Super simple MVP for Android + Navigator as an example

mvp.kt

package com.songsterr.mvp

import android.arch.lifecycle.ViewModel

// Immutable view state
interface ViewState

interface View<in S: ViewState> {
    fun render(state: S) {}
}

abstract class Presenter< S: ViewState, V: View<S>>(protected var state: S): ViewModel() {
    var view: V? = null

    fun takeView(view: V) {
        this.view = view
        view.render(state)
    }

    fun dropView() {
        this.view = null
    }
}

NavigatorFragment.kt

package com.songsterr.mvp

/**
 * Here is a simple navigational module that can only safely start activities for result
 */

import android.content.Context
import android.content.Intent
import android.support.v4.app.Fragment
import android.support.v4.app.FragmentActivity
import org.koin.android.viewmodel.ext.android.sharedViewModel

private const val RC_NAVIGATOR = 0x99
private const val FRAGMENT_TAG = "navigation"

object Navigation {
    @JvmStatic
    fun init(activity: FragmentActivity) {
        val manager = activity.supportFragmentManager
        if (manager.findFragmentByTag(FRAGMENT_TAG) == null) {
            manager
                    .beginTransaction()
                    .add(NavigatorFragment(), FRAGMENT_TAG)
                    .commit()
        }
    }
}

data class NavigatorState(val intent: Intent? = null, val callback: ((Int, Intent?) -> Unit)? = null): ViewState {
    fun isOccupied() : Boolean {
        return intent != null || callback != null
    }
}

class Navigator: Presenter<NavigatorState, NavigatorFragment>(NavigatorState()) {
    fun launchActivity(intent: Intent, callbackForResult: ((Int, Intent?) -> Unit)?): Boolean {
        if (state.isOccupied()) {
            return false
        }

        state = NavigatorState(intent, callbackForResult)

        view?.render(state)

        return true
    }

    fun navigationLaunched() {
        state = NavigatorState(null, callback = state.callback)
    }

    fun navigationResult(resultCode: Int, data: Intent?) {
        state.callback?.invoke(resultCode, data)

        state = NavigatorState()
    }
}

class NavigatorFragment: Fragment(), View<NavigatorState> {
    private val presenter: Navigator by sharedViewModel()

    override fun onAttach(context: Context?) {
        super.onAttach(context)

        presenter.takeView(this)

    }

    override fun onDestroy() {
        super.onDestroy()

        presenter.dropView()
    }

    override fun render(state: NavigatorState) {
        if (state.intent != null) {
            startActivityForResult(state.intent, RC_NAVIGATOR)
            presenter.navigationLaunched()
        }
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        if (requestCode == RC_NAVIGATOR) {
            presenter.navigationResult(resultCode, data)
        } else {
            super.onActivityResult(requestCode, resultCode, data)
        }
    }
}