ChildFragment

abstract class ChildFragment: Fragment() {

    interface Parent {
        fun titleChanged(newTitle: String?)
        fun changeChild(newChild: ChildFragment)
        fun changeContent(newContent: Fragment, addToBackStack: Boolean)
        val child: ChildFragment?
    }

    protected val parent: Parent?
        get() = if(parentFragment is Parent) {
            parentFragment as Parent
        } else null


    var title: String = ""
        set(value) {
            field = value
            parent?.titleChanged(value)
        }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        savedInstanceState?.getBundle(CHILD_FRAGMENT_STATE)?.apply {
            title = getString(TITLE_KEY)
        }
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putBundle(CHILD_FRAGMENT_STATE, Bundle().apply {
            putString(TITLE_KEY, title)
        })
    }

    private companion object {
        private const val CHILD_FRAGMENT_STATE = "CHILD_FRAGMENT_STATE_KEY"
        private const val TITLE_KEY = "CHILD_FRAGMENT_TITLE_KEY"
    }

}