How to monkey patch a ruby class inside a method [closed]

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I am unable to monkey patch a class inside a method body.

Within a method definition, I am trying to use a class in two ways:

1] Create an instance and use the orignal definition of a method in the class I am using

2] Monkey patch (pverride) a method in the class and now use an instance with the new method definition.

Basically I would be using both of the above instances of a class in my program.

The challenge is the method I am overriding gets called during initialization so I have to override it before I create the instance of the class.

Here is a small mock-up:

class A

  def initialize
   do_something
  end

  def do something
     #implementation
  end

end

Now , I want to use A in the same method twice , but once by using a modified version of do_something This is how I am trying to do it:

def my_method

  orig_instance = A.new

  #patch the class
  Class A          # ERROR: CLASS DEF IN METHOD BODY
   class << self
     alias_method :old_do_something, :do_something

     def self.do_something
        # new implementation
     end
  end

  new_instance = A.new

  #restore method
   class << self
     alias_method :do_something,:old_do_something

     def self.do_something
        # new implementation
     end
  end        



end # end of method

I get the (ERROR: CLASS DEF IN METHOD BODY) where I try to monkey patch the class, since I am trying to change the class inside of a method.

How do I achieve monkey patching the class in a method ?

Thanks

Answer 1

Instead of using class Clazz; blabla; end to reopen Clazz and monkey patch it, you can use Module#class_eval, Module#instance_eval and some other meta-programming utilities/methods to do the same trick. And because that block accepted by these methods doesn't create new binding scopes, it is more convenient in meta-programming practice.

def my_method
  puts ">> creating orig_instance"
  orig_instance = A.new

  puts ">> dump orig_instance"
  orig_instance.do_something

  new_do_something = lambda do
    puts "Modified A#do_something"
  end

  # monkey patch class A so that the modified version of do_something get called
  # during initialization of new_instance
  A.class_eval do
    alias_method :old_do_something, :do_something
    define_method :do_something, new_do_something
  end

  puts ">> creating new_instance"
  new_instance = A.new

  puts ">> dump before do_something gets restored"
  new_instance.do_something
  orig_instance.do_something

  # add singleton method for the special instance
  # so that the instance always calls the modified do_something
  new_instance_singleton = class << new_instance; self end
  new_instance_singleton.send :define_method, :do_something, new_do_something

  # restore the modified do_something
  # so that orig_instance and all other instances (except new_instance) have the original definition
  A.class_eval do
    alias_method :do_something, :old_do_something
  end
  puts ">> dump for final result"
  new_instance.do_something
  orig_instance.do_something
end

And the following is the output of my_method call:

>> creating orig_instance
Original A#do_something
>> dump orig_instance
Original A#do_something
>> creating new_instance
Modified A#do_something
>> dump before do_something gets restored
Modified A#do_something
Modified A#do_something
>> dump for final result
Modified A#do_something
Original A#do_something