An introduction to class methods in Ruby

class_methods..rb

class HTML

  def initialize(str)
    @str = str
  end

  # instance method (note it makes use of an instance variable)
  def render
    puts "<div>#{@str}</div>"
  end

  # class method - no instance variables here
  def self.render_div(str)
    puts "<div>#{str}</div>"
  end

  # class method
  def self.render_link(link, title)
    puts "<a href=#{link}>#{title}</a>"
  end

  # alternative syntax, this does exactly the same as above
  class << self
    # class method
    def render_div(str)
      puts "<div>#{str}</div>"
    end

    # class method
    def render_link(link, title)
      puts "<a href=\"#{link}\">#{title}</a>"
    end
  end # note the end
end

html_render = HTML.new("first instance")
another_html_renderer = HTML.new("second instance")

# instance methods
html_render.render
another_html_renderer.render

# class methods
HTML.render_div("Class method")
HTML.render_link("google.com", 'Google')

# We see class methods used all over the place
# for example in ActiveRecord::Base, when we call
# ModelName.find or ModelName.all we are calling a class method
#
#
# class ActiveRecord::Base
#   def self.find(id)
#     # do stuff
#   end

#   def self.all
#     # do stuff
#   end