OrderProtocolRestaurantQueue.swift

// Orders Protocol
protocol Orders {
  associatedtype Item
  func enqueue(item:Item)
  func dequeue() -> Item?
  func showCurrentQueue() -> [Item]
}
// RestaurantOrders class
class RestaurantOrders<Item>: Orders {
  var items : [Item] = [Item]()
  func enqueue(item: Item){
    items.append(item)
  }
  func dequeue() -> Item? {
    if !items.isEmpty {
      return items.removeFirst()
    }
    else {
      return nil
    }
  }
  func showCurrentQueue() -> [Item] {
    return items
  }
}