplication guiApplication(argc, argv);
    qmlRegisterType<UI>("Nodes", 1, 0, "UI");
    QQuickView *view = new QQuickView;
    view->setSource(QUrl::fromLocalFile("main.qml"));
    view->show();
    return guiApplication.exec();
}

main.qml

import Nodes 1.0
import QtQuick 2.0

Rectangle {
    id: button
    width: 500; height: 500

    MouseArea {
        anchors.fill: parent
        onClicked: console.log(UI.Root)
    }
}

main.pro

TEMPLATE = app
TARGET = main
QT += quick
SOURCES += main.cpp

Build and Run

qmake && make && ./main

Output

0

回答2:

I had the exact same problem, and thanks to these solutions I figured out my problem. However, in case anyone else is dealing with the same mixup, here it is a little clearer.

I was using a class from C++ in QML just like here, so I had something like this in main.cpp

qmlRegisterType<EnumClass>("Enums", 1, 0, "EnumClass");

and in the .qml

import Enums 1.0

EnumClass {
    id: ec
}

and tried and tried to access ec.SomeEnum but kept getting undefined even though, the QtCreator autocomplete said ec.SomeEnum should exist.

This simply does not work, and to get this to work I had to use

EnumClass.SomeEnum

instead (just like they do here).