Buffer Observable Until Another Observable Completes

Question

I'm using RxSwift to wrap a mobile app's server sync process. I have an Observable<RemoteEvent> that wraps a websocket connection and emits each message received as an Event. Similarly, I have an Observable<SynchronizationResult> that wraps an API sync process. Once my application opens the WebSocket connection, the server sends a hello message. After that message is received, I want to start the sync process and buffer any events until the sync is completed. This is where I'm struggling. Currently, I have:

self.eventStreamService.observe(connection).scan((nil, [])) { (state, event) -> (Observable<RemoteEvent>?, [RemoteEvent]) in
  guard event.type == "hello" else {
    return (state.0?.concat(Observable.just(event)), state.1 + [event])
  }

  // This is the sync operation
  return (
    self.synchronizationService
      .synchronize(ConnectionSynchronizationContext(connection: connection), lightweight: true)
      .toArray()
      .flatMap { results -> Observable<RemoteEvent> in
        (state.1 + [event]).toObservable()
      },
    []
  )
}
.flatMapLatest { $0.0 ?? Observable.empty() }

Despite this being fairly ugly, it also has a significant bug: any incoming event results in the synchronization Observable being re-subscribed to, which then restarts the whole sync process. I'm sure there must be a better way to do this.

Answer 1

Here's how you could get the functionality you're looking for:

// this is a stub for the purpose of the example
let interval = Observable<Int>.interval(1, scheduler: MainScheduler.instance)
let websocketEvents = interval
    .map { i -> String in
        if i == 1 {
            return "hello"
        } else {
            return String(i)
        }
    }
    .replayAll()

websocketEvents.connect()

func performSync() -> Observable<Void> {
    return Observable<Void>.create { o in
        print("starting sync")
        // actually start sync with server
        // ....
        delay(2.0) {
            print("sync finished")
            o.onNext(())
        }
        return NopDisposable.instance
    }
}

// websocket events as they come, regardless of sync status
websocketEvents
    .subscribeNext { e in
        print("websocket event received: \(e)")
    }

// all websocket events, buffered and only emitted post-sync
websocketEvents
    .filter { $0 == "hello" }
    .flatMapLatest { _ in performSync() }
    .flatMapLatest { _ in websocketEvents }
    .subscribeNext { e in
        print("websocket event post sync: \(e)")
    }

This will output:

websocket event received: 0
websocket event received: hello
starting sync
websocket event received: 2
websocket event received: 3
sync finished
websocket event post sync: 0
websocket event post sync: hello
websocket event post sync: 2
websocket event post sync: 3
websocket event received: 4
websocket event post sync: 4
websocket event received: 5
websocket event post sync: 5