Develop Reference

Is it necessary to use return withContext(Dispatchers.IO)?

I'm using Firebase authentication. In the repository I have this function:
override suspend fun signIn(): Result<Boolean> {
return try {
auth.signInAnonymously().await()
Result.Success(true)
} catch (ex: Exception) {
Result.Failure(ex)
}
}
This function is called from within the ViewModel class:
var response by mutableStateOf<Result<Boolean>>(Result.Success(false))
private set
fun signIn() = viewModelScope.launch {
response = repository.signIn()
}
Which works fine but I was suggested to use in the ViewModel:
fun signIn() = viewModelScope.launch(Dispatchers.IO) {
response = repository.signIn()
}
To add Dispatchers.IO and inside the repository:
override suspend fun signIn(): Result<Boolean> {
return withContext(Dispatchers.IO) {
try {
auth.signInAnonymously().await()
Result.Success(true)
} catch (ex: Exception) {
Result.Failure(ex)
}
}
}
To launch a coroutine using withContext. I and I don't understand why? I'm using Jetpack Compose.

Whoever suggested changing your code is wrong.
It is a general Kotlin coroutines convention that suspend functions never need to be called on a specific dispatcher, specifically because they must never block. They always internally delegate to a specific dispatcher if they need one. (But perhaps as an optimization, a private suspend function might avoid doing it for a function that must be called on the Main dispatcher.)
Since this is a convention, all the libraries from Google, Android, Square, etc. and anyone else who knows what they're doing, only have suspend functions that can be called from any dispatcher.
This includes the await() call you're using with Firebase. Therefore, your repository's signIn function is already perfectly fine as-is. Since it doesn't call any blocking functions, and the suspend function it calls is a proper suspend function that also does not block, it conforms to the standard (it doesn't block).
The function in your ViewModel is also fine. No dispatcher needs to be specified.

Actually, since you are already calling signIn from a coroutine started with Dispatchers.IO you don't have to use return withContext(...).
Since your repository method is suspend, it is able to call coroutines without special blocks like withContext.
// This line tells to launch code on separate IO thread, to avoid UI freezing
// Since default viewModelScope.launch runs on Dispatchers.Main, which is
// also used for rendering
fun signIn() = viewModelScope.launch(Dispatchers.IO) {
response = repository.signIn()
}
In your repository you can just
// Since signIn was called on IO context from viewModel, it will also
// return on IO
override suspend fun signIn(): Result<Boolean> {
return try {
auth.signInAnonymously().await()
Result.Success(true)
} catch (ex: Exception) {
Result.Failure(ex)
}
}

We have a two ways to start coroutine async and launch.
launch will use to perform serial/sequence task in background.
async is used when we expect some result back and also want to perform parallel operation.
Same way withContext is nothing but another way of writing the async where one does not have to write await(). When withContext, is used, it runs the tasks in series instead of parallel. So one should remember that when we have a single task in the background and want to get back the result of that task, we should use withContext.
In your case you can change your code as below
fun signIn() = viewModelScope.launch(Dispatchers.IO) {
val response = async { repository.signIn()}.await()
}
and remove withContext
suspend fun signIn(): Result<Boolean> {
return try {
auth.signInAnonymously().await()
Result.Success(true)
} catch (ex: Exception) {
Result.Failure(ex)
}
}
One more way if you don't want to use return with withContext
override suspend fun signIn() = {
withContext(Dispatchers.IO) {
try {
auth.signInAnonymously().await()
Result.Success(true)
} catch (ex: Exception) {
Result.Failure(ex)
}
}
}
In Nutshell if you expecting some result from your task then you have to use async or withContext.
Hope I am able to solve your problem or issue.

Kotlin Coroutines Flow catch mechanism

In my sample I'm calling network operation and emitting success case but on error e.g 404 app crashes wihout emitting exception. Surrendering with try catch prevent crashes but I want to pass error till the ui layer like success case.
suspend fun execute(
params: Params,
):
Flow<Result<Type>> = withContext(Dispatchers.IO) {
flow {
emit(Result.success(run(params)))
}.catch {
emit(Result.failure(it))
}
}

There is a helpful function runCatching for creating a Result easily, but the problem in coroutines is that you don't want to be swallowing CancellationExceptions. So below, I'm using runCatchingCancellable from my answer here.
This shouldn't be a Flow since it returns a single item.
If run is a not a blocking function (it shouldn't be if you are using Retrofit with suspend functions), your code can simply be:
suspend fun execute(params: Params): Result<Type> = runCatchingCancellable {
run(params)
}
If it is a blocking function you can use:
suspend fun execute(params: Params): Result<Type> = runCatchingCancellable {
withContext(Dispatchers.IO) {
run(params)
}
}
If you were going to return a Flow (which you shouldn't for a returning a single item!!), then you shouldn't make this a suspend function, and you should catch the error inside the flow builder lambda:
fun execute(params: Params): Flow<Result<Type>> = flow {
emit(runCatchingCancellable {
run(params)
})
}
// or if run is blocking (it shouldn't be):
fun execute(params: Params): Flow<Result<Type>> = flow {
emit(runCatchingCancellable {
withContext(Dispatchers.IO) { run(params) }
})
}

If you want to use flows you can use the catch method of flows.
As you said you can use try-catch but it would break the structured concurrency since it would catch the cancellation exception as well or it would avoid the cancellation exception to be thrown.
One thing that you can do is to use an Exception handler at the point where you launch the root coroutine that calls the suspend function.
val coroutineExceptionHandler = CoroutineExceptionHandler { _, exception ->
// handle it
}
scope.launch(handler) { // root coroutine
execute(params)
somethingThatShouldBeExecutedOnlyIfPreviousCallDoesNotThrow()
}
This solution is good for both flows and non-flow coroutines.
In the solution with the runCatching you will have to manually check the result of the first execute to avoid the second one to run.
One interesting thread is here.

How can i work with flow inside repository android?

I wrote code which works fine without kotlin flow, but i want to try kotlin flow and when i used it inside repository but somehow it is not even entering inside i could not find any solution since it is not throwing any error, it is just not entering inside function and i think it is because of flow collector. It is coming till repository but for repo it is not entering.
class NewsRepositoryImpl(private val newsService: NewsService) : NewsRepository {
override suspend fun getNews(search: String): Flow<ResultWrapper<List<Article>>> = flow {
emit(ResultWrapper.Loading)
try {
val news = newsService.getNews(search, BuildConfig.API_KEY)
emit(ResultWrapper.Success(news.articles.map { it.toArticle() }))
} catch (e: Exception) {
emit(ResultWrapper.Error(e.message))
}
}
}

Using flow builder you create a cold stream of data. The code inside it is executed only when the flow is being collected, i.e. terminal operators invoked, for example collect, collectLatest, first... .
coroutineScope.launch {
newsRepository.getNews("news").collectLatest { result ->
// use result
}
}

How to make higher order functions synchronous kotlin?

I am trying to run a test in which I want to wait till higher order function executes. As of now I am not able to figure out any ways to do it. Following is my code.
#Test
fun `test execute routine error`() = runBlocking(coroutineDispatcher) {
val observer = mock<Observer<String>>()
baseViewModel.error.observeForever(observer)
val httpException = HttpException(Response.error<String>(402, mock(ResponseBody::class.java)))
val function = baseViewModel.executeRoutine {
throw httpException
}
verify(observer).onChanged("Something went wrong. Please try again")
}
The problem with above snippet is that it jumps to the last line i.e. verify() before throwing an http exception for executeRoutine.
Update: Execute routine definition
fun executeRoutine(requestType: RequestType = RequestType.POST_LOGIN, execute: suspend () -> Unit) {
viewModelScope.launch {
withContext(Dispatchers.IO) {
_spinner.postValue(true)
try {
execute()
} catch (ex: HttpException) {
val errorHandler = errorHandlerFactory.create(requestType)
_error.postValue(errorHandler.getErrorMessageFrom(ex))
} catch (ex: Exception) {
_error.postValue(ex.localizedMessage)
Timber.e(ex)
} finally {
_spinner.postValue(false)
}
}
}
}

The problem is that the higher order function does execute, it just doesn't do what you think it does -- its execution is launching the task, not waiting for it to complete.
You will have to solve the problem another way, by either having your test wait until the change is observed, or having the callback complete a barrier to allow the test to proceed (e.g. completableJob.complete() at the end of the call back, and completableJob.join() waiting before proceeding with the test).
It might also be desirable to rearchitect your code so you don't have to do anything special, e.g. by making executeRoutine a suspend function executing the code rather than launching the code in another scope.

Android Kotlin Co-routine Exceptions

I'm trying to use Kotlin co-routines to trigger a network request and handle Exceptions. I've looked at a lot of tutorials on co-routines but I'm really struggling to relate what i know to the problem i have.
The problem
Trying to get an exception to be caught in the View, but no Exception is thrown from the ViewModel so the application crashes.
The Code
My Android app has three layers that relate to this issue. I have the View, ViewModel and a Service layer.
In the service layer, request.execute() can throw a UserAuthException.
MyView (View)
private val mViewModel: MyViewModel by viewModels()
private fun getFileId() {
try {
mViewModel.requestFileId()
} catch (e: UserAuthException) {
Timber.i(e)
}
}
MyViewModel (ViewModel)
private val apiService = MyApiService()
fun requestFileId() {
viewModelScope.launch {
ApiService.requestFileId()
}
}
MyApiService (Service Layer)
suspend fun requestFileId(): FileId = withContext(Dispatchers.IO) {
request.execute()
}
Things that i have looked at
I have played around with CoroutineExceptionHandlers, supervisorJobs with no luck, but without the fundamental knowledge of how these things work I'm not really making any progress.
Any help would be appreciated, thanks.

fun requestFileId() {
viewModelScope.launch {
ApiService.requestFileId()
}
}
This is not a suspendable function. It launches a concurrent coroutine and returns right away. Clearly, calling requestFileId() will never throw an exception.
Launching a coroutine is just like starting another thread, it introduces concurrency to your code. If your current code hopes to stay non-concurrent while observing the results of suspendable functions, you may be looking at significant architectural changes to the application to make it behave correctly under concurrency.

In your model, change it to something like this:
fun requestFileId() {
viewModelScope.launch {
try {
ApiService.requestFileId()
} catch (e: Exception) {
// inform your view
}
}
}