Ruby On Rails 面接の質問と回答

Question: How will you implement Single Pattern in Ruby on Rails?

Singleton means single instance. 

So, the goal of a singleton pattern is to write a class definition but only allow the creation of the single instance of that object. 

This can be achieved nicely with the singleton gem as shown below:

require 'singleton'
 class Logger
  include Singleton
  def initialize
    @log = File.open("logfile.txt", "a")
  end
  def log(msg)
    @log.puts(msg)
  end
end

Adding the singleton as a mixin to the 

Logger.instance.log('This is just a test message')

The code above will create a single instance of Logger and simply put the message in the logger file.

Singleton patterns are mostly used for DB instance, Logger instance, etc. —- cases where there should be ONE and only ONE instance of the object that is used. 

Sometimes you might like to actually hold on to the logger object and use it everywhere you can do so by the following command:

logObj = Logger.instance

Notice you cannot use the Logger.new to create an object instance because this is a singleton object and therefore calling ‘new’ would fail.