method_swizzle.rb

#!/usr/bin/env ruby
# Author : Emad Elsaid (https://github.com/blazeeboy)
def swap_methods(from_obj, from_method, to_obj, to_method)
  
  from_alias  = "#{from_method}_#{rand 1000}"
  to_alias    = "#{to_method}_#{rand 1000}"

  # alias methods in both objects
  from_obj.class.class_eval do
    alias_method from_alias, from_method
  end

  to_obj.class.class_eval do
    alias_method to_alias, to_method
  end

  # override methods and call aliases in both direction
  from_obj.define_singleton_method(from_method) do |*params, &block|
    to_obj.send(to_alias, *params, &block)
  end

  to_obj.define_singleton_method(to_method) do |*params, &block|
    from_obj.send(from_alias, *params, &block)
  end

end

# calling swap between two methods on two objects
# should swap them, so if you call obj1.method1 
# will execute obj2.method2 and vice versa
obj1 = "this is my first string object"
obj2 = "this is my second string object"
swap_methods obj1, :to_s, obj2, :to_s

# this should print the second string
puts obj1.to_s
#  and this should print the first one
puts obj2.to_s

# swapping String new method with
# other class new method, so whenever
# you create a new String an instance of
# the other class
class X
  attr_accessor :value
  def initialize(value)
    @value = value
  end
end
swap_methods String, :new, X, :new
x_instance = String.new "Heeeey"
puts x_instance.class

# this code will output the following lines:
# 
# this is my second string object
# 
# this is my first string object
# 
# X
# 
# 
# it normally should be :
# 
# this is my frist string object
# 
# this is my second string object
# 
# String