ModelObserver.swift

public final class ModelObserver {

  deinit {
    tokens.forEach {
      notificationCenter.removeObserver($0)
    }
  }

  // MARK: - Properties

  private var tokens: [NSObjectProtocol] = []

  private let observerIdentifier: String = UUID().uuidString

  private let notificationCenter: NotificationCenter = .default

  // MARK: - Initializers

  init() {

  }

  // MARK: - Functions

  func update<T: ModelObservableType>(_ model: T) {

    let _key = notificationKey(model)

    notificationCenter.post(name: .init(rawValue: _key), object: model)
  }

  func notificationKey<T: ModelObservableType>(_ model: T) -> String {
    let modelName = String(reflecting: type(of: model))
    let identifier = model.identifier

    let key = "\(observerIdentifier).\(modelName).\(identifier)"
    return key
  }

  @discardableResult
  public func add<T: ModelObservableType>(model: T, didUpdate: @escaping (ModelObserver, T) -> Void) -> NSObjectProtocol {

    model.addRecursiveModelObserve(observer: self)
    let key = notificationKey(model)
    let queue = OperationQueue.current ?? OperationQueue.main

    let token = notificationCenter.addObserver(
      forName: NSNotification.Name(rawValue: key),
      object: nil,
      queue: queue
    ) { [weak self] (notification) in
      guard let `self` = self else { return }
      if let model = notification.object.flatMap({ $0 as? T }) {
        didUpdate(self, model)
      }
    }
    
    tokens.append(token)
    didUpdate(self, model)

    return token
  }

  public func remove(token: NSObjectProtocol) {

    if let index = tokens.index(where: { $0.isEqual(token) }) {
      tokens.remove(at: index)
    }
    notificationCenter.removeObserver(token)
  }
}