Run time generated property set in C++ exposed to QML

[don@thegagnes.com]

I would like to set up the following object hierarchy:

Foo has children of type Child named Child1 and Child2. The Child type has fixed properties : prop1 and prop 2 for example. I can see how to create this object hierarchy in C++ using the Q_PROPERTY syntax and then push then into the QML context such that I can property bind to Foo.Child1.prop1. The problem I am trying to solve is that the number of Child objects and their names is determined at runtime. I don't see how I can create a Foo object that exposes a dynamic set of Child objects that still supports property binding.

Docs for Qt Script seems to say that if I parent the child QObjects to Foo and give them names I should be able to access them that way. But that seems to be only for Qt Script. I can't get it to work in QML.

QScriptValue looked promising as well. Since you can create the object dynamically and add properties to it. But once again that seems to be only for Qt Script.

Is there some other way to do it?

[anda_skoa]

http://qt-project.org/doc/qt-5/qqmllistproperty.html

Cheers,
_

[don@thegagnes.com]

Seems so close! But from looking at the callbacks I need to provide it seems like I could only access the list using the syntax Foo[1].prop1. I want to be able to access the child properties by name: Foo.Child.prop1. Is that incorrect?

[anda_skoa]

Hmm. Can you describe your actual use case?

Cheers,
_

[wysota]

I want to be able to access the child properties by name: Foo.Child.prop1.

Give the child an id and invoke it by that id: "childId.prop1"

[don@thegagnes.com]

Question on using "id:" to identify the properties dynamically: How do I set "id" programmatically in C++ code? Is "id" a dynamic property on the object?

Here is the use case: I'm connecting to a piece of hardware. That hardware is configured through a set of parameters. The parameters are named with strings. The parameter set differs depending on what firmware you have installed. I want to expose those parameters as properties so I can create QML ui to edit them.

How I currently do it:
- I have a Parameters object which has a Q_PROPERTY in it for each parameter which is named the parameter string name.
- The Q_PROPERTY exposes a Fact object which has a value property as well as meta data such as min/max ranges, user visible descriptions and so forth.
- I can then bind to the parameter in QML like this: parameters.PROP_1.value
- The top level Parameters object exists as a container for the properties because there will be additional sets of information like this. There is also telemetry data which comes back from the board, which I want to also expose as Facts. By using the top level object it removes possible collision on the namespace between the two sets of values.

What I would like to do:
- I can query the hardware for all the parameter names. Using that I'd like to attach the parameter Fact properties to the Parameters object dynamically at run time.
- This way if the client code gets out of sync with the firmware parameter set it will still work since the property list is added at runtime.

Also, thanks a ton for all the help.

[wysota]

Question on using "id:" to identify the properties dynamically: How do I set "id" programmatically in C++ code?

You don't. Why would you want to do that? You said you wanted to access the object in QML.

Is "id" a dynamic property on the object?

No, id is not reflected on the C++ side.

Here is the use case: I'm connecting to a piece of hardware. That hardware is configured through a set of parameters. The parameters are named with strings. The parameter set differs depending on what firmware you have installed. I want to expose those parameters as properties so I can create QML ui to edit them.

How I currently do it:
- I have a Parameters object which has a Q_PROPERTY in it for each parameter which is named the parameter string name.
- The Q_PROPERTY exposes a Fact object which has a value property as well as meta data such as min/max ranges, user visible descriptions and so forth.
- I can then bind to the parameter in QML like this: parameters.PROP_1.value
- The top level Parameters object exists as a container for the properties because there will be additional sets of information like this. There is also telemetry data which comes back from the board, which I want to also expose as Facts. By using the top level object it removes possible collision on the namespace between the two sets of values.

What I would like to do:
- I can query the hardware for all the parameter names. Using that I'd like to attach the parameter Fact properties to the Parameters object dynamically at run time.
- This way if the client code gets out of sync with the firmware parameter set it will still work since the property list is added at runtime.

In my opinion you should expose the data as a plain-old QAbstractListModel model. You can then connect to that model on QML side using a view (e.g. ListView) or a custom UI.

[don@thegagnes.com]

As far as setting an id from the C++ side I was just trying to figure out how to use your example. The objects are all created in C++ and then pushed into a QML context. Given that, using an id won't work.

If I use a list model I won't be able to do something like this in QML will I?

TextInput {
text: parameters.PROP_1.value
}

[wysota]

As far as setting an id from the C++ side I was just trying to figure out how to use your example. The objects are all created in C++ and then pushed into a QML context. Given that, using an id won't work.

You surely have a QML declaration somewhere which refers to a particular parameter (and you can do that via id), otherwise your QML code wouldn't know how to "call" a particular parameter as it might not exist.

If I use a list model I won't be able to do something like this in QML will I?

TextInput {
text: parameters.PROP_1.value
}

If you put that in a delegate referencing a particular model entry then it would be simpler:

ListView {
  id: view
  model: myModel
  // ...
 
  Row {
    width: view.width
    Text { text: model.name }
    Slider {
      minimumValue: model.minimum
      maximumValue: model.maximum
      value: model.value
      onValueChanged: model.value = value
    }
    Text { text: model.value }
  }
}

ListView {
  id: view
  model: myModel
  // ...

  Row {
    width: view.width
    Text { text: model.name }
    Slider {
      minimumValue: model.minimum
      maximumValue: model.maximum
      value: model.value
      onValueChanged: model.value = value
    }
    Text { text: model.value }
  }
}

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You could even have different types of parameters and construct a proper delegate depending on the type (e.g. putting a slider for numeric parameters and text field for textual ones).

[anda_skoa]

What you could try is a map

class Properties : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVariantMap facts .... )
};

class Properties : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVariantMap facts .... )
};

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where the key is the property name and the value is a Fact object

Or exposing the map directly as a context property (if it does not change during runtime).

Cheers,
_

[don@thegagnes.com]

QVariantMap - That look like exactly what I need. Thank you very much.