Hi all - is there a `Flow<T>.flowOn(dispatcher)` equivalent that that affects downstream? so t...

# coroutines

[xxfast]

Hi all - is there a Flow<T>.flowOn(dispatcher) equivalent that that affects downstream? so that i can do

results
  .map { result -> format(result) }
  .flowOn(<http://Dispatchers.IO|Dispatchers.IO>)
  .collectOn(Dispatchers.Main)
  .collect { presenter.display(it) }

instead of

withContext(Dispatchers.Main) { 
  results
    .map { result -> format(result) }
    .flowOn(<http://Dispatchers.IO|Dispatchers.IO>)
    .collect {
      presenter.display(it)
    }
}

[streetsofboston]

What about:

withContext(Dispatchers.Main) {
  results
    .map { result -> format(result) } 
    .flowon(<http://Dispatchers.IO|Dispatchers.IO>)
    .collect { ... }
}

since collect is a suspending function

[ursus]

scope you run in is your downstream "thread"

[Nick Allen]

flowWith used to exist as part of the Flow preview library but was removed because it didn't play well with other coroutine/flow semantics.

[Nick Allen]

If you simply want to avoid the nesting:

results
  .map { result -> format(result) }
  .flowOn(<http://Dispatchers.IO|Dispatchers.IO>)
  .onEach { presenter.display(it) }
  .flowOn(Dispatchers.Main)
  .collect()

[xxfast]

yeah i prefer less nesting - and like @Nick Allen’s one the most

[xxfast]

is there a negative connotation of using .onEach instead of .collect ?

[ephemient]

no, it's also the only reasonable way to use .launchIn(scope)

[myungpyo.shim]

If you are using it in Android ViewModel, you can use launchIn like below. (as ephemient mentioned above)

private fun sample1() {
    membershipUiData
        .map { /* IO Thread */ }
        .flowOn(<http://Dispatchers.IO|Dispatchers.IO>)
        .onEach { data ->
            /* Main thread */ 
        }
        .launchIn(viewModelScope)
}

private suspend fun sample2() = viewModelScope.launch {
    membershipUiData
        .map { /* IO Thread */ }
        .flowOn(<http://Dispatchers.IO|Dispatchers.IO>)
        .collect { data ->
            /* Main thread */
        }
}

(viewModelScope is composed of Dispatchers.main.immediate + SupervisorJob. So, if you are not using ViewModel, you can define appropriate context by yourself.)

[ephemient]

but consider

viewLifecycleOwner.repeatOnLifecycle(STARTED) {
    flow.collect { ... }
}

instead, https://medium.com/androiddevelopers/a-safer-way-to-collect-flows-from-android-uis-23080b1f8bda