Calling function from object of object in QML results in TypeError

[prex]

I'm trying to connect a function of a C++ object of another C++ object to my QML UI. The first C++ object lives in a separate QThread. The structure is as follows:

• main.qml
• main.cpp
  • Base class, QThread
    • Hardware class

Which means that the Hardware class is just an object created in the QThread Base.

I would like to connect my QML UI to the functions of the Hardware class. So I set the context property:

Base* base = new Base(argc, argv);
engine.rootContext()->setContextProperty("base", base);

I was not able to connect to the functions of the Hardware class, e.g.

onClicked: base.hardware.resetSystem()

fails with

qrc:/SideBarForm.ui.qml:95: TypeError: Cannot call method 'resetSystem' of undefined

But I can successfully call functions of the base class, e.g.

onClicked: base.test()

And calling the function from main.cpp works also fine:

base->hardware->resetSystem();

Furthermore, hardware is public in Base and resetSystem() is a public slot in Hardware:

public:
  std::shared_ptr<Hardware> hardware;

--

public slots: 
  void resetSystem();

What is the proper way of connecting signals and slots from "sub-objects" in QThreads to a QML interface?

[ODБOï]

@prex one simple way is to add a Q_INVOKABLE method (or public slot) to Base

Q_INVOKABLE void resetHardwareSys(){
 hardware->resetSystem();
}

or you can expose your Hardware to QML adding a Q_PROPERTY for it and returning it as a QVariant (it is not enough to set it public to access it in QML )

[prex]

@LeLev Thanks, this works now. So the first solution means going over two public slots. I'm wondering about how to handle signals from C++ to QML then. First using QObject::connect() and then the Connections QML type? I have lots of data from the Hardware class which I need to visualize in the UI.

I was reading many posts about signals & slots not working between QML and QThreads. I'm a little bit confused since it seems to work here...

[prex]

I like the Q_PROPERTY approach in general but I don't understand what you mean by

@LeLev said in Calling function from object of object in QML results in TypeError:

you can expose your Hardware to QML adding a Q_PROPERTY for it and returning it as a QVariant (it is not enough to set it public to access it in QML )

At the moment I have different Q_PROPERTIES defined in my Hardware class, but how can I make them accessible by QML? Or do you mean adding a Q_PROPERTY for the whole Hardware class?

[ODБOï]

@prex said in Calling function from object of object in QML results in TypeError:

you mean adding a Q_PROPERTY for the whole Hardware class?

yes,
for the getter method, don't return the hardware object directly, use QVariant::fromValue to return it as a QVariant

[prex]

@LeLev I‘m not sure if I understand correctly. Should I do something like

Q_PROPERTIES(Hardware hardware ...)

in Base? But how to access this then? Can you please elaborate a little bit more on it?

[ODБOï]

@prex hi

class Hardware : public QObject
{
    Q_OBJECT
public:
    explicit Hardware (QObject *parent = nullptr);
public slots:

    void write(const QString &str){
        qDebug()<< str;
    }
};

class Example : public QObject
{
    Q_OBJECT
    Q_PROPERTY( Hardware * hardware READ hardware NOTIFY hardwareChanged)

public:
    explicit Example(QObject *parent = nullptr);

    Hardware * hardware (){
        return  m_hardware;
    }
signals:
    void hardwareChanged();

private:
    Hardware *m_hardware;
};

 Example e;
    engine.rootContext()->setContextProperty("obj",&e);

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Component.onCompleted: obj.hardware.write("hello")

}

[prex]

@LeLev Thank you very much for the code sample!