"Adding Callbacks From Fragment To Activity Or Activity To Application In Android" @ http://blog.jakelee.co.uk/2018/10/22/adding-callbacks-from-fragment-to-activity-or-activity-to-application-in-android

ActionHandler.kt

interface ActionHandler {
    fun handleAction(actionCode: String)
}

FragmentA.kt

companion object {
    const val FRAGMENT_A_CLOSED = "com.example.fragment_a_closed"
}

fun closeFragment() {
    try {
        (activity as ActionHandler).handleAction(FRAGMENT_A_CLOSED)
    } catch (e: ClassCastException) {
        Timber.e("Calling activity can't get callback!")
    }
    dismiss()
}

MainActivity.kt

class MainActivity: ActionHandler { 
    override fun handleAction(actionCode: String) {
        when {
            actionCode == FragmentA.FRAGMENT_A_CLOSED -> {
                doSomething()
            }
            actionCode == FragmentB.FRAGMENT_B_CLOSED -> {
                doSomethingElse()
            }
            actionCode == FragmentC.FRAGMENT_C_CLOSED -> {
                doAnotherThing()
            }
        }
    }
  
    // Usual activity setup code snipped
}