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Yoshinori Imajo
Yoshinori Imajo

Posted on • Edited on

Using Back-Pressure By flatMap in Combine framework

#ios #combine #swift

In the Combine framework, when using flatMap for irreparable processing requests using Web API, it is better to use the maxPublishers argument and use .flatMap(maxPublishers: .max(1)).

For example, by using .flatMap (maxPublishers: .max(1)) as shown below, the Web API POST will not process input events until the result is subscribed.

let cancelable = inputEvents .flatMap(maxPublishers: .max(1)) { // Example: irreparable processing requests createUser(name: $0.email, $0.password) } .sink { ... }
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Verification code

Verify the operation with two source codes.

  • Example without maxPublishers argument for flatMap
    • .unlimited by default
  • Example using .flatMap (maxPublishers: .max(1))

To conclude, .flatMap (maxPublishers: .max(1)) uses a flatMap to fix the number of Publishers created, even if there are new inputs until the result is subscribed. Do not execute.

Example without maxPublishers argument in flatMap

  • Spec
    • Input events are (1...3).publisher
    • flatMap
      • In the first event, asynchronously delay the number after 1 second and stream the number as a string
      • After the first one, asynchronously stream numbers as strings 
import Foundation import Combine import PlaygroundSupport let cancelable = (1...3).publisher .flatMap { value -> Future<String, Never> in print("🍏flatMap: \(value)") if value == 1 { return Future<String, Never> { promise in let v = value // delay the event of the stream created in the first shot. DispatchQueue.main.asyncAfter(deadline: .now() + 1.0) { print("🐢after: \(v)") promise(.success("\(v)")) } } } else { return Future<String, Never> { promise in let v = value DispatchQueue.main.async { print("🐰after : \(v)") promise(.success("\(v)")) } } } } .sink(receiveCompletion: { completion in switch completion { case .finished: print("🍎sink finished:", completion) case .failure(let error): print("🍎sink failure:", error) } }) { value in print("🍎sink received: \(String(describing: value))") } let page = PlaygroundPage.current page.needsIndefiniteExecution = true DispatchQueue.main.asyncAfter(deadline: .now() + 5.0) { // Instructs Xcode that the playground page has finished execution. print("🕘finished") page.finishExecution() }
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This code has the following output:

🍏flatMap: 1 🍏flatMap: 2 🍏flatMap: 3 🐰after : 2 🍎sink received: 2 🐰after : 3 🍎sink received: 3 🐢after: 1 🍎sink received: 1 🍎sink finished: finished 🕘finished
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Use asncAfter(deadline: .now() + 1.0) to delay the event of the first stream created. However, events flow one after another and switch to a new stream with flatMap.

Example of using .flatMap (maxPublishers: .max(1))

Change the following from the previous code

.flatMap { value -> Future<String, Error> in
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↓

.flatMap(maxPublishers: .max(1)) { value -> Future<String, Error> in
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This code has the following output:

🍏flatMap: 1 🐢after: 1 🍎sink received: 1 🍏flatMap: 2 🐰after : 2 🍎sink received: 2 🍏flatMap: 3 🐰after : 3 🍎sink received: 3 🍎sink finished: finished 🕘finished
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As before, use asncAfter(deadline: .now() + 1.0) to delay the event of the first stream created. It seems that the next event occurs after the delayed event is sink received: 1.

Conclusion

  • If you do not use the maxPublishers argument in flatMap, event creation does not wait for the end.
    • It does not use Back-Pressure.
  • If you specify max(1) for maxPublishers, event creation waits for completion.
    • It is requesting 1 as Back-Pressure.
      • The request repeats with each subscription.
    • Even if you specify max(1), the subscription does not end when you create a one-time event.

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