ViewLifecycleCoroutineScope

[RBusarow]

Fragment's double lifecycle issue affects LifecycleCoroutineScope (Androidx's or Dispatch's) as well.

Consider this example from the official docs:

class MyFragment: Fragment() {
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        viewLifecycleOwner.lifecycleScope.launch {
            val params = TextViewCompat.getTextMetricsParams(textView)
            val precomputedText = withContext(Dispatchers.Default) {
                PrecomputedTextCompat.create(longTextContent, params)
            }
            TextViewCompat.setPrecomputedText(textView, precomputedText)
        }
    }
}

Because the View lifecycle is odd, we either need to wait until onViewCreated in order to access it, or we need to use viewLifecycleOwnerLiveData. Using the former, the developer must use ordinary lifecycle callback functions in order to guarantee the View's existence. This is inconsistent with the rest of lifecycle coroutines, which could be executed from inside init { }.

The alternative approach of observing viewLifecycleOwnerLiveData seems better. It allows the use of init { }. Also, since onViewCreated must happen after onAttach(), this means that an AAC viewModel initialized via by viewModels may be accessed safely. This means we can write the following:

class MyFragment @Inject constructor(
  coroutineScope: MainImmediateCoroutineScope
) : Fragment {

  val viewModel: MyViewModel by viewModels()

  init {
    coroutineScope.withViewLifecycle  { // this: ViewLifecycleCoroutineScope
      // safely reference this directly inside the constructor because this will never invoke before onAttach
      viewModel.someFlow.onEach {
        // ...
      }.launchOnStart() // ViewLifecycleCoroutineScope-specific functions for lifecycle-aware collection of Flow
    }
  }
}