Challenge 3: Fragment-to-Activity Communication — Solution

// fragment_details.xml — add a Button
<Button
    android:id="@+id/closeButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Close" />

// DetailsFragment.kt
class DetailsFragment : Fragment() {
    private lateinit var binding: FragmentDetailsBinding

    interface OnDetailsClosedListener {
        fun onDetailsClosed()
    }

    private var listener: OnDetailsClosedListener? = null

    override fun onAttach(context: Context) {
        super.onAttach(context)
        listener = context as? OnDetailsClosedListener
    }

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding.closeButton.setOnClickListener {
            listener?.onDetailsClosed()
        }
    }
}

// MainActivity.kt
class MainActivity : AppCompatActivity(), DetailsFragment.OnDetailsClosedListener {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        if (savedInstanceState == null) {
            supportFragmentManager.beginTransaction()
                .replace(R.id.fragmentContainer, WelcomeFragment())
                .commit()
        }
    }

    override fun onDetailsClosed() {
        Log.d("MainActivity", "DetailsFragment reported it was closed")
    }
}

Notes:
- context as? OnDetailsClosedListener is a SAFE cast — if MainActivity
  (or whatever hosts DetailsFragment) didn't implement the interface,
  this would yield null instead of crashing, and listener?.onDetailsClosed()
  would then simply do nothing when the Close button is tapped.
- onAttach runs before onCreateView, which is why it's the correct place
  to capture the listener reference — DetailsFragment already knows its
  host Activity by the time any of its Views are set up.
- MainActivity declares "DetailsFragment.OnDetailsClosedListener" as a
  second supertype (alongside AppCompatActivity) and implements
  onDetailsClosed() — this is what makes the "as? OnDetailsClosedListener"
  cast succeed at runtime.