This page presents several best practices that have a positive impact by making your app more scalable and testable when using coroutines.
Inject Dispatchers
Don't hardcode Dispatchers
when creating new coroutines or calling
withContext
.
// DO inject Dispatchers
class NewsRepository(
private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {
suspend fun loadNews() = withContext(defaultDispatcher) { /* ... */ }
}
// DO NOT hardcode Dispatchers
class NewsRepository {
// DO NOT use Dispatchers.Default directly, inject it instead
suspend fun loadNews() = withContext(Dispatchers.Default) { /* ... */ }
}
This dependency injection pattern makes testing easier as you can replace those dispatchers in unit and instrumentation tests with a test dispatcher to make your tests more deterministic.
Suspend functions should be safe to call from the main thread
Suspend functions should be main-safe, meaning they're safe to call from the
main thread. If a class is doing long-running blocking operations in a
coroutine, it's in charge of moving the execution off the main thread using
withContext
. This applies to all classes in your app, regardless of the part
of the architecture the class is in.
class NewsRepository(private val ioDispatcher: CoroutineDispatcher) {
// As this operation is manually retrieving the news from the server
// using a blocking HttpURLConnection, it needs to move the execution
// to an IO dispatcher to make it main-safe
suspend fun fetchLatestNews(): List<Article> {
withContext(ioDispatcher) { /* ... implementation ... */ }
}
}
// This use case fetches the latest news and the associated author.
class GetLatestNewsWithAuthorsUseCase(
private val newsRepository: NewsRepository,
private val authorsRepository: AuthorsRepository
) {
// This method doesn't need to worry about moving the execution of the
// coroutine to a different thread as newsRepository is main-safe.
// The work done in the coroutine is lightweight as it only creates
// a list and add elements to it
suspend operator fun invoke(): List<ArticleWithAuthor> {
val news = newsRepository.fetchLatestNews()
val response: List<ArticleWithAuthor> = mutableEmptyList()
for (article in news) {
val author = authorsRepository.getAuthor(article.author)
response.add(ArticleWithAuthor(article, author))
}
return Result.Success(response)
}
}
This pattern makes your app more scalable, as classes calling suspend functions
don't have to worry about what Dispatcher
to use for what type of work. This
responsibility lies in the class that does the work.
The ViewModel should create coroutines
ViewModel
classes should prefer
creating coroutines instead of exposing suspend functions to perform business
logic. Suspend functions in the ViewModel
can be useful if instead of
exposing state using a stream of data, only a single value needs to be emitted.
// DO create coroutines in the ViewModel
class LatestNewsViewModel(
private val getLatestNewsWithAuthors: GetLatestNewsWithAuthorsUseCase
) : ViewModel() {
private val _uiState = MutableStateFlow<LatestNewsUiState>(LatestNewsUiState.Loading)
val uiState: StateFlow<LatestNewsUiState> = _uiState
fun loadNews() {
viewModelScope.launch {
val latestNewsWithAuthors = getLatestNewsWithAuthors()
_uiState.value = LatestNewsUiState.Success(latestNewsWithAuthors)
}
}
}
// Prefer observable state rather than suspend functions from the ViewModel
class LatestNewsViewModel(
private val getLatestNewsWithAuthors: GetLatestNewsWithAuthorsUseCase
) : ViewModel() {
// DO NOT do this. News would probably need to be refreshed as well.
// Instead of exposing a single value with a suspend function, news should
// be exposed using a stream of data as in the code snippet above.
suspend fun loadNews() = getLatestNewsWithAuthors()
}
Views shouldn't directly trigger any coroutines to perform business logic.
Instead, defer that responsibility to the ViewModel
. This makes your business
logic easier to test as ViewModel
objects can be unit tested, instead of using
instrumentation tests that are required to test views.
In addition to that, your coroutines will survive configuration changes
automatically if the work is started in the viewModelScope
. If you create
coroutines using lifecycleScope
instead, you'd have to handle that manually.
If the coroutine needs to outlive the ViewModel
's scope, check out the
Creating coroutines in the business and data layer section.
Don't expose mutable types
Prefer exposing immutable types to other classes. In this way, all changes to the mutable type is centralized in one class making it easier to debug when something goes wrong.
// DO expose immutable types
class LatestNewsViewModel : ViewModel() {
private val _uiState = MutableStateFlow(LatestNewsUiState.Loading)
val uiState: StateFlow<LatestNewsUiState> = _uiState
/* ... */
}
class LatestNewsViewModel : ViewModel() {
// DO NOT expose mutable types
val uiState = MutableStateFlow(LatestNewsUiState.Loading)
/* ... */
}
The data and business layer should expose suspend functions and Flows
Classes in the data and business layers generally expose functions to perform one-shot calls or to be notified of data changes over time. Classes in those layers should expose suspend functions for one-shot calls and Flow to notify about data changes.
// Classes in the data and business layer expose
// either suspend functions or Flows
class ExampleRepository {
suspend fun makeNetworkRequest() { /* ... */ }
fun getExamples(): Flow<Example> { /* ... */ }
}
This best practice makes the caller, generally the presentation layer, able to control the execution and lifecycle of the work happening in those layers, and cancel when needed.
Creating coroutines in the business and data layer
For classes in the data or business layer that need to create coroutines for different reasons, there are different options.
If the work to be done in those coroutines is relevant only when the user is
present on the current screen, it should follow the caller's lifecycle. In most
cases, the caller will be the ViewModel, and the call will be cancelled when the
user navigates away from the screen and the ViewModel is cleared. In this case,
coroutineScope
or supervisorScope
should be used.
class GetAllBooksAndAuthorsUseCase(
private val booksRepository: BooksRepository,
private val authorsRepository: AuthorsRepository,
) {
suspend fun getBookAndAuthors(): BookAndAuthors {
// In parallel, fetch books and authors and return when both requests
// complete and the data is ready
return coroutineScope {
val books = async { booksRepository.getAllBooks() }
val authors = async { authorsRepository.getAllAuthors() }
BookAndAuthors(books.await(), authors.await())
}
}
}
If the work to be done is relevant as long as the app is opened, and the work is
not bound to a particular screen, then the work should outlive the caller's
lifecycle. For this scenario, an external CoroutineScope
should be used as
explained in the Coroutines & Patterns for work that shouldn’t be cancelled blog post.
class ArticlesRepository(
private val articlesDataSource: ArticlesDataSource,
private val externalScope: CoroutineScope,
) {
// As we want to complete bookmarking the article even if the user moves
// away from the screen, the work is done creating a new coroutine
// from an external scope
suspend fun bookmarkArticle(article: Article) {
externalScope.launch { articlesDataSource.bookmarkArticle(article) }
.join() // Wait for the coroutine to complete
}
}
externalScope
should be created and managed by a class that lives longer than
the current screen, it could be managed by the Application
class or a
ViewModel
scoped to a navigation graph.
Inject TestDispatchers in tests
An instance of
TestDispatcher
should be injected into your classes in tests. There are two available
implementations in the
kotlinx-coroutines-test
library:
StandardTestDispatcher
: Queues up coroutines started on it with a scheduler, and executes them when the test thread is not busy. You can suspend the test thread to let other queued coroutines run using methods such asadvanceUntilIdle
.UnconfinedTestDispatcher
: Runs new coroutines eagerly, in a blocking way. This generally makes writing tests easier, but gives you less control over how coroutines are executed during the test.
See the documentation of each dispatcher implementation for additional details.
To test coroutines, use the
runTest
coroutine builder. runTest
uses a
TestCoroutineScheduler
to skip delays in tests and to allow you to control virtual time. You can also
use this scheduler to create additional test dispatchers as needed.
class ArticlesRepositoryTest {
@Test
fun testBookmarkArticle() = runTest {
// Pass the testScheduler provided by runTest's coroutine scope to
// the test dispatcher
val testDispatcher = UnconfinedTestDispatcher(testScheduler)
val articlesDataSource = FakeArticlesDataSource()
val repository = ArticlesRepository(
articlesDataSource,
testDispatcher
)
val article = Article()
repository.bookmarkArticle(article)
assertThat(articlesDataSource.isBookmarked(article)).isTrue()
}
}
All TestDispatchers
should share the same scheduler. This allows you to
run all your coroutine code on the single test thread to make your tests
deterministic. runTest
will wait for all coroutines that are on the same
scheduler or are children of the test coroutine to complete before returning.
Avoid GlobalScope
This is similar to the Inject Dispatchers best practice. By using
GlobalScope
,
you're hardcoding the CoroutineScope
that a class uses bringing some downsides
with it:
Promotes hard-coding values. If you hardcode
GlobalScope
, you might be hard-codingDispatchers
as well.Makes testing very hard as your code is executed in an uncontrolled scope, you won't be able to control its execution.
You can't have a common
CoroutineContext
to execute for all coroutines built into the scope itself.
Instead, consider injecting a CoroutineScope
for work that needs to outlive
the current scope. Check out the
Creating coroutines in the business and data layer section
to learn more about this topic.
// DO inject an external scope instead of using GlobalScope.
// GlobalScope can be used indirectly. Here as a default parameter makes sense.
class ArticlesRepository(
private val articlesDataSource: ArticlesDataSource,
private val externalScope: CoroutineScope = GlobalScope,
private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default
) {
// As we want to complete bookmarking the article even if the user moves
// away from the screen, the work is done creating a new coroutine
// from an external scope
suspend fun bookmarkArticle(article: Article) {
externalScope.launch(defaultDispatcher) {
articlesDataSource.bookmarkArticle(article)
}
.join() // Wait for the coroutine to complete
}
}
// DO NOT use GlobalScope directly
class ArticlesRepository(
private val articlesDataSource: ArticlesDataSource,
) {
// As we want to complete bookmarking the article even if the user moves away
// from the screen, the work is done creating a new coroutine with GlobalScope
suspend fun bookmarkArticle(article: Article) {
GlobalScope.launch {
articlesDataSource.bookmarkArticle(article)
}
.join() // Wait for the coroutine to complete
}
}
Learn more about GlobalScope
and its alternatives in the
Coroutines & Patterns for work that shouldn’t be cancelled blog post.
Make your coroutine cancellable
Cancellation in coroutines is cooperative, which means that when a coroutine's
Job
is cancelled, the coroutine isn't cancelled until it suspends or checks
for cancellation. If you do blocking operations in a coroutine, make sure
that the coroutine is cancellable.
For example, if you're reading multiple files from disk, before you start
reading each file, check whether the coroutine was cancelled. One way to
check for cancellation is by calling the
ensureActive
function.
someScope.launch {
for(file in files) {
ensureActive() // Check for cancellation
readFile(file)
}
}
All suspend functions from kotlinx.coroutines
such as withContext
and
delay
are cancellable. If your coroutine calls them, you shouldn't need to do
any additional work.
For more information about cancellation in coroutines, check out the Cancellation in coroutines blog post.
Watch out for exceptions
Unhandled exceptions thrown in coroutines can make your app crash. If exceptions
are likely to happen, catch them in the body of any coroutines created with
viewModelScope
or lifecycleScope
.
class LoginViewModel(
private val loginRepository: LoginRepository
) : ViewModel() {
fun login(username: String, token: String) {
viewModelScope.launch {
try {
loginRepository.login(username, token)
// Notify view user logged in successfully
} catch (exception: IOException) {
// Notify view login attempt failed
}
}
}
}
For more information, check out the blog post Exceptions in coroutines, or Coroutine exceptions handling in the Kotlin documentation.
Learn more about coroutines
For more coroutines resources, see the Additional resources for Kotlin coroutines and flow page.