Sending a signal to a QML item from C++ (Qt5)

Question 1

However, myElement is always null (and I get a runtime warning about a non existent slot).

You are trying to find the child based on id, whereas it is based on the objectName property. You would need to set the objectName property to the desired to actually find it.

Also, you do not seem to declare the signal in your QML Text item. I am not sure if it is a custom C++ item or a built-in one. You have not shared enough code unfortunately to understand that bit. Either way, declare your signal as per documentation.

Therefore, try this code out:

Text {
    id: testData
    objectName: "testData"
    // ^^^^^^^^^^^^^^^^^^^
    signal taskClicked (HolidayTask task)
    // ^^^^^^^^^^^^^^^^^^^
    onTaskClicked:{
        testData.text = task.name
    }
}

Once that is done, you are almost ready. You need to have your HolidayTask registered to QML for sure, and you also need to change the connect syntax in your main.cpp as follows:

connect(m_view, SIGNAL(taskClicked(HolidayTask* task), myElement, SIGNAL(taskClicked(HolidayTask* task)));

In short, you need to trigger your QML signal handler this way, and not via SLOT.

Also, note that your connect syntax is broken as it is missing the closing brackets at the end. That needs to be fixed.

I would even consider removing the pointer assignment and use value or reference based passing for this.

Question 2

You can connect a signal from C++ to QML by:

 view->rootContext()->setContextProperty("testData",this);
 QObject::connect(this,SIGNAL(taskClicked(HolidayTask* task)),(QObject *)view->rootObject(),SLOT(onTaskClicked(HolidayTask* task)));

When your signal is named taskClicked, The slot in QML should be onTaskClicked.

Also in QML you should name the object testData by:

objectName: "testData"

Question 3

QML Connections: Cannot assign to non-existent property "onTaskClicked"

The error tells you that your Text item do not have signal taskClicked or property onTaskClicked! You need to declare a slot inside your text item. To do that, you simply declare a function:

Text {
   id: testData
   objectName: "testData" // as Laszlo said

   function onTaskClicked( task ) {
       testData.text = task.name;
   }
}

But that also won't work because you create a SLOT( onTaskClicked(QVariant) ) instead of SLOT(taskClicked(HolidayTask*)). In order to exchange data with QML you need to change your signal to SIGNAL(taskClicked(QVariant)):

Q_SIGNALS:
    void taskClicked(QVariant task);

And emit it with:

emit taskClicked( QVariant::fromValue( task ) );

Remember that in order to be able to use HolidayTask it must be a QObject that is registered with qmlRegisterType.

You can also simply call that qml function.

If you are not able to use QVariant you can declare a signal inside you Text object:

Text {
   id: testData
   objectName: "testData" // as Laszlo said

   signal taskClicked ( HolidayTask task )

   onTaskClicked: {
       testData.text = task.name;
   }
}

And then connect from a C++ SIGNAL to qml SIGNAL:

auto root = view->rootObject(); 
auto myElement = root->findChild<QObject*>(QLatin1Literal("testData");
connect(m_view, SIGNAL(taskClicked(HolidayTask*), myElement,
    SIGNAL(taskClicked(HolidayTask*));