Design Patterns in Ruby
This post is based on design patterns, that are described in the excellent book - “Design Patterns in Ruby” (by Russ Olsen).
NOTE: I asked about permission to use content from “Design Patterns in Ruby” (by Russ Olsen) in this post.
NOTE: Not all of the patterns are covered in the book “Design Patterns in Ruby” (by Russ Olsen). There really was a number of reasons. Read the post “The Lost Patterns” for more information.
Read “Object-Oriented Design Patterns in Life”.
Patterns
- Creational
- Structural
- Behavioral
Abstract Factory
Creates an instance of several families of classes. Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Example:
class Animal
attr_accessor :name
def initialize(name)
@name = name
end
def eat
end
def speak
end
def sleep
end
end
class Tiger < Animal
def eat
puts "Tiger #{name} is eating anything it wants."
end
def speak
puts "Tiger #{name} Roars!"
end
def sleep
puts "Tiger #{name} sleeps anywhere it wants."
end
end
class Plant
attr_accessor :name
def initialize(name)
@name = name
end
def grow
end
end
class Tree < Plant
def grow
puts "The tree #{name} grows tall."
end
end
class Habitat
def initialize(organism_factory, number_animals: 0, number_plants: 0)
@organism_factory = organism_factory
@animals = []
number_animals.times do |i|
animal = @organism_factory.new_animal("Animal#{i}")
@animals << animal
end
@plants = []
number_plants.times do |i|
plant = @organism_factory.new_plant("Plant#{i}")
@plants << plant
end
end
def simulate_one_day
@plants.each { |plant| plant.grow }
@animals.each { |animal| animal.speak }
@animals.each { |animal| animal.eat }
@animals.each { |animal| animal.sleep }
end
end
class JungleOrganismFactory
def new_animal(name)
Tiger.new(name)
end
def new_plant(name)
Tree.new(name)
end
end
class PondOrganismFactory
def new_animal(name)
Frog.new(name)
end
def new_plant(name)
Algae.new(name)
end
end
jungle = Habitat.new(JungleOrganismFactory.new, number_animals: 1, number_plants: 4)
jungle.simulate_one_day
pond = Habitat.new(PondOrganismFactory.new, number_animals: 2, number_plants: 4)
pond.simulate_one_day
Read “Design Patterns in Ruby: Abstract Factory”.
Builder
Separates object construction from its representation. Separate the construction of a complex object from its representation so that the same construction processes can create different representations.

Example:
class CPU
end
class BasicCPU < CPU
end
class TurboCPU < CPU
end
class Motherboard
attr_accessor :cpu
attr_accessor :memory_size
def initialize(cpu=BasicCPU.new, memory_size=1024)
@cpu = cpu
@memory_size = memory_size
end
end
class Drive
attr_reader :type # either :cd, :dvd or :hard_disk
attr_reader :size # in Mb
attr_reader :writable # true if this drive is writable
def initialize(type, size, writable)
@type = type
@size = size
@writable = writable
end
end
class Computer
attr_accessor :display
attr_accessor :motherboard
attr_reader :drives
def initialize(display=:crt, motherboard=Motherboard.new, drives=[])
@motherboard = motherboard
@drives = drives
@display = display
end
end
class ComputerBuilder
attr_reader :computer
def initialize
@computer = Computer.new
end
def basic_cpu
computer.motherboard.cpu = BasicCPU.new
end
def turbo_cpu
computer.motherboard.cpu = TurboCPU.new
end
def display=(display)
computer.display = display
end
def memory_size=(size_in_mb)
computer.motherboard.memory_size = size_in_mb
end
def add_cd(writer=false)
computer.drives << Drive.new(:cd, 760, writer)
end
def add_dvd(writer=false)
computer.drives << Drive.new(:dvd, 4000, writer)
end
def add_hard_disk(size_in_mb)
computer.drives << Drive.new(:hard_disk, size_in_mb, true)
end
end
builder = ComputerBuilder.new
builder.turbo_cpu
builder.add_hard_disk(1_000_000)
builder.memory_size = 16000
builder.add_cd(true)
builder.add_dvd
computer = builder.computer
Read “Design Patterns in Ruby: Builder”.
Read “Builder design pattern in Ruby”.
Factory Method
Creates an instance of several derived classes. Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Example:
class Animal
attr_accessor :name
def initialize(name)
@name = name
end
def eat
end
def speak
end
def sleep
end
end
class Duck < Animal
def eat
puts "Duck #{name} is eating."
end
def speak
puts "Duck #{name} says Quack!"
end
def sleep
puts "Duck #{name} sleeps quietly."
end
end
class Frog < Animal
def eat
puts "Frog #{name} is eating."
end
def speak
puts "Frog #{name} says Crooooaaaak!"
end
def sleep
puts "Frog #{name} doesn't sleep, he croaks all night!"
end
end
class Plant
attr_accessor :name
def initialize(name)
@name = name
end
def grow
end
end
class Algae < Plant
def grow
puts "The Algae #{name} soaks up the sun and grows."
end
end
class WaterLily < Plant
def grow
puts "The water lily #{name} floats, soaks up the sun and grows."
end
end
class Pond
def initialize(number_animals: 0, number_plants: 0)
@animals = []
number_animals.times do |i|
animal = new_organism(:animal, "Animal#{i}")
@animals << animal
end
@plants = []
number_plants.times do |i|
plant = new_organism(:plant, "Plant#{i}")
@plants << plant
end
end
def simulate_one_day
@plants.each { |plant| plant.grow }
@animals.each { |animal| animal.speak }
@animals.each { |animal| animal.eat }
@animals.each { |animal| animal.sleep }
end
end
class DuckWaterLilyPond < Pond
def new_organism(type, name)
if type == :animal
Duck.new(name)
elsif type == :plant
WaterLily.new(name)
else
raise "Unknown organism type: #{type}"
end
end
end
class FrogAlgaePond < Pond
def new_organism(type, name)
if type == :animal
Frog.new(name)
elsif type == :plant
Algae.new(name)
else
raise "Unknown organism type: #{type}"
end
end
end
DuckWaterLilyPond.new(number_animals: 4, number_plants: 2)
FrogAlgaePond.new(number_animals: 3, number_plants: 7)
Read “Design Patterns in Ruby: Factory Method”.
Prototype
A fully initialized instance to be copied or cloned. Specify the kinds of objects to create using a prototypical instance and create new objects by copying this prototype.
Read “Design Patterns in Ruby: Prototype”.
Singleton
A class of which only a single instance can exist. Ensure a class only has one instance and provide a global point of access to it.
You can use Singleton
module for building Singleton pattern in Ruby.
Read “Design Patterns in Ruby: Singleton”.
Adapter
Match interfaces of different classes. Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.

Example:
class Encrypter
def initialize(key)
@key = key
end
def encrypt(reader, writer)
key_index = 0
while not reader.eof?
clear_char = reader.getc
encrypted_char = clear_char.ord ^ @key[key_index].ord
writer.putc(encrypted_char.chr)
key_index = (key_index + 1) % @key.size
end
end
end
class StringIOAdapter
def initialize(string)
@string = string
@position = 0
end
def getc
if @position >= @string.length
raise EOFError
end
ch = @string[@position]
@position += 1
return ch
end
def eof?
return @position >= @string.length
end
end
encrypter = Encrypter.new('SECRET_KEY')
reader = File.open('message.txt')
writer = File.open('message.encrypted','w')
encrypter.encrypt(reader, writer)
reader = StringIOAdapter.new('We attack at dawn')
writer = File.open('out.txt', 'w')
encrypter.encrypt(reader, writer)
Read “Design Patterns in Ruby: Adapter”.
Bridge
Separates an object’s interface from its implementation. Decouple an abstraction from its implementation so that the two can vary independently.
Read “Design Patterns in Ruby: Bridge”.
Composite
A tree structure of simple and composite objects. Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Example:
class Task
attr_accessor :name, :parent
def initialize(name)
@name = name
@parent = nil
end
def get_time_required
0.0
end
end
class CompositeTask < Task
def initialize(name)
super(name)
@sub_tasks = []
end
def add_sub_task(task)
@sub_tasks << task
task.parent = self
end
def remove_sub_task(task)
@sub_tasks.delete(task)
task.parent = nil
end
def get_time_required
time = 0.0
@sub_tasks.each { |task| time += task.get_time_required }
time
end
end
class AddDryIngredientsTask < Task
def initialize
super('Add dry ingredients')
end
def get_time_required
1.0
end
end
class MixTask < Task
def initialize
super('Mix that batter up')
end
def get_time_required
3.0
end
end
class AddLiquidsTask < Task
def initialize
super('Add Liquids')
end
def get_time_required
4.0
end
end
class MakeBatterTask < CompositeTask
def initialize
super('Make batter')
add_sub_task(AddDryIngredientsTask.new)
add_sub_task(AddLiquidsTask.new)
add_sub_task(MixTask.new)
end
end
class FillPanTask < Task
def initialize
super('Fill pan')
end
def get_time_required
2.0
end
end
class FrostTask < Task
def initialize
super('Frost')
end
def get_time_required
2.0
end
end
class BakeTask < Task
def initialize
super('Bake')
end
def get_time_required
2.0
end
end
class LickSpoonTask < Task
def initialize
super('Bake')
end
def get_time_required
100.0
end
end
class MakeCakeTask < CompositeTask
def initialize
super('Make cake')
add_sub_task(MakeBatterTask.new)
add_sub_task(FillPanTask.new)
add_sub_task(BakeTask.new)
add_sub_task(FrostTask.new)
add_sub_task(LickSpoonTask.new)
end
end
Read “Design Patterns in Ruby: Composite”.
Decorator
Add responsibilities to objects dynamically. Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

You can use SimpleDelegator
class for building Decorator pattern in Ruby.
Read my post “Decorator Pattern in Ruby” for more information.
Read “Design Patterns in Ruby: Decorator”.
Facade
A single class that represents an entire subsystem. Provide a unified interface to a set of interfaces in a system. Facade defines a higher-level interface that makes the subsystem easier to use.
Read “Design Patterns in Ruby: Facade”.
Read “Facade design pattern in Ruby on Rails”.
Flyweight
A fine-grained instance used for efficient sharing. Use sharing to support large numbers of fine-grained objects efficiently. A flyweight is a shared object that can be used in multiple contexts simultaneously. The flyweight acts as an independent object in each context — it’s indistinguishable from an instance of the object that’s not shared.
Read “Design Patterns in Ruby: Flyweight”.
Proxy
An object representing another object. Provide a surrogate or placeholder for another object to control access to it.

Example:
class Account
attr_reader :balance
def initialize(starting_balance=0)
@balance = starting_balance
end
def deposit(amount)
@balance += amount
end
def withdraw(amount)
@balance -= amount
end
end
require 'etc'
class AccountProtectionProxy
def initialize(real_account, owner_name)
@subject = real_account
@owner_name = owner_name
end
def deposit(amount)
check_access
return @subject.deposit(amount)
end
def withdraw(amount)
check_access
return @subject.withdraw(amount)
end
def balance
check_access
return @subject.balance
end
def check_access
if Etc.getlogin != @owner_name
raise "Illegal access: #{Etc.getlogin} cannot access account."
end
end
end
account = Account.new(100)
account.deposit(50)
account.withdraw(10)
proxy = AccountProtectionProxy.new(account, 'russolsen')
proxy.deposit(50)
proxy.withdraw(10)
Read “Design Patterns in Ruby: Proxy”.
Chain of responsibility
A way of passing a request between a chain of objects. Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
Read “Design Patterns in Ruby: Chain of Responsibility”.
Read “Chain of responsibility design pattern in Ruby”.
Command
Encapsulate a command request as an object. Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Example:
class Command
attr_reader :description
def initialize(description)
@description = description
end
def execute
end
end
class CreateFile < Command
def initialize(path, contents)
super("Create file: #{path}")
@path = path
@contents = contents
end
def execute
f = File.open(@path, "w")
f.write(@contents)
f.close
end
end
class DeleteFile < Command
def initialize(path)
super("Delete file: #{path}")
@path = path
end
def execute
File.delete(@path)
end
end
require 'fileutils'
class CopyFile < Command
def initialize(source, target)
super("Copy file: #{source} to #{target}")
@source = source
@target = target
end
def execute
FileUtils.copy(@source, @target)
end
end
Read “Design Patterns in Ruby: Command”.
Interpreter
A way to include language elements in a program. Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.

Example:
class Expression
def |(other)
Or.new(self, other)
end
def &(other)
And.new(self, other)
end
end
require 'find'
class All < Expression
def evaluate(dir)
results = []
Find.find(dir) do |p|
next unless File.file?(p)
results << p
end
results
end
end
require 'find'
class FileName < Expression
def initialize(pattern)
@pattern = pattern
end
def evaluate(dir)
results = []
Find.find(dir) do |p|
next unless File.file?(p)
name = File.basename(p)
results << p if File.fnmatch(@pattern, name)
end
results
end
end
class Not < Expression
def initialize(expression)
@expression = expression
end
def evaluate(dir)
All.new.evaluate(dir) - @expression.evaluate(dir)
end
end
require 'find'
class Bigger < Expression
def initialize(size)
@size = size
end
def evaluate(dir)
results = []
Find.find(dir) do |p|
next unless File.file?(p)
results << p if(File.size(p) > @size)
end
results
end
end
require 'find'
class Writable < Expression
def evaluate(dir)
results = []
Find.find(dir) do |p|
next unless File.file?(p)
results << p if(File.writable?(p))
end
results
end
end
class Or < Expression
def initialize(expression1, expression2)
@expression1 = expression1
@expression2 = expression2
end
def evaluate(dir)
result1 = @expression1.evaluate(dir)
result2 = @expression2.evaluate(dir)
(result1 + result2).sort.uniq
end
end
class And < Expression
def initialize(expression1, expression2)
@expression1 = expression1
@expression2 = expression2
end
def evaluate(dir)
result1 = @expression1.evaluate(dir)
result2 = @expression2.evaluate(dir)
(result1 & result2)
end
end
module ExpressionSugar
def all
All.new
end
def bigger(size)
Bigger.new(size)
end
def file_name(pattern)
FileName.new(pattern)
end
def except(expression)
Not.new(expression)
end
def writable
Writable.new
end
end
expression1 = Or.new(
And.new(Bigger.new(2000), Not.new(Writable.new)),
FileName.new('*.mp3')
)
expression1.evaluate('.')
include ExpressionSugar
expression2 = (bigger(2000) & except(writable)) | file_name('*.mp3')
expression2.evaluate('.')
Read “Design Patterns in Ruby: Interpreter”.
Iterator
Sequentially access the elements of a collection. Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
In Ruby the Enumerable
mixin provides collection classes with several traversal and searching methods, and with the ability to sort. The class must provide a method each
, which yields successive members of the collection. If Enumerable#max
, #min
, or #sort
is used, the objects in the collection must also implement a meaningful <=>
operator, as these methods rely on an ordering between members of the collection.
Example:
class Account
attr_accessor :name, :balance
def initialize(name, balance)
@name = name
@balance = balance
end
def <=>(other)
balance <=> other.balance
end
end
class Portfolio
include Enumerable
def initialize
@accounts = []
end
def each(&block)
@accounts.each(&block)
end
def add_account(account)
@accounts << account
end
end
my_portfolio = Portfolio.new
my_portfolio.add_account(Account.new('Bonds', 200))
my_portfolio.add_account(Account.new('Stocks', 100))
my_portfolio.add_account(Account.new('Real Estate', 1000))
my_portfolio.any? { |account| account.balance > 2000 }
my_portfolio.all? { |account| account.balance >= 10 }
my_portfolio.each { |account| puts "#{account.name}: #{account.balance}" }
my_portfolio.map { |account| account.balance }
my_portfolio.max
my_portfolio.min
Read “Design Patterns in Ruby: Iterator”.
Mediator
Defines simplified communication between classes. Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly and it lets you vary their interaction independently.
Read “Design Patterns in Ruby: Mediator”.
Memento
Capture and restore an object’s internal state. Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.
Read “Design Patterns in Ruby: Memento”.
Observer
A way of notifying change to a number of classes. Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Example:
module Subject
def initialize
@observers = []
end
def add_observer(observer)
@observers << observer
end
def delete_observer(observer)
@observers.delete(observer)
end
def notify_observers
@observers.each do |observer|
observer.update(self)
end
end
end
class Employee
include Subject
attr_reader :name, :address
attr_reader :salary
def initialize(name, title, salary)
super()
@name = name
@title = title
@salary = salary
end
def salary=(new_salary)
@salary = new_salary
notify_observers
end
end
class Payroll
def update(changed_employee)
puts "Cut a new check for #{changed_employee.name}!"
puts "His salary is now #{changed_employee.salary}!"
end
end
class TaxMan
def update(changed_employee)
puts "Send #{changed_employee.name} a new tax bill!"
end
end
fred = Employee.new('Fred', 'Crane Operator', 30000.0)
payroll = Payroll.new
tax_man = TaxMan.new
fred.add_observer(payroll)
fred.add_observer(tax_man)
fred.salary = 90000.0
The Ruby standard library comes with a fine, prebuilt Observable
module that provides all of the support you need to make your object, well, observable.
You can use Observable
module for building Observer pattern in Ruby.
Read “Design Patterns in Ruby: Observer”.
State
Alter an object’s behavior when its state changes. Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
Strategy
Encapsulates an algorithm inside a class. Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Example:
class Formatter
def output_report(context)
raise 'Abstract method called'
end
end
class HTMLFormatter < Formatter
def output_report(context)
puts('<html>')
puts(' <head>')
puts(" <title>#{context.title}</title>")
puts(' </head>')
puts(' <body>')
context.text.each do |line|
puts(" <p>#{line}</p>")
end
puts(' </body>')
puts('</html>')
end
end
class PlainTextFormatter < Formatter
def output_report(context)
puts("***** #{context.title} *****")
context.text.each do |line|
puts(line)
end
end
end
class Report
attr_reader :title, :text
attr_accessor :formatter
def initialize(formatter)
@title = 'Monthly Report'
@text = ['Things are going', 'really, really well.']
@formatter = formatter
end
def output_report
@formatter.output_report(self)
end
end
report = Report.new(HTMLFormatter.new)
report.output_report
report = Report.new(PlainTextFormatter.new)
report.output_report
Read “Design Patterns in Ruby: Strategy”.
Template Method
Defer the exact steps of an algorithm to a subclass. Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.

Example:
class Report
def initialize
@title = 'Monthly Report'
@text = ['Things are going', 'really, really well.']
end
def output_report
output_start
output_head
output_body_start
output_body
output_body_end
output_end
end
def output_body
@text.each do |line|
output_line(line)
end
end
def output_start
raise 'Called abstract method: output_start'
end
def output_head
raise 'Called abstract method: output_head'
end
def output_body_start
raise 'Called abstract method: output_body_start'
end
def output_line(line)
raise 'Called abstract method: output_line'
end
def output_body_end
raise 'Called abstract method: output_body_end'
end
def output_end
raise 'Called abstract method: output_end'
end
end
class HTMLReport < Report
def output_start
puts('<html>')
end
def output_head
puts(' <head>')
puts(" <title>#{@title}</title>")
puts(' </head>')
end
def output_body_start
puts('<body>')
end
def output_line(line)
puts(" <p>#{line}</p>")
end
def output_body_end
puts('</body>')
end
def output_end
puts('</html>')
end
end
class PlainTextReport < Report
def output_start
end
def output_head
puts("**** #{@title} ****")
puts
end
def output_body_start
end
def output_line(line)
puts line
end
def output_body_end
end
def output_end
end
end
report = HTMLReport.new
report.output_report
report = PlainTextReport.new
report.output_report
Read “Design Patterns in Ruby: Template Method”.
Visitor
Defines a new operation to a class without change. Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
Read “Visitor design pattern in Ruby”.
Acknowledgment

Thanks for the book @russolsen!