Introduction to Combine using Instagram analogy

CombineBasics.swift

import Foundation
import Combine

struct Post {
  let imageName: String
  let description: String?
  
  init(imageName: String, description: String? = nil) {
    self.imageName = imageName
    self.description = description
  }
}

enum UserError: Error {}

struct User: Publisher {
  typealias Output = Post
  typealias Failure = UserError
  
  func receive<S>(subscriber: S) where S: Subscriber, Failure == S.Failure, Output == S.Input {
    subscriber.receive(subscription: UserSubscription(subscriber: subscriber))
  }
}

class UserSubscription<S: Subscriber>: Subscription where User.Failure == S.Failure, User.Output == S.Input {
  private let posts = [
    Post(imageName: "Mountain"),
    Post(imageName: "Coffee"),
    Post(imageName: "Desert"),
    Post(imageName: "City")
  ]
  private var subscriber: S?
  
  init(subscriber: S) { self.subscriber = subscriber }
  
  func request(_ demand: Subscribers.Demand) {
    var posts = self.posts
    var currentDemand = demand
    while currentDemand != .none && !posts.isEmpty {
      let post = posts.removeFirst()
      if let addedDemand = subscriber?.receive(post) {
        currentDemand -= 1
        currentDemand += addedDemand
      }
    }
    subscriber?.receive(completion: .finished)
  }
  
  func cancel() { subscriber = nil }
}

class Follower: Subscriber {
  typealias Input = Post
  typealias Failure = UserError
  
  func receive(subscription: Subscription) {
    subscription.request(.unlimited)
  }
  
  func receive(_ post: Post) -> Subscribers.Demand {
    print(post)
    return .none
  }
  
  func receive(completion: Subscribers.Completion<UserError>) {
    print(completion)
  }
}

let user = User()
var follower = Follower()
user.receive(subscriber: follower)