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Android Fragments and Navigation Component

Fragment lifecycle, Fragment communication via shared ViewModel, Jetpack Navigation Component, Safe Args, View Binding, and the single-Activity architecture pattern.

Key Facts

  • Fragment = reusable UI portion within an Activity, has its own lifecycle
  • Fragment lifecycle: onAttach -> onCreate -> onCreateView -> onViewCreated -> ... -> onDestroyView -> onDestroy -> onDetach
  • onDestroyView destroys the view but Fragment survives - can recreate view later
  • Navigation Component manages fragment transactions, back stack, and argument passing
  • Safe Args plugin generates type-safe classes for passing arguments between destinations
  • View Binding replaces findViewById with generated binding classes
  • Modern pattern: Single Activity + Multiple Fragments

Patterns

Fragment with View Binding

class UserListFragment : Fragment() {
    private var _binding: FragmentUserListBinding? = null
    private val binding get() = _binding!!

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                               savedInstanceState: Bundle?): View {
        _binding = FragmentUserListBinding.inflate(inflater, container, false)
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        binding.recyclerView.adapter = adapter
        binding.recyclerView.layoutManager = LinearLayoutManager(requireContext())
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null  // prevent memory leak
    }
}

Shared ViewModel for Fragment Communication

class SharedViewModel : ViewModel() {
    val selectedUser = MutableLiveData<User>()
}

// Fragment A - sets data
class ListFragment : Fragment() {
    private val sharedVM: SharedViewModel by activityViewModels()
    fun onUserClicked(user: User) { sharedVM.selectedUser.value = user }
}

// Fragment B - observes data
class DetailFragment : Fragment() {
    private val sharedVM: SharedViewModel by activityViewModels()
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        sharedVM.selectedUser.observe(viewLifecycleOwner) { user ->
            binding.tvName.text = user.name
        }
    }
}

<navigation app:startDestination="@id/listFragment">
    <fragment android:id="@+id/listFragment"
        android:name="com.example.ListFragment">
        <action android:id="@+id/action_list_to_detail"
            app:destination="@id/detailFragment" />
    </fragment>
    <fragment android:id="@+id/detailFragment"
        android:name="com.example.DetailFragment">
        <argument android:name="userId" app:argType="integer" />
    </fragment>
</navigation>

Safe Args Navigation

// Navigate with type-safe args
val action = ListFragmentDirections.actionListToDetail(userId = user.id)
findNavController().navigate(action)

// Receive args
class DetailFragment : Fragment() {
    private val args: DetailFragmentArgs by navArgs()
    override fun onViewCreated(...) { val id = args.userId }
}

Bottom Navigation with NavController

val navController = findNavController(R.id.navHostFragment)
binding.bottomNav.setupWithNavController(navController)

View Binding Setup

android { buildFeatures { viewBinding true } }

Gotchas

  • _binding = null in onDestroyView() is critical - without it, Fragment holds reference to destroyed views
  • Use viewLifecycleOwner (not this) when observing LiveData in Fragments - prevents updates to detached views
  • requireContext() throws if Fragment is not attached - use context (nullable) in callbacks
  • addToBackStack(null) in manual Fragment transactions enables back navigation
  • Fragment recreation on config change: use ViewModel for persistent state, not arguments

See Also