Roles

The prototype pattern is used to create a new object by duplicating existing objects called prototypes, and they have a cloning capability.

This pattern is used in the following use cases:

Design

The following diagram shows the generic class of the prototype pattern:

Participants

Participant to this pattern are as follows:

Collaborations

The client asks to one or more prototypes to clone themselves.

Illustration

A simple and real example of where this pattern can be applied is the famous game HeartStone from Blizzard (the creator of World of Warcraft). In this strategy card game, when you spend "mana" to use spells, weapons, or put a minion on the board, there is a special minion that has the ability to clone a particular card. When a player uses this card, it selects the minion that he or she wants to clone and the card becomes an exact copy of the selected card. The following card represent the "HeartStone" card that have this behavior:

Implementation

The following code represent the implementation of the pattern using Swift:

import UIKit

class AbstractCard {
  var name: String?
  var mana: Int?
  var attack: Int?
  var defense: Int?
  
  init(name:String?, mana:Int?, attack:Int?, defense:Int?) {
    self.name = name
    self.attack = attack
    self.defense = defense
    self.mana = mana
  }
  
  func clone() -> AbstractCard {
    return AbstractCard(name: self.name, mana: self.mana, attack: self.attack, defense: self.defense)
  }
}

class Card: AbstractCard {
  
  override init(name:String?, mana:Int?, attack:Int?, defense:Int? ) {
    super.init(name: name,mana: mana,attack: attack,defense: defense)

  }
}

Usage

The following code simulate how the client will interact with the Card object which implement the prototype pattern:

// Simulate our client

// This is the card that we will copy
let raidLeader = Card(name: "Raid Leader", mana: 3, attack: 2, defense: 2)

// Now we use our faceless Manipulator card to clone the raidleader
let facelessManipulator = raidLeader.clone()

print("\(facelessManipulator.name, facelessManipulator.mana, facelessManipulator.attack, facelessManipulator.defense)")

Since the code is written in a Playground file, you should consider it as the code that you'll put in the Client class.

First, we instantiate a new card named Raid Leader. This is a concrete prototype class. Let say that you have the "Faceless Manipulator" card and you want to use it to clone the "Raid Leader" card, then you simply need to use the raidLeader.clone() method that will return a new instance with the exact same properties as "Raid Leader".

By checking the details on the right-hand side of the Playground file, you'll see that the facelessManipulator constant has exactly the same properties as raidLeader (line 39), as shown in the following screenshot: