Driver.swift

import Combine

extension Publisher where Failure == Never {
  
  /// Converts a publisher into a `Driver` 
  func driver() -> Driver<Output> { Driver(self) }
}

extension Driver {
  func driver() -> Driver<Output> { self }
}

/// Publisher that is useful to drive the user interface;
/// - Always returns values on main thread.
/// - All subscribers share the upstream publisher
/// - Is stateful, upon subscription the most recent value is always replayed (if available).
///
/// Inspired by the RxCocoa implementation;
/// https://github.com/ReactiveX/RxSwift/blob/main/RxCocoa/Traits/Driver/Driver.swift
///
struct Driver<Output>: Publisher {
  typealias Failure = Never

  private let publisher: AnyPublisher<Output, Never>
    
  init<P>(_ upstream: P) where P: Publisher, P.Output == Output, P.Failure == Never {
    self.publisher = upstream
      .map(Optional.some)
      .multicast { CurrentValueSubject<Output?, Failure>(.none) }
      .autoconnect()
      .compactMap { $0 }
      .receive(on: DispatchQueue.main)
      .eraseToAnyPublisher()
  }

  func receive<S>(subscriber: S) where S: Subscriber,  S.Input == Output, S.Failure == Failure {
    publisher.receive(subscriber: subscriber)
  }
}