UI initialization with Inheritance

[IARI]

Consider the followingFragment class and its child class:

open class A : Fragment("") {
    override val root = vbox {
        addStuff(this)
        ...
    }
   open fun addStuff(b: VBox) {
      ...
   }
}

class B : A {
   val labelText = SimpleStringProperty()   
   override fun addStuff(b: VBox) {
       super.addStuff(b)
       b.apply {
          label(labelText)
       }
   }
}

I want Classes inheriting from A to be able to add their own stuff in some subpart of the A.root.
This setup leads to an NPE (labelText is null) due to the order in which initialization happens with Kotlin (as explained here).

How can this be achieved properly?

[IARI]

I have found that initializing the root lazy (by lazy { ... }) does not work out of the box.
The problem is in FX.kt in line 512:

        is UIComponent -> root?.addChildIfPossible(node)

It seems that here it is assumed, that root is not initialized by a getter or delegate - or at least that before initialization root at least yields null at least one time.
If that is not the case (as with by lazy { ... }) it will result in a stackoverflow.

A workaround I found for now is to avoid the builder implementation in FX.kt, by doing something like this:

    override val root by lazy {
        VBox().apply {
            ...
        }
    }

works, but its not exactly what I would call a nice solution ;)

[edvin]

You're right, the framework expects the root to be created at once, and there are probably more cases of that than the one you ran into. You might have better success by overriding onDock in your subclass. In general I've not found much need for subclassing UIComponents at all, and I wonder if it's the right solution for your use case as well. If you can elaborate, maybe we can come up with a better solution :)

[IARI]

Thanks for the advice.
My Application is basically about templatese for (latex) documents.
There is a template for letters, a model datastructure that is used to fill the template, and a view (LetterView : Fragment) , which is used to input or edit the model.

There are special types of Letters (like an invoice) whose models contain additional information.
So far, the most straight forward way to realize the corresponding view seemed to be inheritance for me.

Would you recommend a different approach? composition?