Coroutines on Android

Android gives coroutines lifecycle-aware scopes so async work is cancelled automatically when the screen goes away — no leaks.

viewModelScope

A ViewModel has a built-in viewModelScope that is cancelled when the ViewModel is cleared:

class UserViewModel(private val repo: UserRepository) : ViewModel() {
    private val _state = MutableStateFlow<UiState>(UiState.Loading)
    val state: StateFlow<UiState> = _state.asStateFlow()

    fun load() {
        viewModelScope.launch {              // cancelled when the VM is cleared
            _state.value = try {
                UiState.Success(repo.fetch())
            } catch (e: Exception) {
                UiState.Error(e.message ?: "failed")
            }
        }
    }
}

Exposing state with StateFlow

Expose immutable StateFlow to the UI and keep the MutableStateFlow private. The example app’s CounterViewModel shows the pattern:

class CounterViewModel : ViewModel() {
    private val _count = MutableStateFlow(0)
    val count: StateFlow<Int> = _count.asStateFlow()

    fun increment() = _count.update { it + 1 }
}

Because the state lives in the ViewModel, it survives configuration changes (like rotation) automatically.

Collecting flows in Compose

Use collectAsStateWithLifecycle() so collection pauses when the screen is not visible:

@Composable
fun CounterScreen(viewModel: CounterViewModel = viewModel()) {
    val count by viewModel.count.collectAsStateWithLifecycle()
    Text("Count: $count")
    Button(onClick = { viewModel.increment() }) { Text("Increment") }
}

lifecycleScope

Activities and Fragments expose lifecycleScope for UI-tied coroutines. Prefer keeping logic in the ViewModel; use lifecycleScope for purely view-related work.

The runnable example

examples/android/ contains CounterViewModel and CounterScreen. The ViewModel’s state logic is covered by a JVM unit test (CounterViewModelTest) that runs in CI — no emulator required:

@Test
fun increment_increasesCount() {
    val vm = CounterViewModel()
    vm.increment(); vm.increment()
    assertEquals(2, vm.count.value)
}

Exercises

1. Add a decrement() to CounterViewModel and a button for it, then extend the unit test.

2. Write a ViewModel that exposes a UiState StateFlow and loads data in viewModelScope, mapping success/failure to Success/Error.

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