Challenge 1: Convert Chapter 6's Fragments to a Nav Graph — Solution

// res/navigation/nav_graph.xml
<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/nav_graph"
    app:startDestination="@id/welcomeFragment">

    <fragment
        android:id="@+id/welcomeFragment"
        android:name=".WelcomeFragment"
        android:label="Welcome">
        <action
            android:id="@+id/action_welcome_to_details"
            app:destination="@id/detailsFragment" />
    </fragment>

    <fragment
        android:id="@+id/detailsFragment"
        android:name=".DetailsFragment"
        android:label="Details" />

</navigation>

// activity_main.xml — replaces the FrameLayout from Chapter 6
<?xml version="1.0" encoding="utf-8"?>
<androidx.fragment.app.FragmentContainerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/navHostFragment"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:navGraph="@navigation/nav_graph"
    app:defaultNavHost="true" />

// MainActivity.kt — no more manual FragmentManager call needed at all;
// NavHostFragment loads app:startDestination automatically.
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        // No FragmentManager.beginTransaction() call needed here anymore —
        // the NavHostFragment reads app:startDestination from the nav
        // graph and loads WelcomeFragment automatically.
    }
}

// WelcomeFragment.kt — the button click listener changes from Chapter 6
binding.viewDetailsButton.setOnClickListener {
    findNavController().navigate(R.id.action_welcome_to_details)
}

Notes:
- MainActivity.onCreate no longer needs the
  "if (savedInstanceState == null) { ...FragmentManager... }" block from
  Chapter 6 at all — NavHostFragment itself handles showing the start
  destination and correctly not re-adding it on rotation.
- findNavController().navigate(R.id.action_welcome_to_details) replaces
  Chapter 6's "parentFragmentManager.beginTransaction().replace(...).commit()"
  — the ID here is the <action> ID from the nav graph, not a destination
  ID directly.
- The system Back button now automatically returns from detailsFragment
  to welcomeFragment with zero extra code, since app:defaultNavHost="true"
  wires it into the nav graph's own back stack.