QML: modifying a property of a Child object that is defined in a different QML file (not main.qml)

Question

Basically, I have something like:

Main.qml:

ApplicationWindow{
width: 500
height: 500

    Page{
    id: page0

        DataPage{
          id: datapage0  
        }
    }
}

DataPage.qml:

Page{
id: displayPage

    DataDisplay{
    id: dataShow
    }
}

DataDisplay.qml:

Label{
text: "data: "
}

TextArea{
id: dataArea
text: ""
}

I've removed the stuff I think isn't relevant (such as anchors, height, width, etc.). Now, in main.qml, I have a signal coming from the c++ backend:

Connections{
target: modb
onPostData: {
    page0.datapage0.dataShow.dataArea.text = string;
}

And I get the following error: TypeError: Cannot read property 'dataArea' of undefined

So, I wanted to ask: how do I connect that signal to the child object that is defined in DataDisplay.qml? I'm able to get info into main.qml using signals, but seem to be unable to dereference child objects

Edit:

main.cpp:

QQmlContext* ctx0 = engine.rootContext();
ctx0->setContextProperty("ark", &ark);

QQmlContext* ctx1 = engine.rootContext();
ctx1->setContextProperty("comm", ark.comm);

QQmlContext* ctx2 = engine.rootContext();
ctx2->setContextProperty("modb", ark.modb);

is how I set the Context (of 3 classes, as you can see). I'm able to get signals from any of the three into main.qml, as well as call slots in any of the three in main.qml; I haven't yet tried to call slots from the c++ classes in the other qml files, but I assume it would work because I can access the parent's properties from the child

Have you considered adding the `Connections` block inside DataDisplay.qml? Please post how you got the `modb` connected to qml (did you use `setContext`?) — Amfasis, Oct 30 '18 at 12:16

dtech · Accepted Answer · 2018-10-30T16:36:48.180

2

1 - you have 3 pointers pointing to the same object. One is enough. Really!

2 - as long as ark is properly implemented, you can access ark.comm and ark.modb from QML, no need to expose them individually.

3 - you don't seem to understand the scope of ids. Take a look at this exhaustive answer. dataShow is simply not visible from wherever you made the connection.

4 - context properties are not very efficient, that's more of a "quick and dirty" approach to expose C++ to qml. For optimal performance consider a more efficient approach.

All in all, you exhibit the typical symptoms of "getting ahead of yourself". Take the time to learn before you practice.

edited Oct 30 '18 at 16:36

answered Oct 30 '18 at 16:31

dtech

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You've hit the nail on the head! I am indeed rushing into this due to the urgency of the project. I've read over your links and used the Singleton thing (which is working). Do you have any idea why they've implemented QML this way? Is there any specific reason why accessing signals/slots from other files requires extra work when using the context approach? – Numi Oct 31 '18 at 09:03
Because for context properties, qml has took down the component tree until it finds the context property, which in your case is declared for the very root context. This takes time, especially if your object tree is deep. There is also the danger that the context property might be shadowed by a similarly named property, which will be the one resolved. With singletons those problems are solved, access is direct and unambiguous. – dtech Oct 31 '18 at 09:11

score 1 · Answer 2 · answered Oct 30 '18 at 14:30

1

As you indeed assume you can use the modb variable in the other qml's as well, since it is added to the rootContext. I would advise this option.

Another option you could try is just using dataArea.text = string since the id's go all over the place (it's javascript after all), you should use strong id's in this case.

Another option is to define property alias's to pass the string through over the objects (See Qt docs). Or use property string, but that's even more work.

answered Oct 30 '18 at 14:30

Amfasis

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2
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I tried creating a 'Connections' in the other page targeting modb, but it gives me a syntax error. Is there some specific way to create Connections in qml files that aren't main.qml? – Numi Oct 30 '18 at 15:38
most certainly not – Amfasis Oct 30 '18 at 19:45

QML: modifying a property of a Child object that is defined in a different QML file (not main.qml)

2 Answers2