Which is better to put the Coroutine call in the Repository or the ViewModel?

The question is where to specify that repository jobs should run on IO threads pools:

• A) In Repository
• B) In ViewModel

Frankly I am not sure which way is better.

B) Having it in ViewModel means more transparency of what runs on which thread. This is also the way I mostly worked with RxJava. As a negative your ViewModel will be cluttered with thread switching (withContext()), while it's actually obvious that all repository job should run on the background thread. So is this extra information any useful?

A) Having it in Repository means more flexibility regarding thread switching in repository and cleaner code in ViewModel. Code will be less explicit from ViewModel point of view about threads. As a negative all repository methods will need to be suspended, so thus usable only from coroutines.

I think either way is fine honestly but if I have to choose I prefer keeping thread related code in the repository layer. (Version A in the example)

My justification:

(A) View Models should not assume how Repositories internally work. Version B implies that the View Model assumes that the Repository will run on the calling thread.

(B) Also, the repository should not depend on other components to use a specific thread to call its method. The repository should be complete by itself.

(C) To avoid code duplication. Having multiple View models calling one repository method is a very common situation. Version A wins in this case because you only need Corountine in one place, the repository.

I suggest looking at some of the Google Samples to see where they like to handle Coroutine and data access threading codes.

1. Sunflower uses Coroutine code in their repository. This is probably particularly useful for you because it includes examples of fire-and-forget type of requests.

2. GitHubBrowserRepo does not use Coroutine but the repository keeps reference to the Executor instances that are used when accessing the data.