Kotlin Coroutines

Built on structured concurrency: every coroutine runs in a scope; cancellation and errors propagate through the parent–child hierarchy. This reference focuses on the two disciplines most often gotten wrong: who owns the scope, and exception handling that doesn't break cancellation.

Main-safety: the function doing blocking work owns the withContext(ioDispatcher); callers never switch dispatchers before calling a suspend function.

Scope ownership — prefer suspend fun, let the caller own the scope

A stored CoroutineScope on a non-UI class (repository, manager, use case, data source) is a strong review signal: the class would have to prove it owns cancellation, error reporting, restart, and lifecycle — most can't. The fix is almost always make the API suspend and let the caller own the scope.

// DO — suspend fun; the caller owns the scope, cancellation propagates, exceptions surface
class ArticlesRepository(private val dataSource: ArticlesDataSource, private val io: CoroutineDispatcher) {
    suspend fun bookmark(article: Article) = withContext(io) { dataSource.bookmark(article) }
}
class BookmarkViewModel(private val repo: ArticlesRepository) : ViewModel() {
    fun onBookmark(a: Article) { viewModelScope.launch { repo.bookmark(a) } }
}