Fragments और Fragment Lifecycle

// HomeFragment.kt
class HomeFragment : Fragment() {

    private var _binding: FragmentHomeBinding? = null
    private val binding get() = _binding!!

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

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding.btnDetails.setOnClickListener {
            (requireActivity() as MainActivity).showDetails("Maths")
        }
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
}

<!-- activity_main.xml -->
<androidx.fragment.app.FragmentContainerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragmentContainer"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        if (savedInstanceState == null) {             // don't add again after rotation
            supportFragmentManager.commit {
                add(R.id.fragmentContainer, HomeFragment())
            }
        }
    }

    fun showDetails(subject: String) {
        supportFragmentManager.commit {
            setReorderingAllowed(true)
            replace(R.id.fragmentContainer, DetailsFragment.newInstance(subject))
            addToBackStack("details")                 // Back returns to HomeFragment
        }
    }
}

class DetailsFragment : Fragment(R.layout.fragment_details) {   // layout in the constructor

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val binding = FragmentDetailsBinding.bind(view)
        val subject = requireArguments().getString(ARG_SUBJECT) ?: "-"
        binding.tvTitle.text = "Notes of $subject"
    }

    companion object {
        private const val ARG_SUBJECT = "subject"
        fun newInstance(subject: String) = DetailsFragment().apply {
            arguments = bundleOf(ARG_SUBJECT to subject)
        }
    }
}

// One ViewModel shared by every fragment of the Activity
class SharedViewModel : ViewModel() {
    val selectedSubject = MutableLiveData<String>()
}

// ListFragment
private val shared: SharedViewModel by activityViewModels()
adapter.onClick = { shared.selectedSubject.value = it.name }

// DetailsFragment
private val shared: SharedViewModel by activityViewModels()
shared.selectedSubject.observe(viewLifecycleOwner) { binding.tvTitle.text = it }