ruby-patterns

Examples of Patterns in Ruby

Adapter
Builder
Command
Composite
Decorator
Facade
Factory
Interpreter
Iterator
Observer
Proxy
Singleton
State
Strategy
Template

Adapter

Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces.^[link]

class Quest
  attr_accessor :difficulty, :hero

  def initialize(difficulty)
    @difficulty = difficulty
    @hero = nil
  end

  def finish
    @hero.exp += calculate_experience
  end

  def calculate_experience
    @difficulty * 50 / @hero.level
  end
end

class Hero
  attr_accessor :level, :exp, :quests

  def initialize
    @level = 1
    @exp = 0
    @quests = []
  end

  def take_quest(quest)
    @quests << (quest.hero = self)
  end

  def finish_quest(quest)
    quest.finish
    @quests.delete quest
  end
end

class OldQuest
  attr_accessor :hero, :difficulty, :experience

  def initialize
    @difficulty = 3
    @experience = 10
  end

  def done
    difficulty * experience
  end
end

class QuestAdapter
  attr_accessor :hero

  def initialize(old_quest, difficulty)
    @old_quest = old_quest
    @old_quest.difficulty = difficulty
    @hero = nil
  end

  def finish
    @hero.exp += @old_quest.done
  end
end

# Usage
hero = Hero.new
quest = Quest.new 5
hero.take_quest quest
hero.finish_quest quest
puts hero.exp
# => 250
some_old_quest = OldQuest.new
old_quest_adapted = QuestAdapter.new(some_old_quest, 5)
hero.take_quest old_quest_adapted
hero.finish_quest old_quest_adapted
puts hero.exp
# => 300

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Builder

Separate the construction of a complex object from its representation so that the same construction process can create different representations.^[link]

class BoardBuilder
  def initialize(width, height)
    @board = Board.new
    @board.width = width
    @board.height = height
    @board.tiles = []
    @board.monsters = []
  end

  def add_tiles(n)
    n.times{ @board.tiles << Tile.new }
  end

  def add_monsters(n)
    n.times{ @board.monsters << Monster.new }
  end

  def board
    @board
  end
end

class Board
  attr_accessor :width, :height, :tiles, :monsters
  def initialize
  end
end

class Tile; end
class Monster; end

# Usage
builder = BoardBuilder.new 2, 3
puts builder.board
# => Board Object
board = builder.board
puts board.width
# => 2
builder.add_tiles(3)
builder.add_monsters(2)
puts board.tiles.size
# => 3
puts board.monsters.size
# => 2

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Command

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.^[link]

class Turn
  def initialize
    @commands = []
  end

  def run_command(command)
    command.execute
    @commands << command
  end

  def undo_command
    @commands.pop.unexecute
  end
end

class Hero
  attr_accessor :money, :health

  def initialize
    @money = 0
    @health = 100
  end
end

class GetMoneyCommand
  def initialize(hero)
    @hero = hero
  end

  def execute
    @hero.money += 10
  end

  def unexecute
    @hero.money -= 10
  end
end

class HealCommand
  def initialize(hero)
    @hero = hero
  end

  def execute
    @hero.health += 10
  end

  def unexecute
    @hero.health -= 10
  end
end

# Usage
hero = Hero.new
get_money = GetMoneyCommand.new hero
heal = HealCommand.new hero
turn = Turn.new
turn.run_command(heal)
puts hero.health
# => 110
turn.run_command(get_money)
puts hero.money
# => 10
turn.undo_command
puts hero.money
# => 0

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Composite

Composition over inheritance. Compose objects into tree structures to represent part-whole hierarchies.^[link]

class CompositeQuest
  def initialize
    @tasks = []
  end

  def <<(task)
    @tasks << task
  end

  def reward
    @tasks.inject(0){ |sum, task| sum += task.reward }
  end
end

class MegaQuest < CompositeQuest
end

class Quest < CompositeQuest
end

class MonsterTask
  attr_reader :reward

  def initialize
    @reward = 100
  end
end

class PuzzleTask
  attr_reader :reward

  def initialize
    @reward = 200
  end
end

# Usage
quest1 = Quest.new
quest1 << MonsterTask.new
quest1 << PuzzleTask.new
puts quest1.reward
# => 300

quest2 = Quest.new
quest2 << MonsterTask.new
quest2 << PuzzleTask.new
megaquest = MegaQuest.new
megaquest << quest1
megaquest << quest2
puts megaquest.reward
# => 600

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Decorator

Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.^[link]

class ItemDecorator
  def initialize(item)
    @item = item
  end
  # this needs to be delegated with other efective way
  def use
    @item.use
  end
end

class MagicItemDecorator < ItemDecorator
  def price
    @item.price * 3
  end

  def description
    @item.description + "Magic"
  end
end

class MasterpieceItemDecorator < ItemDecorator
  def price
    @item.price * 2
  end

  def description
    @item.description + "Masterpiece"
  end
end

class Item
  attr_reader :price, :description
  def initialize
    @price = 10
    @description = "Item "
  end

  def use
    "do something"
  end
end

# Usage
item = Item.new
magic_item = MagicItemDecorator.new(item)
puts magic_item.price
# => 30
puts magic_item.description
# => Item Magic

masterpiece_item = MasterpieceItemDecorator.new(item)
puts masterpiece_item.price
# => 20
puts masterpiece_item.description
# => Item Masterpiece

# all next lines puts "do something"
item.use
magic_item.use
masterpiece_item.use

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Facade

The goal of the Facade Pattern is to provide a unified interface to a set of interfaces in a subsystem. This means you'd just have some object that can send back other objects.^[link]

class Hero
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def join(level)
    puts "#{self.name} join #{level}\n"
  end

  def attack(enemy)
    puts "#{self.name} kick #{enemy}\n"
  end
end

class Enemy
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def dead(hero)
    puts "#{self.name} killed by #{hero}"
  end
end

class Level
  attr_reader :stage

  def initialize(stage)
    @stage = stage
  end

  def to_s
    stage
  end
end

class GameFacade
  attr_reader :hero, :enemy, :level

  def initialize
    @hero  = Hero.new('Sonic')
    @enemy = Enemy.new('Eggman')
    @level = Level.new('Green Hill')
  end

  def start_game
    hero.join(level)
    hero.attack(enemy.name)
    enemy.dead(hero.name)
  end
end

game = GameFacade.new
game.start_game
# => Sonic join Green Hill
#    Sonic kick Eggman
#    Eggman killed by Sonic

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Factory

Define an interface for creating an object, but let subclasses decide which class to instantiate.^[link]

class Party
  attr_reader :members
  def initialize(factory)
    @members = []
    @factory = factory
  end

  def add_warriors(n)
    n.times{ @members << @factory.create_warrior }
  end

  def add_mages(n)
    n.times{ @members << @factory.create_mage }
  end
end

class HeroFactory
  def create_warrior
    Warrior.new
  end

  def create_mage
    Mage.new
  end
end

class Hero
  def initialize
  end
end

class Warrior < Hero
end

class Mage < Hero
end

# Usage
party = Party.new(HeroFactory.new)
party.add_warriors(3)
party.add_mages(2)
puts party.members.size
# => 5
puts party.members.count{ |member| member.class == "Mage" }
# => 2

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Interpreter

This pattern provides an interpreter to deal with an abstract language. Using classes we can understand the inputs for parse them.^[link]

class Word
  def initialize(value)
    @value = value
  end

  def execute
    @value
  end
end

class Plus
  def initialize(first, second)
    @first = first
    @second = second
  end

  def execute
    @first.execute + @second.execute
  end
end

class Minus
  def initialize(first, second)
    @first = first
    @second = second
  end

  def execute
    index = @first.execute =~ /#{@second.execute}/
    second_index = index + @second.execute.length
    @first.execute[0,index] + @first.execute[second_index..-1]
  end
end

class Interpreter
  def self.parse(input)
    @waiting_second_word = false
    words = []
    operations = []
    input.split.each_with_index do |value|
      if value =~ /^[^+-].*/ && !@waiting_second_word
        words << Word.new(value)
      else
        if symbol = operations.pop()
          first = words.size > 1 ? Word.new(words.map(&:execute).join(" ")) :
            words.pop
          second = Word.new(value)
          case symbol
          when /\A\+/
            words << Word.new(Plus.new(first, second).execute)
          when /\A\-/
            words << Word.new(Minus.new(first, second).execute)
          end
          @waiting_second_word = false
        else
          @waiting_second_word = true
          operations << value
        end
      end
    end
    words.pop.execute
  end
end

puts Interpreter.parse("NA + NA + NA + BATMAN")
#=> NANANABATMAN

puts Interpreter.parse("you know nothing Jon Snow - nothing")
#=> you know Jon Snow

puts Interpreter.parse("hello + world - llowo")
#=>herld

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Iterator

Iterator helps you to iterate through a complex object using an iterator method.^[link]

class Parent
  attr_reader :first_name

  def initialize(first_name, gender)
    @first_name = first_name
    @gender = gender
  end
end

class Child < Parent
end

class Family
  def initialize(surname)
    @surname = surname
    @children = []
  end

  def add_father(first_name)
    @father = Parent.new first_name, "M"
  end

  def add_mother(first_name)
    @mother = Parent.new first_name, "F"
  end

  def add_child(first_name, gender)
    @children << Child.new(first_name, gender)
  end

  def each_member
    [@father, @mother, @children].flatten.each do |member|
      yield member
    end
  end
end

# Usage
family = Family.new "Jackson"
family.add_father("Robert")
family.add_mother("Susan")
family.add_child("Lucas", "M")
family.add_child("James", "M")
family.add_child("Rose", "F")
family.each_member{ |member| puts member.first_name }
# => Robert
#    Susan
#    Lucas
#    James
#    Rose

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Observer

Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.^[link]

module Observable
  attr_reader :observers

  def initialize(attrs = {})
    @observers = []
  end

  def add_observer(observer)
    @observers << observer
  end

  def notify_observers
    @observers.each{ |observer| observer.update }
  end
end

class Tile
  include Observable

  def initialize(attrs = {})
    super
    @cursed = attrs.fetch(:cursed, false)
  end

  def cursed?
    @cursed
  end

  def activate_curse
    notify_observers
  end
end

class Hero
  attr_reader :health

  def initialize
    @cursed = false
    @health = 10
  end

  def damage(hit)
    @health -= hit
  end

  def cursed?
    @cursed
  end

  def discover(tile)
    if tile.cursed?
      @cursed = true
      tile.add_observer(self)
    end
  end

  def update
    damage(4)
  end
end

# Usage
hero = Hero.new
tile = Tile.new cursed: true
hero.discover(tile)
tile.activate_curse
puts hero.health
# => 6
puts hero.cursed?
# => true

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Proxy

Provide a surrogate or placeholder for another object to control access to it.^[link]

require 'forwardable'

class Hero
  attr_accessor :keywords

  def initialize
    @keywords = []
  end
end

class ComputerProxy
  # Forwardable allows objects to run methods on behalf
  # of it's members, in this case the Computer object
  extend Forwardable

  # We delegate the ComputerProxy's use of
  # the Computer object's add method
  def_delegators :real_object, :add

  def initialize(hero)
    @hero = hero
  end

  def execute
    check_access
    real_object.execute
  end

  def check_access
    unless @hero.keywords.include?(:computer)
      raise "You have no access"
    end
  end

  def real_object
    @real_object ||= Computer.new
  end
end

class Computer
  def initialize
    @queue = []
  end

  def add(command)
    @queue << command
  end

  def execute
    "executing commands"
  end
end

# Usage
hero = Hero.new
proxy = ComputerProxy.new(hero)
proxy.add("some command")
proxy.execute
# => raise error
hero.keywords << :computer
proxy.execute
# => executing commands

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Singleton

Define a unique instance of an object.^[link]

class HeroFactory
  @@instance = nil

  def self.instance
    @@instance ||= HeroFactory.send(:new)
  end

  def create_warrior
    Warrior.new
  end

  def create_mage
    Mage.new
  end

  private_class_method :new
end

# Usage
factory = HeroFactory.instance
another_factory = HeroFactory.instance
puts factory == another_factory
# => true
HeroFactory.new
# => Raise Exception

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State

This pattern tries to simplify complicated control flows changing an object's behavior dynamically.^[link]

class Operation
  attr_reader :state
  def initialize
    @state = OperationOpenState.new
  end

  def trigger(state)
    @state = @state.next(state)
  end
end

class OperationOpenState
  def next(state)
    if valid?(state)
      OperationPendingPaymentState.new
    else
      raise IllegalStateJumpError
    end
  end

  def valid?(state)
    state == :pending_payment
  end
end

class OperationPendingPaymentState
  def next(state)
    OperationConfirmState.new if valid?(state)
  end

  def valid?(state)
    state == :confirm
  end
end
class IllegalStateJumpError < StandardError; end
class OperationConfirmState; end

#Usage
operation = Operation.new
puts operation.state.class
#=> OperationOpenState
operation.trigger :pending_payment
puts operation.state.class
#=> OperationPendingPaymentState
operation.trigger :confirm
puts operation.state.class
#=> OperationConfirmState

operation = Operation.new
operation.trigger :confirm
#=> raise IllegalStateJumpError

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Strategy

Define a family of algorithms, encapsulate each one, and make them interchangeable.^[link]

class Hero
  attr_reader :damage, :health, :skills
  attr_accessor :printer

  def initialize(printer)
    @damage = 10
    @health = 5
    @printer = printer
    @skills = [:stealth, :driving, :intimidation]
  end

  def print_stats
    if block_given?
      yield(damage, health, skills)
    else
      printer.print(damage, health, skills)
    end
  end
end

class BattleStats
  def print(damage, health, skills)
    "Damage: #{damage}\nHealth: #{health}"
  end
end

class SkillStats
  def print(damage, health, skills)
    skills.inject(""){ |result, skill| result + skill.to_s.capitalize + "\n" }
  end
end

# Usage
Hero.new(BattleStats.new).print_stats
# => Damage: 10
#    Health: 5

Hero.new(SkillStats.new).print_stats
# => Stealth
#    Driving
#    Intimidation

Hero.new(any_printer).print_stats do |damage, health, skills|
  "Looks: I'm printing a customize message about my hero with damage #{damage} and number of skills: #{skills.size}"
end

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Template

Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template methods lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.^[link]

class Hero
  attr_reader :damage, :abilities

  def initialize
    @damage = damage_rating
    @abilities = occupation_abilities
  end

  def greet
    greeting = ["Hello"]
    greeting << unique_greeting_line
    greeting
  end

  def unique_greeting_line
    raise "You must define unique_greeting_line"
  end

  def damage_rating
    10
  end

  def occupation_abilities
    []
  end

  def attack
    "Atack dealing #{damage} damage"
  end
end

class Warrior < Hero
  def damage_rating
    15
  end

  def occupation_abilities
    [:strike]
  end

  def unique_greeting_line
    "Warrior is ready to fight!"
  end
end

class Mage < Hero
  def damage_rating
    7
  end

  def occupation_abilities
    [:magic_spell]
  end

  def unique_greeting_line
    "Mage is ready to make powerful spells!"
  end
end

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ruby-patterns

Table of Contents

Adapter

Builder

Command

Composite

Decorator

Facade

Factory

Interpreter

Iterator

Observer

Proxy

Singleton

State

Strategy

Template

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