Crazy hack to intercept all changes to instance variables

intercept_ivars.rb

module IVarInterceptor
  def self.included(klass)
    def klass.new(*args, &block)
      super.extend IVarInterceptor
    end
  end

  def self.extended(object)
    object.singleton_class.class_eval do
      original_methods = instance_methods.each_with_object({}) do |method_name, hash|
        next if [ :instance_variable_get, :instance_variables, :object_id ].include?(method_name)
        next if BasicObject.instance_methods.include?(method_name)
        next if IVarInterceptor.instance_methods.include?(method_name)
        hash[method_name] = instance_method(method_name)
        undef_method method_name
      end

      define_method :method_missing do |method_name, *args, &block|
        return_value, changed_ivars = capture_ivar_changes do
          original_methods[method_name].bind(self).call(*args, &block)
        end

        if changed_ivars.any?
          puts "The following instance variables changed as a result of calling ##{method_name}: "

          changed_ivars.each do |ivar, change|
            puts "  - #{ivar} changed from #{change.fetch(:from, "(unset)").inspect} to #{change.fetch(:to, "(unset)").inspect}"
          end
        else
          puts "No instance variables changed as a result of calling ##{method_name}."
        end

        return_value
      end
    end
  end

  def capture_ivar_changes
    ivar_values_1 = current_ivar_values
    return_value = yield
    ivar_values_2 = current_ivar_values

    changed_ivars = {}

    (ivar_values_1.keys | ivar_values_2.keys).each do |ivar|
      case
        when ivar_values_1.has_key?(ivar) && !ivar_values_2.has_key?(ivar)
          changed_ivars[ivar] = { :from => ivar_values_1[ivar] }
        when ivar_values_2.has_key?(ivar) && !ivar_values_1.has_key?(ivar)
          changed_ivars[ivar] = { :to => ivar_values_2[ivar] }
        when ivar_values_1[ivar] != ivar_values_2[ivar]
          changed_ivars[ivar] = { :from => ivar_values_1[ivar], :to => ivar_values_2[ivar] }
      end
    end

    return return_value, changed_ivars
  end

  def current_ivar_values
    instance_variables.each_with_object({}) do |ivar, values|
      values[ivar] = instance_variable_get(ivar)
    end
  end
end

class Point
  include IVarInterceptor
  attr_accessor :x, :y

  def product
    @product ||= x * y
  end
end

p = Point.new
p.x = 7
p.y = 3

puts "The product is: #{p.product}"

p.x = 2

puts "The product is: #{p.product}"

output

$ ruby intercept_ivars.rb
The following instance variables changed as a result of calling #x=: 
  - @x changed from "(unset)" to 7
The following instance variables changed as a result of calling #y=: 
  - @y changed from "(unset)" to 3
No instance variables changed as a result of calling #x.
No instance variables changed as a result of calling #y.
The following instance variables changed as a result of calling #product: 
  - @product changed from "(unset)" to 21
The product is: 21
The following instance variables changed as a result of calling #x=: 
  - @x changed from 7 to 2
No instance variables changed as a result of calling #product.
The product is: 21