Trouble with Connections

Jpraccio

I am trying to send a signal from C++ to QML.
It seems I have done everything correct according to the documentation, however the signal is never received.

I have the C++ object as a context property:

    context->setContextProperty("midiInterface", &midiInterface);

have defined a signal in the MidiInterface class:

signals:
    void updateTriggered();

And here's the connection:

    Connections {
        target: midiInterface

        onUpdateTriggered: {
            console.log("trigger update received")
            generateNewBlossom();
        }
    }

When the signal is emitted which I have verified through the debugger, the function is not called and there is no console output.

Can't seem to find any documentation on why this may be happening.

mrjj

Hi
Are you sure that syntax works when its a c++ signal ?
The docs says
"A Connections object creates a connection to a QML signal."

Anyway, I might just have over complicated it using the method
https://andrew-jones.com/blog/qml2-to-c-and-back-again-with-signals-and-slots/
section "Emitting a signal from C++ and listening to it from QML"

if "onUpdateTriggered:" can in fact auto bind, its much more elegant :)

Jpraccio

Hey, thanks for the response,

I got that from this doc and a few stack overflows:
http://doc.qt.io/qt-5/qtqml-cppintegration-contextproperties.html

Right at the bottom.

mrjj

Hi
You are right. it does say
"ApplicationData has a signal named dataChanged()"
and you even followed the naming exactly.

So i assume your code is very close to the sample except other names?

Jpraccio

Yes, just the names are different.

Jpraccio

Also, I can see using your method, but I have no clue how I would get to the QML object

mrjj

@Jpraccio
Hi
I just tested the sample project and it worked.. :)
So there must be something a little bit off some place in your code or something else.

Test project
https://www.dropbox.com/s/2ixu3sx172e97rm/untitled54.zip?dl=0

6thC

Have you registered it? For cpp instanced objects I register mine with:

qmlRegisterUncreatableType<T>

http://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterUncreatableType

Jpraccio

I have it able to receive the signal if I put the connection in the top level Window qml. The qml object that needs the signal is nested a bit down in the hierarchy. Is this a know behavior?

Jpraccio

So it seems like nothing is received down the hierarchy from the main window. I have used the Connections inside my Window qml and it's triggered fine. Trying to pass a signal from the Window down to the Component using QML and a Connection object inside the Component fails.

So the components are inside a stack view. They are Component classes that contain a rectangle. The signal needs to happen within these components. Don't know if it matter, but these are not the initialItem of the StackView. I can get some code together if desired.

J.Hilk

@Jpraccio I think, my knowledge is limited here, that that behaviour depends heavly on how you marry your cpp class and your qml root file.

if for example you use qmlRegisterType<MyClass>("MyClass",1,0, "mClass"); than you can create an instande in each qml file you import myclass in, and can use Connect and signal slots there easily enough.

If you create it as a fixed class and register it as a rootContext() / ContextProperty than you should be able to access it everywhere in your qml files.

//main.cpp
MyClass mClass;
QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("MyClass", &mClass);

//any qml file
Connections{
    MyClass.onMySignal: console.log("SIGNAL!!!")
}
//The actual signal in MyClass is named: mySignal
//I believe the "on" prefix is needed

Jpraccio

I have the c++ class working at this point, I can send a signal to the window qml. Now trying to send a signal into the qml objects from the window. It seems these specific qml components just can't receive them. It looks like this:

// Window qml
Window {
    id: mainWindow
    signal mySig()
    Connections {
         target: myCpp
         onAction: {
              // this works
              mySig()
         }
    }

   StackView {
       initalItem: itemA
       ItemA {
          // this has a connection that works
       }

     ItemB {
         // this won't receive the above signal
     }
   }
}

// ItemB qml
Component {
     Rectangle {
         Connections {
             target: mainWindow
             onMySig: {
                  // nothing happens here
                  // regardles of using the cpp signal or the qml signal
              }
          }
     }

}

Jpraccio

I made some changes that forced the stack view to transition and got a new error:

QQmlEngine: Illegal attempt to connect to Chord(0x112847200) that is in a different thread than the QML engine QQmlApplicationEngine(0x7ffeefbfe1e0.

Is it possible this is a threading issue of some sort? I am not intentionally spawning any threads myself

Jpraccio

Solved by pushing ItemB onto the stack view.