Navigation Components - onSupportNavigateUp()

Question

Is it really necessary to call

override fun onSupportNavigateUp() = findNavController(R.id.nav_host_fragment).navigateUp()

in my MainActivity ?

Because when I attach the NavHostFragment in my xml it starts from there, but I have seen the Google I/O lecture where they use it to make the start point of the navigation

My question

Where is it necessary to use it ?

Thanks

Answer 1

onSupportNavigateUp comes from AppCompatActivity. You should override the method in the same activity where you define your NavHostFragment (probably your MainActivity). You override it so that the NavigationUI can correctly support the up navigation or even the drawer layout menu. AppCompatActivity and NavigationUI are two indepenent components, so you override the method in order to connect the two. Note that if you set a toolbar with the navigation component, i.e., if you do something similar to

override fun onCreate(savedInstanceState: Bundle?) {
  setContentView(R.layout.activity_main)

  // ...

  val navController = findNavController(R.id.nav_host_fragment)
  val appBarConfiguration = AppBarConfiguration(navController.graph)
  findViewById<Toolbar>(R.id.toolbar).setupWithNavController(navController, appBarConfiguration)
}

you don't need to override the onSupportNavigationUp method as Navigation will automatically handle the click events.

You also don't need to override it if you want to handle the toolbar yourself.