gistfile1.rb

### Problem, rigid dependency injection ###
class Demo
  def initialize
    @instancev = Something.new
  end
  
  def method_1
   do_that
   @instancev.something
  end
  
  def method_2
   filter_something
   @instancev.orother
  end
end

### Solution (Factory method) ###

class Demo
  def something
    Something
  end
  
  def will_use_something
    s = something.new
    s.awesome_stuff
  end
  
  # `will_use_something` has no side effects
  def change_something_state
    s = somthing.new.mutate
    s.awesome_stuff
  end
end

### Solution (Singleton method) ###

class Demo
  def something
    @something ||= Something.new
  end
  
  def will_use_something
    something.awesome_stuff
  end
  
  # Side effects as `will_use_something` might return something else now.
  def change_something_state
    something.mutate!
  end
end