Sign in with Apple + Combine (CombineCocoa)

ASAuthorizationController+Combine.swift

import AuthenticationServices
import Combine
import CombineCocoa

extension ASAuthorizationController {
  private var delegateProxy: AuthorizationControllerDelegateProxy {
    .createDelegateProxy(for: self)
  }
  
  var didCompleteWithAuthroizationPublisher: AnyPublisher<ASAuthorization, Never> {
    let selector = #selector(ASAuthorizationControllerDelegate.authorizationController(controller:didCompleteWithAuthorization:))
    return self.delegateProxy.interceptSelectorPublisher(selector)
      .compactMap { $0[1] as? ASAuthorization }
      .eraseToAnyPublisher()
  }
  var didCompleteWithErrorPublisher: AnyPublisher<Error, Never> {
    let selector = #selector(ASAuthorizationControllerDelegate.authorizationController(controller:didCompleteWithError:))
    return self.delegateProxy.interceptSelectorPublisher(selector)
      .compactMap { $0[1] as? Error }
      .eraseToAnyPublisher()
  }
  
  func performRequestsPublisher(options: RequestOptions = []) -> AnyPublisher<ASAuthorization, Error> {
    Deferred {
      defer { self.performRequests(options: options) }
      return Publishers
        .Merge(
          self.didCompleteWithAuthroizationPublisher
            .setFailureType(to: ASAuthorizationError.self)
            .mapError { $0 as Error },
          self.didCompleteWithErrorPublisher
            .flatMap { Fail(error: $0) }
        )
    }
    .eraseToAnyPublisher()
  }
}

private class AuthorizationControllerDelegateProxy: DelegateProxy, ASAuthorizationControllerDelegate, DelegateProxyType {
  func setDelegate(to object: ASAuthorizationController) {
    object.delegate = self
  }
}