main.swift

protocol Player {
  var name: String { get }
  var hitpoints: Int { get set }
}

protocol DeathWathable {
  func playerDied(_ player: Player)
}

@propertyDelegate
class DeathAlerter<Value: Player> {
  var watchers = [DeathWathable]()
  
  init(initialValue: Value) {
    self.value = initialValue
  }
  
  var value: Value {
    didSet {
      guard value.hitpoints <= 0 else {
        return
      }
      
      for watcher in watchers {
        watcher.playerDied(value)
      }
    }
  }
  
  func addWatcher(_ watcher: DeathWathable) {
    self.watchers.append(watcher)
  }
}

struct P: Player {
  let name: String
  
  var hitpoints = 100 {
    didSet {
      if hitpoints < 0 {
        hitpoints = 0
      }
    }
  }
  
  init(name: String, hitpoints: Int = 100) {
    self.name = name
    self.hitpoints = hitpoints
  }
}

class Arena {
  @DeathAlerter var p1 = P(name: "Player 1")
  @DeathAlerter var p2 = P(name: "Player 1")
}

class EventBoard: DeathWathable {
  func playerDied(_ player: Player) {
    print("\(player.name) died")
  }
}

let eventBoard = EventBoard()
let arena = Arena()

arena.$p1.addWatcher(eventBoard)
arena.$p2.addWatcher(eventBoard)

arena.p1.hitpoints -= 100 // Will print Player 1 Died