An example of a method decorator in Ruby

decorator.rb

module TrackMethods
  def track(meth)
    self.class_eval do
      alias_method "old_#{meth}", meth

      define_method meth do |*args|
        puts "Calling #{meth} with #{args.join(', ')}"
        self.send "old_#{meth}", *args
      end
    end
  end
end

class Account
  extend TrackMethods

  def initialize(balance)
    @balance = balance
  end

  def deposit(n)
    @balance += n
    puts "Balance is now #{@balance}"
  end

  track :deposit
end

a = Account.new(10)
a.deposit 5