DecorableFragment

abstract class DecorableFragment: Fragment() {

    private lateinit var topContent: LinearLayout
    private lateinit var content: LinearLayout
    private lateinit var bottomContent: LinearLayout

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        val view = inflater.inflate(R.layout.fragment_decorable, container, false)

        topContent = view.findViewById(R.id.fragment_decorable_top_content)
        content = view.findViewById(R.id.fragment_decorable_content)
        bottomContent = view.findViewById(R.id.fragment_decorable_bottom_content)
        onCreateContent(inflater, content, savedInstanceState)
        onCreateDecoration(DecorationInstaller(topContent), DecorationInstaller(bottomContent), savedInstanceState)

        return view
    }

    open fun onCreateContent(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?) {
    }

    open fun onCreateDecoration(
            topInstaller: DecorationInstaller,
            bottomInstaller: DecorationInstaller,
            savedInstanceState: Bundle?
    ) {
    }

    class DecorationInstaller (private val container: ViewGroup) {

        private fun ViewGroup.addDecoration(decoration: Decoration): View {
            if(!decoration.initialized) {
                decoration.create(LayoutInflater.from(context), this)
            }

            addView(decoration.decorationView)
            return decoration.decorationView!!
        }

        fun add(decoration: Decoration): View {
            return container.addDecoration(decoration)
        }
    }
}