[Question] Thread switching in Repository and Network layer

[blyscuit]

Issue

• The code uses receive(on:) in multiple places. What are the thought behind adding these receive(on:)?
• If a repository or a use case calls two network request with Publishers.merge, what thread are the calls being made on?
• Do you think main thread is being blocked while API is called?

[lngyeen]

@blyscuit

1. I checked the source code and there are a total of 4 places where this function is used:

• The first place is in the LoginViewModel, this ensures that changes affecting the loginButtonEnabled variable are always made in the main thread, which is nothing special.
• The second place is in the NetworkJSONAPIClient class, you can check the publishData() function, its default queue is the main queue, but in this class I also have to use the Japx library to decode JSONAPI, so I set the queue for publishData( ) to .global(qos: .userInteractive), this will ensure all subsequent map functions are executed in the global queue and do not block the main thread. Finally, I switch the thread back to main to return the data in the main thread as a default utility like the default queue of publishData().
• The last place that contains 2 receive(on:) is in the SurveyRepositoryWithCachingImpl class. As I said above, NetworkJSONAPIClient returns data to the main queue by default. However, in SurveyRepositoryWithCachingImpl class I still have to do some additional work such as reading and writing cache data to the file. I didn't want these to block the main thread so I decided to move them back to the background thread and then back to the main thread to return the final data.

2. If a repository or a use case calls two network request with Publishers.merge, so it depend on which thread the each Publisher on Publishers.merge run on. For example: 1st one emit values 1-3-5 and run on main thread, 2nd one emit 2-4-6 and run on background thread so in Publishers.merge the observation code will run on main thread when it receive values 1-3-5 and on background thread when receive values 2-4-6. Therefore, to unify the observation thread for Publishers.merge, we need to use the receive(on:) for Publishers.merge.

3. No, I believe API calls will not block the main thread.