ios – Find out how to Chain Service Requests Serially with Writer and Sharing First Writer

I’ve three capabilities that must be referred to as serially. Two of them are publishers. The third one creates sockets. After calling the primary perform(request writer) I must then flip round and name different requests.

I’m having a tough time with the syntax for chaining and mapping the result of 1 request to the opposite.

My calls so as are

//1
func getCredentialsPublisher(userInfo: String)-> AnyPublisher<CredentialsResponse, Error>
//2
func getSession(token: String) -> AnyPublisher<Session, Error>
//3
func setupSockets(sessionID: String) -> SocketManager?

The getCredentialsPublisher compiles and returns the anticipated data.

The difficulty I’m having is attempting to transform getSession writer to return a SocketManager.

func startSession(uniqueId: String) -> AnyPublisher<SocketManager?, By no means>
     return getSession(token: uniqueId)
         .flatMap { response in
             let socket =  self.setupSockets(sessionID: response.id)
             return  Simply(socket)
             .catch { _ in
                 Simply(nil)
              }
          }
         .eraseToAnyPublisher()
}

this provides me a “Can’t convert return expression of sort ‘AnyPublisher<P.Output, Error>’ to return sort ‘AnyPublisher<SocketManager?, By no means>'”

Any concepts why?

since sockets are created as soon as, is it okay to have “By no means” for the failure?

In my viewModel, I take advantage of it like this

var credentialsSubscriber: AnyCancellable?
var startSessionSubscriber: AnyCancellable?

credentialsSubscriber =.getCredentialsPublisher(userInfo: object)
    .sink { lead to
        print("end result from get credentials: (end result).")
     } receiveValue: { response in
          //A. begin sockets
          startSessionSubscriber = startSession(uniqueId: response.id)
               .sink( receiveValue: { socket in  
                   self.socketManager = socket
           }
          //B. do one thing else that must be achieved when credentials are acquired.
     }

is that this the correct technique to chain calls 1 and a pair of? Do I would like totally different subscribers for A. and B.?