A future publisher backed by a passthrough subject.

Publishers+Future.swift

//
//  Publishers+Future.swift
//
//  Created by Honghao Zhang on 9/21/20.
//

public extension Publishers {

  /// A deferred future backed by a passthrough subject.
  /// Note: This is because Future can create a memory leak.
  static func future<Output, Failure: Error>(_ attemptToFulfill: @escaping (@escaping (Result<Output, Failure>) -> Void) -> Void) -> AnyPublisher<Output, Failure> {
    Deferred<PassthroughSubject<Output, Failure>> {
      let subject = PassthroughSubject<Output, Failure>()
      attemptToFulfill { result in
        switch result {
        case .success(let value):
          subject.send(value)
          subject.send(completion: .finished)
        case .failure(let error):
          subject.send(completion: .failure(error))
        }
      }
      return subject
    }
    .eraseToAnyPublisher()
  }
}