How to Replicate Promisekit-Style Chained Async Flow Using Combine + Swift

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

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>
}

Chaining calls when using Future in Swift similar to PromiseKit

You can use a .flatMap operator, which takes a value from upstream and produces a publisher. This would look something like below.

Note, that it's also better to return a type-erased AnyPublisher at the function boundary, instead of the specific publisher used inside the function

func loginwithFacebook() -> AnyPublisher<UserProfileCompact, Error> {
   loginWithFacebook().flatMap { authCredential in
      loginWithFirebase(authCredential)
   }
   .eraseToAnyPublisher()
}

Chaining n requests in Combine

For this one, a lot depends on how you want to use the User objects. If you want them to all emit as individual users as they come in, then merge is the solution. If you want to keep the array order and emit them all as an array of users once they all come in, then combineLatest is what you need.

Since you are dealing with an array, and neither merge nor combineLatest have array versions, you will need to use reduce. Here's examples:

func combine(ids: [Int]) -> AnyPublisher<[User], Error> {
    ids.reduce(Optional<AnyPublisher<[User], Error>>.none) { state, id in
        guard let state = state else { return fetchUser(for: id).map { [$0] }.eraseToAnyPublisher() }
        return state.combineLatest(fetchUser(for: id))
            .map { $0.0 + [$0.1] }
            .eraseToAnyPublisher()
    }
    ?? Just([]).setFailureType(to: Error.self).eraseToAnyPublisher()
}

func merge(ids: [Int]) -> AnyPublisher<User, Error> {
    ids.reduce(Optional<AnyPublisher<User, Error>>.none) { state, id in
        guard let state = state else { return fetchUser(for: id).eraseToAnyPublisher() }
        return state.merge(with: fetchUser(for: id))
            .eraseToAnyPublisher()
    }
    ?? Empty().eraseToAnyPublisher()
}

Notice that in the combine case, if the array is empty, the Publisher will emit an empty array then complete. In the merge case, it will just complete without emitting anything.

Also notice that in either case if any of the Publishers fail, then the entire chain will shut down. If you don't want that, you will have to catch the errors and do something with them...

PromiseKit 6 iOS chaining

The problem is because you're chaining off of a done, which doesn't like that you're trying to then do a call to then off of that.

Instead, you'll want to save the promise and use it for the later calls. You can do something like this:

let promise = firstly {
    stellar.promiseFetchPayments(for: "")
}

promise.done { records in
    print(records)
}

promise.then {
    // call some other method 
}.done { data in 
    // more data 
}.catch { error in
    print(error)
}

You can even return that promise from a method to use in other places, or pass it around to another method.

Setting backpressure in OperationQueue (or alternative API, e.g. PromiseKit, Combine Framework)

If you're using Combine, flatMap can provide the back pressure. FlatMap creates a publisher for each value it receives, but exerts back pressure when it reaches the specified maximum number of publishers that haven't completed.

Here's a simplified example. Assuming you have the following functions:

func loadImage(url: URL) -> AnyPublisher<UIImage, Error> {
   // ...
}

func doImageProcessing(image: UIImage) -> AnyPublisher<Void, Error> {
   // ...
}

let urls: [URL] = [...] // many image URLs

let processing = urls.publisher
    .flatMap(maxPublishers: .max(5)) { url in 
        loadImage(url: url)
           .flatMap { uiImage in
              doImageProcessing(image: uiImage)
           }
    }

In the example above, it will load 5 images, and start processing them. The 6th image will start loading when one of the earlier ones is done processing.

Combine uncollect operator?

You can use flatMap together with a new publisher

[1,2,3,4,5,6].publisher
    .collect()
    .flatMap { $0.publisher }
    .sink { print($0) }

1

2

3

4

5

6

Combine using optional struct variable

You can use the publisher of Optional, which gives you a publisher that publishes one element if the optional is not nil, and an empty publisher otherwise.

You can then replaceEmpty(with: Data()), and map to an EventModel.

func loadEvent(_ event: SCEvent) -> AnyPublisher<EventModel, Error> {
    event.imagePath.publisher
        .flatMap(DataDownloader.downloadData(fromPath:))
        .replaceEmpty(with: Data())
        .map {
            EventModel(name: event.name, imageData: $0)
        }
        .eraseToAnyPublisher()
}

However, I don't think replacing with a Data() is a good idea. A better design would be to replace with nil, in which case you'll have to map to an optional first:

struct EventModel {
    let name: String
    let imageData: Data?
}

...

        .flatMap(DataDownloader.downloadData(fromPath:))
        .map { $0 as Data? } // here
        .replaceEmpty(with: nil)

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