How to replicate PromiseKit-style chained async flow using Combine + Swift

Question

I was using PromiseKit successfully in a project until Xcode 11 betas broke PK v7. In an effort to reduce external dependencies, I decided to scrap PromiseKit. The best replacement for handling chained async code seemed to be Futures using the new Combine framework.

I am struggling to replicate the simple PK syntax using Combine

ex. simple PromiseKit chained async call syntax

getAccessCodeFromSyncProvider.then{accessCode in startSync(accessCode)}.then{popToRootViewController}.catch{handleError(error)}

I understand:

A Swift standard library implementation of async/await would solve this problem (async/await does not yet exist, despite lots of chatter and involvement from Chris Latter himself)

I could replicate using Semaphores (error-prone?)

flatMap can be used to chain Futures

The async code I'd like should be able to be called on demand, since it's involved with ensuring user is logged in. I'm wrestling with two conceptual problems.

1. If I wrap Futures in a method, with sink to handle result, it seems that the method goes out of scope before subscriber is called by sink.

2. Since Futures execute only once, I worry that if I call the method multiple times I'll only get the old, stale, result from the first call. To work around this, maybe I would use a PassthroughSubject? This allows the Publisher to be called on demand.

Questions:

1. Do I have to retain every publisher and subscriber outside of the calling method
2. How can I replicate simple chained async using the Swift standard library and then embed this in a swift instance method I can call on-demand to restart the chained async calls from the top??

//how is this done using Combine?
func startSync() {
 getAccessCodeFromSyncProvider.then{accessCode in startSync(accessCode)}.catch{\\handle error here}
}

Answer 1

This is not a real answer to your whole question — only to the part about how to get started with Combine. I'll demonstrate how to chain two asynchronous operations using the Combine framework:

    print("start")
    Future<Bool,Error> { promise in
        delay(3) {
            promise(.success(true))
        }
    }
    .handleEvents(receiveOutput: {_ in print("finished 1")})
    .flatMap {_ in
        Future<Bool,Error> { promise in
            delay(3) {
                promise(.success(true))
            }
        }
    }
    .handleEvents(receiveOutput: {_ in print("finished 2")})
    .sink(receiveCompletion: {_ in}, receiveValue: {_ in print("done")})
        .store(in:&self.storage) // storage is a persistent Set<AnyCancellable>

First of all, the answer to your question about persistence is: the final subscriber must persist, and the way to do this is using the .store method. Typically you'll have a Set<AnyCancellable> as a property, as here, and you'll just call .store as the last thing in the pipeline to put your subscriber in there.

Next, in this pipeline I'm using .handleEvents just to give myself some printout as the pipeline moves along. Those are just diagnostics and wouldn't exist in a real implementation. All the print statements are purely so we can talk about what's happening here.

So what does happen?

start
finished 1 // 3 seconds later
finished 2 // 3 seconds later
done

So you can see we've chained two asynchronous operations, each of which takes 3 seconds.

How did we do it? We started with a Future, which must call its incoming promise method with a Result as a completion handler when it finishes. After that, we used .flatMap to produce another Future and put it into operation, doing the same thing again.

So the result is not beautiful (like PromiseKit) but it is a chain of async operations.

Before Combine, we'd have probably have done this with some sort of Operation / OperationQueue dependency, which would work fine but would have even less of the direct legibility of PromiseKit.

Slightly more realistic

Having said all that, here's a slightly more realistic rewrite:

var storage = Set<AnyCancellable>()
func async1(_ promise:@escaping (Result<Bool,Error>) -> Void) {
    delay(3) {
        print("async1")
        promise(.success(true))
    }
}
func async2(_ promise:@escaping (Result<Bool,Error>) -> Void) {
    delay(3) {
        print("async2")
        promise(.success(true))
    }
}
override func viewDidLoad() {
    print("start")
    Future<Bool,Error> { promise in
        self.async1(promise)
    }
    .flatMap {_ in
        Future<Bool,Error> { promise in
            self.async2(promise)
        }
    }
    .sink(receiveCompletion: {_ in}, receiveValue: {_ in print("done")})
        .store(in:&self.storage) // storage is a persistent Set<AnyCancellable>
}

As you can see, the idea that is our Future publishers simply have to pass on the promise callback; they don't actually have to be the ones who call them. A promise callback can thus be called anywhere, and we won't proceed until then.

You can thus readily see how to replace the artificial delay with a real asynchronous operation that somehow has hold of this promise callback and can call it when it completes. Also my promise Result types are purely artificial, but again you can see how they might be used to communicate something meaningful down the pipeline. When I say promise(.success(true)), that causes true to pop out the end of the pipeline; we are disregarding that here, but it could be instead a downright useful value of some sort, possibly even the next Future.

(Note also that we could insert .receive(on: DispatchQueue.main) at any point in the chain to ensure that what follows immediately is started on the main thread.)

Slightly neater

It also occurs to me that we could make the syntax neater, perhaps a little closer to PromiseKit's lovely simple chain, by moving our Future publishers off into constants. If you do that, though, you should probably wrap them in Deferred publishers to prevent premature evaluation. So for example:

var storage = Set<AnyCancellable>()
func async1(_ promise:@escaping (Result<Bool,Error>) -> Void) {
    delay(3) {
        print("async1")
        promise(.success(true))
    }
}
func async2(_ promise:@escaping (Result<Bool,Error>) -> Void) {
    delay(3) {
        print("async2")
        promise(.success(true))
    }
}
override func viewDidLoad() {
    print("start")
    let f1 = Deferred{Future<Bool,Error> { promise in
        self.async1(promise)
    }}
    let f2 = Deferred{Future<Bool,Error> { promise in
        self.async2(promise)
    }}
    // this is now extremely neat-looking
    f1.flatMap {_ in f2 }
    .receive(on: DispatchQueue.main)
    .sink(receiveCompletion: {_ in}, receiveValue: {_ in print("done")})
        .store(in:&self.storage) // storage is a persistent Set<AnyCancellable>
}

Answer 2

matt's answer is correct, use flatMap to chain promises. I got in the habit of returning promises when using PromiseKit, and carried it over to Combine (returning Futures).

I find it makes the code easier to read. Here's matt's last example with that recommendation:

var storage = Set<AnyCancellable>()

func async1() -> Future<Bool, Error> {
  Future { promise in
    delay(3) {
      print("async1")
      promise(.success(true))
    }
  }
}

func async2() -> Future<Bool, Error> {
  Future { promise in
    delay(3) {
      print("async2")
      promise(.success(true))
    }
  }
}

override func viewDidLoad() {
  print("start")

  async1()
    .flatMap { _ in async2() }
    .receive(on: DispatchQueue.main)
    .sink(receiveCompletion: {_ in}, receiveValue: {_ in print("done")})
    .store(in:&self.storage) // storage is a persistent Set<AnyCancellable>
}

Note that AnyPublisher will work as a return value as well, so you could abstract away the Future and have it return AnyPublisher<Bool, Error> instead:

func async2() -> AnyPublisher<Bool, Error> {
  Future { promise in
    delay(3) {
      print("async2")
      promise(.success(true))
    }
  }.eraseToAnyPubilsher()
}

Answer 3

Also if you want to use the PromiseKit-like syntax, here are some extensions for Publisher

I am using this to seamlessly switch from PromiseKit to Combine in a project

extension Publisher {
    
    func then<T: Publisher>(_ closure: @escaping (Output) -> T) -> Publishers.FlatMap<T, Self>
    where T.Failure == Self.Failure {
        flatMap(closure)
    }
    
    func asVoid() -> Future<Void, Error> {
        return Future<Void, Error> { promise in
            let box = Box()
            let cancellable = self.sink { completion in
                if case .failure(let error) = completion {
                    promise(.failure(error))
                } else if case .finished = completion {
                    box.cancellable = nil
                }
            } receiveValue: { value in
                promise(.success(()))
            }
            box.cancellable = cancellable
        }
    }
    
    @discardableResult
    func done(_ handler: @escaping (Output) -> Void) -> Self {
        let box = Box()
        let cancellable = self.sink(receiveCompletion: {compl in
            if case .finished = compl {
                box.cancellable = nil
            }
        }, receiveValue: {
            handler($0)
        })
        box.cancellable = cancellable
        return self
    }
    
    @discardableResult
    func `catch`(_ handler: @escaping (Failure) -> Void) -> Self {
        let box = Box()
        let cancellable = self.sink(receiveCompletion: { compl in
            if case .failure(let failure) = compl {
                handler(failure)
            } else if case .finished = compl {
                box.cancellable = nil
            }
        }, receiveValue: { _ in })
        box.cancellable = cancellable
        return self
    }
    
    @discardableResult
    func finally(_ handler: @escaping () -> Void) -> Self {
        let box = Box()
        let cancellable = self.sink(receiveCompletion: { compl in
            if case .finished = compl {
                handler()
                box.cancellable = nil
            }
        }, receiveValue: { _ in })
        box.cancellable = cancellable
        return self
    }
}

fileprivate class Box {
    var cancellable: AnyCancellable?
}

And here's an example of use:

func someSync() {
    Future<Bool, Error> { promise in
        delay(3) {
            promise(.success(true))
        }
    }
    .then { result in
        Future<String, Error> { promise in
            promise(.success("111"))
        }
    }
    .done { string in
        print(string)
    }
    .catch { err in
        print(err.localizedDescription)
    }
    .finally {
        print("Finished chain")
    }
}