singleton.md

Singleton Pattern

Problem

We need to have a single instance of a certain class across the whole application.

Solution

In the Singleton pattern, the access to the constructor is restricted so that it cannot be instantiated. So, the creation of the single instance is done inside the class and is held as a class variable. It can be accessed through a getter across the application.

Example

Let's consider the implementation of a logger class:

class SimpleLogger
  attr_accessor :level

  ERROR = 1
  WARNING = 2
  INFO = 3
  
  def initialize
    @log = File.open("log.txt", "w")
    @level = WARNING
  end

  def error(msg)
    @log.puts(msg)
    @log.flush
  end

  def warning(msg)
    @log.puts(msg) if @level >= WARNING
    @log.flush
  end

  def info(msg)
    @log.puts(msg) if @level >= INFO
    @log.flush
  end
end

Logging is a feature used across the whole application, so it makes sense that there should only be a single instance of the logger. We can make sure that nobody instantiates the SimpleLogger class twice by making its constructor private:

class SimpleLogger

  # Lots of code deleted...
  @@instance = SimpleLogger.new

  def self.instance
    return @@instance
  end

  private_class_method :new
end

SimpleLogger.instance.info('Computer wins chess game.')

We can get the same behavior by including the Singleton module, so that we can avoid duplicating code if we create several singletons:

require 'singleton'

class SimpleLogger
  include Singleton
  # Lots of code deleted...
end