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>
}
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()
}