mutableStateOf approach using

JetpackMutableStateOf.kt

//--------------Utils Class start--------------------
sealed class MenuOptions {
    object TaskList : MenuOptions()
    object AddTask : MenuOptions()
    data class ModifyTask(val task: Task) : MenuOptions()
}

object AppMain {
    private var current_screen: MenuOptions = MenuOptions.TaskList
    var route_screen by mutableStateOf(current_screen)
    var taskList by mutableStateOf(mutableListOf<Task>())
}

fun routingActivity(activity: MenuOptions) {
    route_screen = activity
}

//--------------Utils Class end----------------------


//--------------Main Activity------------------------
class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            TODOAppGreeting()
        }
    }

    override fun onBackPressed() {
        if (AppMain.route_screen !is MenuOptions.TaskList)
            routingActivity(MenuOptions.TaskList)
        else
            super.onBackPressed()

    }
}

@Composable
fun TODOAppGreeting() {
    MaterialTheme {
        AppContent(AppState())
    }
}

@Composable
fun AppContent(app: AppMain) {
    Crossfade(current = app.route_screen) { activity ->
        Surface(color = MaterialTheme.colors.background) {
            when (activity) {
                is MenuOptions.AddTask -> AddTask()
                is MenuOptions.TaskList -> TaskList()
                is MenuOptions.ModifyTask -> AddTask(activity.task)
            }
        }

    }
}

@Composable
fun AppState(): AppMain {
    //Initialize state
    return remember { AppMain }
}