Ruby: Module.#prepend でメソッド呼び出しをフック ref: http://qiita.com/lnznt/items/5653f04749c9f82f7021

file10.txt

p C.ancestors  #=> [C::Logging, C, Object, Kernel, BasicObject]

file2.txt

$ ruby try1.rb 
hook: hello                # hook の出力
hello

file4.txt

$ ruby tx.rb 
hook2: before!: hello               # ２つ目のhookの出力
before!: hello                      # １つ目のhookの出力
hello
after!: hello                       # １つ目のhookの出力
hook2: after!: hello                # ２つ目のhookの出力

file5.txt

p C.ancestors   #=> [#<Module:0x007fac4fdb7510>, #<Module:0x007fac4fdb7628>, C, Object, Kernel, BasicObject]

file7.txt

$ ruby try3.rb 
hello
goodbye

file9.txt

class C
  module Logging                # 名前(Logging)を付けて、素直にモジュール定義する
    extend Forwardable
    attr_writer :logger
    def logger ; @logger ||= Logger.new(nil) ; end
    delegate %i(debug info warn error fatal) => :logger
         :
  end
  prepend Logging               # prepend する
         :

hook.rb

module Hook
  def hook(&block)
    module_eval { prepend Module.new &block }
  end
end

class Class
  include Hook
end

try.log

# Logfile created on 2015-01-26 00:39:51 +0900 by logger.rb/44203
I, [2015-01-26T00:50:39.131894 #19689]  INFO -- : call 'foo(hello)'
I, [2015-01-26T00:50:39.132019 #19689]  INFO -- : call 'bar(goodbye)'

try1.rb

class C
  hook do                               # hook 設定
    def foo(message)
      puts "hook: #{message}"
      super
    end
  end

  def foo(message)
    puts message
  end
end

obj = C.new
obj.foo("hello")

try2.rb

class C
  hook do                  # １つ目のhook 設定
    def foo(message)
      puts "#{before_message}: #{message}"
      super
      puts "#{after_message}: #{message}"
    end
  end

  hook do                   # ２つ目のhook 設定
    def foo(message)
      puts "hook2: #{before_message}: #{message}"
      super
      puts "hook2: #{after_message}: #{message}"
    end
  end

  def foo(message)
    puts message
  end

  def before_message
    "before!"
  end

  def after_message
    "after!"
  end
end

c = C.new
c.foo("hello")

try3.rb

require 'forwardable'
require 'logger'

class C
  hook do # ロギング   <-- このコメントが必要なところに'不吉な匂い'が... (「おわりに」で述べます)
    extend Forwardable
    attr_writer :logger
    def logger ; @logger ||= Logger.new(nil) ; end
    delegate %i(debug info warn error fatal) => :logger

    def foo(message)
      info { "call 'foo(#{message})'" }          # ログ出力
      super
    end
    def bar(message)
      info { "call 'bar(#{message})'" }          # ログ出力
      super
    end
  end

  def foo(message)
    puts message
  end

  def bar(message)
    puts message
  end
end

c = C.new
c.logger = Logger.new 'try.log'
c.foo("hello")
c.bar("goodbye")