Fragments & the Single-Activity Pattern

What is a Fragment?

A Fragment is a reusable piece of a screen with its own layout and lifecycle, hosted inside an Activity. Think of an Activity as a frame, and Fragments as the swappable pictures inside it. Modern Android apps usually have one Activity and many Fragments, swapped by the Navigation Component.

Why fragments exist

• Reuse — the same fragment can appear on a phone and inside a tablet’s split view.
• Modularity — each screen is a small, focused class.
• Navigation — moving between fragments is lighter than launching new activities.

A basic fragment

class HomeFragment : Fragment(R.layout.fragment_home) {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val binding = FragmentHomeBinding.bind(view)
        binding.title.text = "Home"
        binding.button.setOnClickListener {
            findNavController().navigate(R.id.action_home_to_detail)
        }
    }
}

The fragment lifecycle (and a key gotcha)

Fragments have their own lifecycle, similar to activities, but with an extra layer: the fragment’s view can be destroyed while the fragment object lives on. The important callbacks are onCreateView() (build the view), onViewCreated() (set up the view — do your work here), and onDestroyView() (clean up view references).

Gotcha: never keep a binding object past onDestroyView() — it causes memory leaks. Set it to null there if you stored it.

The single-activity pattern

Instead of one Activity per screen, you keep one MainActivity that hosts a NavHostFragment, and every screen is a fragment inside it. This is the approach Google recommends today and pairs perfectly with the Navigation Component (covered in a later lesson).

Passing data between fragments

Use Safe Args (type-safe arguments generated from your navigation graph) rather than raw bundles, or share a ViewModel scoped to the activity so two fragments can read the same data.

// shared ViewModel approach
private val sharedVm: SharedViewModel by activityViewModels()

Common mistakes

• Doing setup in onCreateView() instead of onViewCreated().
• Holding the view binding after the view is destroyed (leak).
• Using fragment constructors with arguments — pass data via the navigation graph instead.

Summary: Fragments are reusable screen pieces with their own view lifecycle. One Activity + many Fragments + Navigation Component is the modern standard.