ActivityWithBackAndDrawer.kt

class MainActivity : AppCompatActivity(),
        NavigationView.OnNavigationItemSelectedListener, FragmentManager.OnBackStackChangedListener {

    private lateinit var drawer: DrawerLayout
    private lateinit var toggle: ActionBarDrawerToggle
    private lateinit var navigation: NavigationView
          
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // init UI
      
        val toolbar = // ...
        setSupportActionBar(toolbar)
        drawer = // ...

        toggle = ActionBarDrawerToggle(this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close)
        toggle.setToolbarNavigationClickListener { super.onBackPressed() }
        drawer.addDrawerListener(toggle)
        toggle.syncState()

        if (savedInstanceState == null) {
            openFragmentRoot(State.currentScreen)
        } else {
            onBackStackChanged() // sync state
        }
    }

    override fun onResume() {
        super.onResume()
        supportFragmentManager.addOnBackStackChangedListener(this)
    }

    override fun onPause() {
        supportFragmentManager.removeOnBackStackChangedListener(this)
        super.onPause()
    }

    override fun onBackPressed() =
            if (drawer.isDrawerOpen(GravityCompat.START)) {
                drawer.closeDrawer(GravityCompat.START)
            } else {
                super.onBackPressed()
            }

    override fun onBackStackChanged() {
        val burgerAccessible = supportFragmentManager.backStackEntryCount == 0
        if (toggle.isDrawerIndicatorEnabled != burgerAccessible) {
            toggle.isDrawerIndicatorEnabled = burgerAccessible
            if (!burgerAccessible) {
                supportActionBar!!.let {
                    it.setDisplayHomeAsUpEnabled(true)
                    it.setHomeAsUpIndicator(R.drawable.abc_ic_ab_back_material)
                }
            }
        }
    }

    override fun onNavigationItemSelected(item: MenuItem): Boolean {
        drawer.closeDrawer(GravityCompat.START)
        openFragmentRoot(item.itemId)
        return true
    }

    internal fun openFragmentRoot(@NavItem navItemId: Int) {
        supportFragmentManager.let {
            while (it.backStackEntryCount > 0) it.popBackStackImmediate()

            it
                    .beginTransaction()
                    .replace(R.id.content, NAV_FRAGMENTS[navItemId]!!())
                    .commitNow()

            State.currentScreen = navItemId
            navigation.menu.findItem(navItemId).isChecked = true // needed when was no explicit click (eg. restoring from prefs)

            onBackStackChanged()
        }
    }

}