It’s time to migrate from RxSwift to Combine. Long story short

Apple introduced Combine in pair with SwiftUI a year ago. However, it’s not necessary to convert all your UI to SwiftUI to start using this new reactive framework.

Usually, iOS apps support only the last two released iOS versions – so this fall some of you will finally drop support of iOS 12. Why not convert your existing RxSwift codebase to Combine then?

I’m going to tell you my story of forking an existing network library based on top of RxSwift and refactoring it with Combine.

https://github.com/chipp/miniGnomon

Transcript

1. It’s time to migrate from RxSwift to Combine Long story short by Vladimir Burdukov 1 Mobile Z-days, Sep. 2020
2. Introduction My name is Vladimir Burdukov. I'm a λ-addict since 2013 and RxSwift user since 2015. One of authors of small Rx-based networking library Gnomon. Lead Software Engineer at EPAM by day. Rust maniac by night. 2 Mobile Z-days, Sep. 2020
3. Agenda 1. Brief overview and comparison of RxSwift and Combine core elements. 2. Demo session. 3. Issues which I experienced during my transition to Combine. 3 Mobile Z-days, Sep. 2020
4. RxSwift vs Combine RxSwift Combine Stream class Observable<Element> protocol Publisher (Output, Failure: Error) Receiver protocol ObserverType (Element) protocol Subscriber (Input, Failure: Error) Cancellation Disposable Cancellable and Subscription Subjects PublishSubject, BehaviorSubject PassthroughSubject, CurrentValueSubject 4 Mobile Z-days, Sep. 2020
5. Demo 5 Mobile Z-days, Sep. 2020
6. BlockingSubscriber CFRunLoop with/without timeout Implements Subscriber protocol func receive(subscription: Subscription) func receive(_ input: Input) -> Subscribers.Demand func receive(completion: Subscribers.Completion<Failure>) 6 Mobile Z-days, Sep. 2020
7. receive(subscription: Subscription) func receive(subscription: Subscription) { self.subscription = subscription } 7 Mobile Z-days, Sep. 2020
8. receive(_ input: Input) func receive(_ input: Input) -> Subscribers.Demand { inputs.append(input) demand -= 1 if demand == .none { subscription?.cancel() stop() } return demand } 8 Mobile Z-days, Sep. 2020
9. receive(completion: ) func receive(completion: Subscribers.Completion<Failure>) { self.completion = completion stop() } 9 Mobile Z-days, Sep. 2020
10. CombineLatestMany struct CombineLatestMany: Publisher {} class CombineLatestManySubscription: Subscription {} class CombineLatestManySubscription.Side: Subscriber {} 10 Mobile Z-days, Sep. 2020
11. 11 Mobile Z-days, Sep. 2020
12. CombineLatestMany Publisher struct CombineLatestMany<Upstream: Publisher>: Publisher { typealias Output = [Upstream.Output] typealias Failure = Upstream.Failure func receive<S>(subscriber: S) /* generic constraints */ {} } 12 Mobile Z-days, Sep. 2020
13. CombineLatestMany Publisher func receive<S>(subscriber: S) /* generic constraints */ { let subscription = CombineLatestManySubscription( publishers: publishers, subscriber: subscriber ) for (index, publisher) in publishers.enumerated() { publisher.subscribe( Side<Upstream.Output>(subscription, index: index) ) } subscriber.receive(subscription: subscription) } 13 Mobile Z-days, Sep. 2020
14. CombineLatestMany Publisher func receive<S>(subscriber: S) /* generic constraints */ { let subscription = CombineLatestManySubscription( publishers: publishers, subscriber: subscriber ) for (index, publisher) in publishers.enumerated() { publisher.subscribe( Side<Upstream.Output>(subscription, index: index) ) } subscriber.receive(subscription: subscription) } 13 Mobile Z-days, Sep. 2020
15. CombineLatestMany Publisher func receive<S>(subscriber: S) /* generic constraints */ { let subscription = CombineLatestManySubscription( publishers: publishers, subscriber: subscriber ) for (index, publisher) in publishers.enumerated() { publisher.subscribe( Side<Upstream.Output>(subscription, index: index) ) } subscriber.receive(subscription: subscription) } 13 Mobile Z-days, Sep. 2020
16. CombineLatestMany Publisher func receive<S>(subscriber: S) /* generic constraints */ { let subscription = CombineLatestManySubscription( publishers: publishers, subscriber: subscriber ) for (index, publisher) in publishers.enumerated() { publisher.subscribe( Side<Upstream.Output>(subscription, index: index) ) } subscriber.receive(subscription: subscription) } 13 Mobile Z-days, Sep. 2020
17. receive(subscription: Subscription) func receive(subscription: Subscription) { self.subscription = subscription } 14 Mobile Z-days, Sep. 2020
18. Issues Opaque types don't work for Publishers. func validateInput(_ input: String) -> some Publisher { guard !input.isEmpty else { return Fail(failure: ValidationError.emptyInput) } let request: Request<String> = generateRequest(input) return client.models(for: request).map(\.result) } 15 Mobile Z-days, Sep. 2020
19. Opaque types func validateInput(_ input: String) -> some Publisher { guard !input.isEmpty else { return Fail(failure: ValidationError.emptyInput) // Fail<?, ValidationError> } let request: Request<String> = generateRequest(input) return client.models(for: request).map(\.result) // Publishers.Map<AnyPublisher<...>, String> } 16 Mobile Z-days, Sep. 2020
20. Opaque types func validateInput(_ input: String) -> some Publisher { guard !input.isEmpty else { return Fail(failure: ValidationError.emptyInput) // Fail<?, ValidationError> } let request: Request<String> = generateRequest(input) return client.models(for: request).map(\.result) // Publishers.Map<AnyPublisher<...>, String> } 16 Mobile Z-days, Sep. 2020
21. Opaque types func validateInput(_ input: String) -> some Publisher { guard !input.isEmpty else { return Fail(failure: ValidationError.emptyInput) // Fail<?, ValidationError> } let request: Request<String> = generateRequest(input) return client.models(for: request).map(\.result) // Publishers.Map<AnyPublisher<...>, String> } 16 Mobile Z-days, Sep. 2020
22. Opaque types What is Output and Failure types for that publisher? let publisher: some Publisher = validateInput("Mobile People") // AnyPublisher<String, ValidationError> publisher.map(\.count) 17 Mobile Z-days, Sep. 2020
23. eraseToAnyPublisher() extension Publisher { associatedtype Output associatedtype Failure: Error func eraseToAnyPublisher() -> AnyPublisher<Output, Failure> } 18 Mobile Z-days, Sep. 2020
24. No testing toolchain from Apple Fortunately, iOS open-source community is very cool. EntwineTest is an analogue of RxTest with TestScheduler, TestPublisher and TestSubscriber. CombineExpectations – some kind of RxBlocking analogue based on XCTestCase. (?) CombineBlocking 19 Mobile Z-days, Sep. 2020
25. Lack of familiar Rx operators No CombineLatestMany nor ZipMany nor withLatest(from:). Entwine – implements some of RxSwift operators. Feel free to take inspiration from OpenCombine to implement your own operators. 20 Mobile Z-days, Sep. 2020
26. Conclusion It's good time to start transition of your code from RxSwift to Combine. Combine doesn't have so awesome community at the moment mostly because It's a closed source framework. There are many things to do both for Apple and the OSS community. 21 Mobile Z-days, Sep. 2020