Hi all A little confused about thread in couroutin...

# coroutines

[Jason]

Hi all
A little confused about thread in couroutine.
Basically, we all use <http://Dispatchers.IO|Dispatchers.IO> for call api from network. But I am confused that where need we specified the dispatcher.
For example:
I have a UserViewModel class and FetchUserUseCase class.

Pattern 1

1. FetchUserUseCase.kt

suspend  fun fetch(): UserResponse {
   return runCatching {
            withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
                clientApi.get()
            }
        }
}

2. UserViewModel.kt

fun fetch() {
   viewModelScope.launch {
     fetchUserUseCase.fetch()
   }
}

Pattern 2

1. FetchUserUseCase.kt

suspend  fun fetch(): UserResponse {
   return coroutineScope {
          clientApi.get()
   }
}

2. UserViewModel.kt

fun fetch() {
   viewModelScope.launch {
      runCatching {
         withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
           fetchUserUseCase.fetch()
         }
      }
   }
}

Above 2 way of writing is the same ?
Where need we specified dispatcher ? ViewModel or UseCase

[Jason]

@elizarov Do you have any idea for this case?

[elizarov]

I prefer the first one — encapsulated https://medium.com/@elizarov/blocking-threads-suspending-coroutines-d33e11bf4761

[Jason]

Thank you. Let me considering

[dekans]

withContext already creates its CoroutineScope AFAIU, no need for an additionnal one.

[streetsofboston]

There was a very similar discussion here
https://kotlinlang.slack.com/archives/C5GB1NDGE/p1574205551088100?thread_ts=1574205551.088100&cid=C5GB1NDGE

[Jason]

By the way, how to test UseCase in option 1 ?
We can not change dispatchers ???

[streetsofboston]

You could do scope = viewModelScope + context where context is provided as an constructor input parameter. You could provide a TestCoroutineDispatcher as input for context.

[Jason]

No, in UseCase the Dispatchers.IO is specified so how to change it before test ?

val testScope = TestCoroutineScope(testDispatcher)

    beforeEachTest {
        Dispatchers.setMain(testDispatcher)
    }
    afterEachTest {
        Dispatchers.resetMain()
        testScope.cleanupTestCoroutines()
    }
    return testScope

[Jason]

This job has not completed yet
java.lang.IllegalStateException: This job has not completed yet

[streetsofboston]

Yeah... That's a know issue... If you Google this error you'll find it.

[zak.taccardi]

side note, UseCase pattern doesn’t really make sense in Kotlin. It’s just a higher order function https://twitter.com/ZakTaccardi/status/1199764248861261824?s=20