decorator.rb

class Object
  def self.my_decorator(method_name)
    alias_method "#{method_name}_without_my_decorator", method_name
    
    define_method method_name do
      puts "My decorator logic starts"
      result = send("#{method_name}_without_my_decorator")
      puts "My decorator logic ends"
      result
    end
  end
end

class MyClass
  def hello
    puts "Hello world!"
    123
  end
  my_decorator :hello
end

MyClass.new.hello

# My decorator logic starts
# Hello world!
# My decorator logic ends
# => 123