Observer sample class using @propertyWrapper and class extension

Observer.swift

//
//  Observer.swift
//  TestAspect
//
//  Created by Daniel Vela on 12/01/2020.
//  Copyright © 2020 veladan. All rights reserved.
//

protocol Observer {
  associatedtype Value
  func update(value: Value)
}

class AnyObserver<Value>: Observer {
  private let updateClosure: (Value) -> Void

  init<ObserverType: Observer>(with observer: ObserverType) where ObserverType.Value == Value {
    self.updateClosure = observer.update
  }

  func update(value: Value) {
    self.updateClosure(value)
  }
}

extension Observer {
  func toAnyObserver() -> AnyObserver<Value> {
    return AnyObserver<Value>(with: self)
  }
}

protocol Subject {
  associatedtype Value
  mutating func add(observer: AnyObserver<Value>)
  mutating func remove(observer: AnyObserver<Value>)
  func fireNotification()
}

class UserView {
  func render(user: User) {
    print("User is now: \(user)")
  }
}

extension UserView: Observer {
  func update(value: User) {
    self.render(user: value)
  }
}

@propertyWrapper
struct Observed<Value>: Subject {
  init(initialValue: Value) {
    self.value = initialValue
  }

  var value: Value {
    didSet {
      self.fireNotification()
    }
  }

  var observers = [AnyObserver<Value>]()

  mutating func add(observer: AnyObserver<Value>) {
    self.observers.append(observer)
  }

  mutating func remove(observer: AnyObserver<Value>) {
    self.observers.removeAll { $0 === observer }
  }

  func fireNotification() {
    self.observers.forEach { $0.update(value: self.value) }
  }
}

struct SampleClass {
  @Observed
  var user = User()

  mutating func run() {
    let userView = UserView()
    $user.add(observer: userView.toAnyObserver())

    user.name = "Phoenix"
  }

}