Passing data structures from QML to C++

mzimmers

Hi all -

I'm posting this in Brainstorming because I'm not sure whether it's a C++ question or a QML question, or somewhere in between.

I have a C++ class that maintains a QList of a struct I've defined. I need to add a (populated) element to this list from QML. How is this typically accomplished? Should I try to make my QML "struct aware" (something I don't know how to do, or am I better off updating the C++ class one element at a time?

I hope this makes sense. Here's what my struct looks like (currently defined outside of any class):

struct Activity {
    Q_GADGET
    Q_PROPERTY(QString name MEMBER m_name)
    Q_PROPERTY(QList<QString> equipment MEMBER m_equipment)
    Q_PROPERTY(bool recommended MEMBER m_recommended)
public:
    QString m_name;
    QList<QString> m_equipment;
    bool m_recommended;
};
Q_DECLARE_METATYPE(Activity)

JKSH

@mzimmers said in Passing data structures from QML to C++:

Here's my attempt to create an Activity instance:
Activity {}

Use a helper C++ function that returns a default Activity object: https://stackoverflow.com/questions/42797359/how-to-create-new-instance-of-a-q-gadget-struct-in-qml

mzimmers

When I try to access this struct in QML, I get an error. Here's my attempt to create an Activity instance:

Activity {}

At run-time, I get this error: Activity is not a type.

I also have a method in my C++ class:

Q_INVOKABLE void appendItem(Activity item) { m_list->appendItem(item); }

which I try to use like so:

activityModel.appendItem("")

which causes the run-time error:
Could not convert argument 0 at expression for onClicked: TypeError: Passing incompatible arguments to C++ functions from JavaScript is not allowed.
I tried this and got the same error as above:

activityModel.appendItem({name: "from", equipment: ["QML", "YAY"], recommended: false})

So...I'm not sure where to go from here. I suppose I could declare an Activity object in my class, that would be used only for the QML interface, but that sounds like a hack. Any other ideas on this?

Thanks...

JoeCFD

Struct is a class in C++ and treat it as a class.
https://stackoverflow.com/questions/45650277/best-way-to-access-a-cpp-structure-in-qml

mzimmers

I tried that:

class Activity { //: public QObject {
//    Q_GADGET
//    Q_OBJECT
//    Q_PROPERTY(QString name MEMBER m_name)
//    Q_PROPERTY(QList<QString> equipment MEMBER m_equipment)
//    Q_PROPERTY(bool recommended MEMBER m_recommended)
public:
    QString m_name;
    QList<QString> m_equipment;
    bool m_recommended;
};

and I've registered Activity in main.cpp:

    qRegisterMetaType<Activity>("Activity");

but when I try to use it in my QML file:

import ActivityList // where Activity is defined
Activity {}

I get an error: "Activity is not a type."

JoeCFD

@mzimmers you need Q_OBJECT in your class.
qmlRegisterType may be good enough.

mzimmers

But if I base my class off of Q_OBJECT, I lose the copy constructor and assignment operator. Do I need to create those for my Activity class?

JoeCFD

@mzimmers Why do you lose them?

mzimmers

https://stackoverflow.com/questions/19092513/copy-constructor-of-derived-qt-class

JKSH

@mzimmers said in Passing data structures from QML to C++:

Here's my attempt to create an Activity instance:
Activity {}

Use a helper C++ function that returns a default Activity object: https://stackoverflow.com/questions/42797359/how-to-create-new-instance-of-a-q-gadget-struct-in-qml

JKSH

(Further questions forked to https://forum.qt.io/topic/142227/update-c-list-model-from-qml )