Ruby design patterns: Singleton

Ruby design patterns: Singleton

In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects. The term comes from the mathematical concept of a singleton.*

NOTE: There are some who are critical of the singleton pattern and consider it to be an anti-pattern in that it is frequently used in scenarios where it is not beneficial, introduces unnecessary restrictions in situations where a sole instance of a class is not actually required, and introduces global state into an application.*

*- after wikipedia

So, implementation of singleton pattern is controversial, but imho as they say: It is good to know your enemy ;)

Let’s get started:

require 'singleton'

class Creature
  include Singleton
  attr_accessor :type

  def animal
    'snake'
  end
end

p first = Creature.instance
p second =  Creature.instance

p first.type = {dangerous: true}
p second.type
p Creature.instance.animal
p second.animal

First of all we need to require singleton lib (ruby build-in) and include it in our class in which we want to implement singleton pattern. attr_accessor or this simple method is rather self explanatory, so let’s move further. We instantiate two variables with Creature.instance For the variable first we declare type, but in case of second we aren’t declaring any type instead we only call it. In the next line We call a method animal from Creature.instance and on the last line we call a method animal on a second variable.

This are the outputs:

#=> <Creature:0x000000010ca700>
#=> <Creature:0x000000010ca700>
#{:dangerous=>true}
#{:dangerous=>true}
#"snake"
#"snake"

Variables and instances are identical. As mentioned above the instantiation of a class is restricted to one object.

Cheers