gistfile1.ru

class Dragon; end

# 使用 def 定義 class method
Dragon.instance_eval do
  def foo
    puts "bar"
  end
end
Dragon.foo # bar

# 使用 def 定義 instance method
Dragon.class_eval do
  def foo
    puts "bar"
  end
end
Dragon.new.foo # bar

# 使用 self.def 定義 class method
Dragon.class_eval do # 這裡改用 instance_eval 也可以耶(why?)
  def self.foo
    puts "bar"
  end
end
Dragon.foo # bar

# ------------------------------------------------------------------------------------

# 使用 define_method 定義 instance method
class Dragon
  define_method("foo") { puts "bar" }
end
Dragon.new.foo # bar

Dragon.class_eval do
  define_method("foo") { puts "bar" }
end
Dragon.new.foo # bar

# 使用 define_method 定義 class method
class Dragon
  class << self
    define_method("foo") { puts "bar" }
  end
end
Dragon.foo # bar

# 使用 define_method 嘗試定義 class method
Dragon.instance_eval do
  define_method("foo2") { puts "bar" }
end
Dragon.foo2 rescue puts "fails" # 失敗找不到 method (why?)

# 使用 define_method 定義 class method
metaclass = (class << Dragon; self; end)
metaclass.instance_eval do # 這裡改用 class_eval 也可以耶(why?)
    define_method("foo") { puts "bar" }
end
Dragon.foo # bar

# 使用 define_method 定義 class method (不用 metaclass 的話，只好定義在 Class 上)
Dragon.class.instance_eval do # 這裡用 class_eval 也可以
    define_method("foo") { puts "bar" }
end
Dragon.foo # bar
String.foo # bar，所有類別都被污染啦!! bad idea!!

# ------------------------------------------------------------------------------------
# Recommended reading:
# 
#  http://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/
#  http://yugui.jp/articles/846
#
# My conclusion:

# mechanism      	self(current object)  method definition(current class)    new scope?
# class Dragon	        Dragon	              Dragon	                           yes
# class << Dragon	Dragon’s metaclass    Dragon’s metaclass	           yes
# Dragon.class_eval	Dragon	              Dragon	                           no
# Dragon.instance_eval	Dragon	              Dragon’s metaclass	           no

# "def method_name" define instance method for current class
# "define_method" defind instance method for self (must be class object)
# "def object.method_name" defines instance method for object's metaclass (a.k.a singleton method)

# New scope means that code inside the block does not have access to local variables outside of the block.

# class_eval, instance_eval 和 define_method 這幾個 method 不會有新的 scope 是 Ruby 跑 dynamic code 的利器，因為可以很方便讀取到 block 外的 local variable。