A practical example on why Composition over Inheritance matters for your codebase

1-ManualFragment.kt

/**
 * Solution 1 = Manual
 */
internal class MyManualFragment(
    private val viewModel: ViewModel
) : Fragment() {

    private var startedScope: CoroutineScope? = null

    override fun onStart() {
        super.onStart()

        startedScope = CoroutineScope(Dispatchers.Main.immediate)

        startedScope!!.launch {
            val ui = Ui(requireView())

            viewModel.states
                .onEach { ui.render(it) }
                .launchWhenStartedIn(viewLifecycleOwner)
        }
    }

    override fun onStop() {
        startedScope!!.job.cancel()
        super.onStop()
    }
}

2-InheritanceFragment.kt

/**
 * Solution 2 = Inheritance
 */
internal class InheritanceFragment(
    private val viewModel: ViewModel
) : CoroutineFragment() {

    override fun onViewCreated(scope: CoroutineScope, view: View, savedInstanceState: Bundle?) {
        scope.launch {
            val ui = Ui(requireView())

            viewModel.states
                .onEach { ui.render(it) }
                .launchWhenStartedIn(viewLifecycleOwner)
        }
    }
}

3-CompositionFragment.kt

/**
 * Solution 3: Composition
 */
internal class CompositionFragment(
    viewModel: ViewModel
) : Fragment() {
    init {
        viewLifecycleScope.launchWhenCreated {
            val ui = Ui(requireView())

            viewModel.states
                .onEach { ui.render(it) }
                .launchWhenStartedIn(this)
        }
    }
}

abstract class CoroutineFragment : Fragment() {

    open fun onCreate(scope: CoroutineScope, savedInstanceState: Bundle?) {}

    open fun onCreateView(
        scope: CoroutineScope,
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return null
    }

    open fun onViewCreated(scope: CoroutineScope, view: View, savedInstanceState: Bundle?) {}

    open fun onStart(scope: CoroutineScope) {}

    open fun onResume(scope: CoroutineScope) {}
}

4-SharedCode.kt

typealias State = Int

interface ViewModel {
    val states: Flow<State>
}

class Ui(private val view: View) {
    fun render(state: State) {
    }
}