为Singleton方法而不是Class方法添加的Methods_

Question

I'm really not understanding this Ruby Monk exercise :

There's an important nuance that's worth understanding when dealing with singleton_method_added and method_added.

The most natural tack the mind takes with these two callbacks is to assume that method_added will observe instance methods and singleton_method_added, class methods. This, however, is not entirely true; Ruby's singleton object backs normal objects and contains object specific changes. For classes, these are class methods. But when adding a method to a single, regular object, adding instance methods to that object alone modifies the singleton object for that object.

Let me show you why by having you solve this exercise.

The exercise is:

AN_OBJECT = Object.new

def AN_OBJECT.methods_added
  @@methods_added ||= []
end

def AN_OBJECT.singleton_methods_added
  @@singleton_methods_added ||= []
end

My attempt was to do this:

def AN_OBJECT.methods_added
  unless AN_OBJECT.is_a?(Class)
    @@methods_added ||= []
  end
end

def AN_OBJECT.singleton_methods_added
  if AN_OBJECT.instance_of?(Object)
    @@singleton_methods_added ||= []
  end
end

But the specs still fail.

I'm really having trouble wrapping my head around what exactly they want me to do. What is a singleton object? From what I understand, a singleton class is kind of a 'metaclass' that exists solely to give behavior to a certain instance of a class.

Is a singleton object a kind of 'metaobject' that exists solely to give behavior to a certain instance of an object? This is really confusing for me.

Answer 1

Eureka!

AN_OBJECT = Object.new

def AN_OBJECT.methods_added
  @@methods_added ||= []
end

def AN_OBJECT.singleton_methods_added
  @@singleton_methods_added ||= []
end

def AN_OBJECT.singleton_method_added(method_name)
  @@singleton_methods_added ||= []
  @@singleton_methods_added << method_name
end

If you look at the previous excercise if you scroll up, it should give you enough hint on how to handle it. It's a tragedy that I posted this so much later..