Hi i am new in coroutines :slightly_smiling_face: ...

# coroutines

[Gioele Dev]

Hi i am new in coroutines 🙂 i have a simply question.. from viewModel i get data from server with retrofit, viewModel -> useCase -> Repository... retrofit.
When create coroutine i use the classic viewModelScope.launch {
... in some book i read that for network i have to use or better to use

viewModelScope.launch( <http://Dispatchers.IO|Dispatchers.IO>)

is it correct ?

[Morgane Soula]

It is but you should inject your Dispatchers in order to test your viewModel easily

[Gioele Dev]

i dont' understand , is correct use a different Dispatchers or no?

[uli]

Yes and no 😉

[uli]

True in the sense that to do blocking IO, you should dispatch via Dispatchers.IO so that you only block one of many io threads.

[uli]

Not true in the sense that this dispatching to a background thread is the responsibility of the method doing the actual blocking.

[uli]

The contract in coroutines is, that a suspend method does not block. i.e. this is not guaranteed but is expected from the author of suspend methods.
So if someone needs to block, let’s say for a blocking read from the network they would write:

suspend fun Socket.readMyData(int count) : ByteArray = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
  return read(count)
}

So no one downstream needs to guess, what the method is doing and which dispatcher is the right one to call it on.

[uli]

So, to answer your question: “No need to call retrofit on the IO dispatcher. If you use the suspend methods of retrofit, it’s retrofit’s responsibility to not block your main thread.”

[uli]

The same should always hold true for coroutines based libraries like retrofit, room, …