Single Activity implementation

AppActivity.kt

class AppActivity: AppCompatActivity() {
    companion object {
        fun newInstance(context: Context) = Intent(context, AppActivity::class.java)
    }

    internal val navigationFragment = NavigationFragment()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val rootFrameLayout = FrameLayout(this).apply {
            id = View.generateViewId()
            layoutParams = LayoutParams(MATCH_PARENT, MATCH_PARENT)
        }
        setContentView(rootFrameLayout)
        supportFragmentManager.beginTransaction()
            .replace(rootFrameLayout.id, navigationFragment)
            .commit()
    }

    override fun onBackPressed() {
        if (navigationFragment.childFragmentManager.fragments.isNotEmpty()) {
            navigationFragment.childFragmentManager.popBackStack()
        } else {
            super.onBackPressed()
        }
    }
}

class NavigationFragment: Fragment() {
    internal lateinit var root: View

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        root = FrameLayout(context).apply {
            id = View.generateViewId()
            layoutParams = LayoutParams(MATCH_PARENT, MATCH_PARENT)
        }
        return root
    }

    internal fun setRootFragment(fragment: Fragment) {
        childFragmentManager.beginTransaction().replace(root.id, fragment).commit()
    }

    fun pushFragment(fragment: Fragment) {
        childFragmentManager.beginTransaction()
            .replace(root.id, fragment)
            .addToBackStack(null)
            .commit()
    }
}