“Read-only property” error when using QQmlListProperty with custom class in QML

yiyangfei

I am following the "example":http://qt-project.org/doc/qt-5/qtqml-tutorials-extending-chapter6-plugins-example.html to expose a custom type as a QQmlListProperty in QML. At runtime, when initializing the list property, I get the error Invalid property assignment: "overlayImages" is a read-only property.

This is what I want to do in QML:

@TestPattern {
id: pattern

overlayImages: [
    OverlayImage {
        x: 100
        y: 100
        alpha: 0.1
        source: "../../images/fail.png"
        visible: true
    }
]

}@

Here are my custom type declarations:

@class TestPatternQuickPrivate;
class QMLTESTPATTERN_EXPORT TestPatternQuick : public QQuickItem
{
Q_OBJECT
Q_DECLARE_PRIVATE(TestPatternQuick)
Q_DISABLE_COPY(TestPatternQuick)
Q_PROPERTY(QQmlListProperty<OverlayImage> overlayImages READ overlayImages)

public:
    TestPatternQuick(QQuickItem *parent = Q_NULLPTR);
    virtual ~TestPatternQuick();    
    QQmlListProperty<OverlayImage> overlayImages();

private:
    static void appendOverlayImage(QQmlListProperty<OverlayImage> *list, OverlayImage *overlayImage);
    QScopedPointer<TestPatternQuickPrivate> d_ptr; //QScopedPointer guarantees no throw
};

class OverlayImagePrivate;
class QMLTESTPATTERN_EXPORT OverlayImage : public QQuickItem
{
    Q_OBJECT
    Q_DECLARE_PRIVATE(OverlayImage)
    
public:
    OverlayImage(QQuickItem *parent = Q_NULLPTR);
    OverlayImage(const OverlayImage &other);
    virtual ~OverlayImage();
    OverlayImage &operator=(const OverlayImage &other);

private:
    QScopedPointer<OverlayImagePrivate> d_ptr; //QScopedPointer guarantees no throw
};@

...and C++ plugin registration implementation:

@void QmlTestPatternPlugin::registerTypes(const char *uri)
{
qmlRegisterType<TestPatternQuick>(uri, 1 /major/, 0 /minor/, "TestPattern");
qmlRegisterType<OverlayImage>(uri, 1 /major/, 0 /minor/, "OverlayImage");
}@

Can anyone shed light on what I'm still missing?

dheerendra

You are making the overlay images are read only.

@Q_PROPERTY(QQmlListProperty<OverlayImage> overlayImages READ overlayImages)@

You need to add the write function for this. Have look at how to declare Q_PROPERTY with read, write, signals etc.

yiyangfei

Based on the example, it appears that the QQmlListProperty uses function pointers rather than getter/setter to access the property. I thought this was strange too and not very Qt.

dheerendra

There is one specific example given in the Qt installation under examples. You can look at. It may give you good pointers for you.

Qt5.3.1/Examples/Qt-5.3/qml/referenceexamples/properties/birthdayparty*

swegmann

I'm currently running into the very same issue as yiyangfei and I even looked at the example Dheerendra specified. In this "Birthdayparty example":http://doc.qt.io/qt-5/qtqml-referenceexamples-properties-example.html however also no WRITE part for the Q_PROPERTY is specified. And frustratingly the Birthday example compiles and runs whereas my solution complains about the property being read-only. I wonder if it has to do with the way BirthdayParty is instantiated!?

swegmann

I'm currently running into the very same issue as yiyangfei and I even looked at the example Dheerendra specified. In this "Birthdayparty example":http://doc.qt.io/qt-5/qtqml-referenceexamples-properties-example.html however also no WRITE part for the Q_PROPERTY is specified. And frustratingly the Birthday example compiles and runs whereas my solution complains about the property being read-only. I wonder if it has to do with the way BirthdayParty is instantiated!?

swegmann

Ok, I think at least in my case I found the reason for the problems, maybe this also helps someone else:

The type I used as template parameter for QQmlListProperty<> was in a namespace. And even though it was in the same namespace as the class containing the QQmlListProperty I still needed to specify the full namespaces in the Q_PROPERTY declaration.

There is actually a wiki page about this including the "Birthday example":http://qt-project.org/wiki/How_to_use_a_C_class_declared_in_a_namespace_with_Q_PROPERTY_and_QML.

swegmann

Ok, I think at least in my case I found the reason for the problems, maybe this also helps someone else:

The type I used as template parameter for QQmlListProperty<> was in a namespace. And even though it was in the same namespace as the class containing the QQmlListProperty I still needed to specify the full namespaces in the Q_PROPERTY declaration.

There is actually a wiki page about this including the "Birthday example":http://qt-project.org/wiki/How_to_use_a_C_class_declared_in_a_namespace_with_Q_PROPERTY_and_QML.

linoleo

@swegmann I ran into the same problem, and fully qualifying my namespaced payload type in the QQmlListProperty<> template inside the Q_PROPERTY() declaration indeed solved it for me. Thanks!

jdub

I ran into exactly the same problem and this comment saved me, @swegmann. Thank you. I reported it as a bug on the bugtracker: https://bugreports.qt.io/browse/QTBUG-84263