Ruby 中的 module_function 和 extend self异同

在阅读开源的 Ruby 代码和编写可维护性的代码经常遇到这两者的使用，那么他们两者的共同点和区别是什么呢？

module_function

Ruby 的 module 是 method 和 constants 的集合。module 中的method 又可分为 instance method 和 module method, 当一个 module 被 include 进一个 class ,那么 module 中的 method (注：没有被 module_function 标记的 method)就是 class 中的 instance method, instance method 需要所在的 class 被实例化之后才能被调用；被 module_function 标记的 method（不管该 method 是 public 或者 private）就是 module method 且 instance method 也会变成 private method，对于被 include 所在的 class 来说是 private method，object.module_name 会出错。module method 都能被 module_name.method_name 调用，没有被 module_function 标记的 public method 不能被 module_name.method_name 调用。

module 中的 module_function 会把 module 中的 method 变成 module method 且对于被 include 所在的 class 来说，module method 在 module 中是 private method 故 module_name.module_method 能调用，而不能被 object.module_name 调用。

module 中的 public method 对于被 include 所在的 class 来说是 instance method,故 object.public_method_in_module 能调用。如果想要非 module method 能够被 module 调用(module_name.not_module_method) ，需要引入 extend self （下文会讨论 extend self）

# test.rb
module MyModule
 def public_meth
  p "a public method, if the module is included to a class , can be call as object.public_meth"
 end
 def module_method
  p "a module method,can be called as module_name.module_method. but can not be call as object.module_method"
 end
 private
 def private_method_to_module_function
  p "a private_method, but can be call as module_name.module_method, because it was assigned to module_function"
 end
 def private_method
  p "I am a private method"
 end
 module_function :module_method, :private_method_to_module_function
end

MyModule.module_method
MyModule.private_method_to_module_function
begin
 MyModule.public_meth
rescue
 p "public method can not be called by module_name.public_meth"
end
begin
 MyModule.private_method
rescue NoMethodError
 p "private method can not be called by module_name.module_method"
end

class MyClass
 include MyModule
end

obj = MyClass.new
obj.public_meth

begin
 obj.private_method
rescue NoMethodError
 p "private method in module can not be call by object.method_name"
end

begin
 obj.module_method
rescue NoMethodError
 p "module method can not be called by object.method_name, for object, module method is private instance method"
end

#调用
ruby test.rb
"a module method,can be called as module_name.module_method. but can not be call as object.module_method"
"a private_method, but can be call as module_name.module_method, because it was assigned to module_function"
"public method can not be called by module_name.public_meth"
"private method can not be called by module_name.module_method"
"a public method, if the module is included to a class , can be call as object.public_meth"
"private method in module can not be call by object.method_name"
"module method can not be called by object.method_name, for object, module method is private instance method"

总结就是

•The method will be copied to class' singleton class
•The instance method's visibility will become private

extend self

Include is for adding methods to an instance of a class and extend is for adding class methods

extend 本质是给 class 或者 module 添加 class method

extend self 让 module 中的 instance method 能够被 module_name.instance_method 调用，保留 module 中原本 method 的 public 或 private 属性，但又不像 module_function 一样把被标记的 method 变成 private 。

#!/usr/bin/env ruby
# encoding: utf-8
# test_extend.rb
module MyModule
 extend self
 def public_meth
  p "a public_method extended by self can be called by module_name.public_meth and object.public_meth, included by a class"
  private_method
 end
 private
 def private_method
  p "a private method, can be call in module internal"
 end
end

class MyClass
 include MyModule
end

MyModule.public_meth

begin
 MyModule.private_method
rescue NoMethodError
 p "private method in extend self module can not be called module_name.private_method"
end

obj = MyClass.new
obj.public_meth

begin
 obj.private_method
rescue NoMethodError
 p "private method can not be called by object.private_method"
end

# 调用 ruby test_extend.rb
"a public_method extended by self can be called by module_name.public_meth and object.public_meth, included by a class"
"a private method, can be call in module internal"
"private method in extend self module can not be called module_name.private_method"
"a public_method extended by self can be called by module_name.public_meth and object.public_meth, included by a class"
"a private method, can be call in module internal"
"private method can not be called by object.private_method"

总结就是：
•No method copying involved
•No changes to method visibility

module_function 改变 module 内 原来 method 的 public/private 属性并把改 method 变成 module method ,能够被 module_name.module_method 调用。

extend self 就是在 module 自继承，不改变 module 中 method 的 public/private 属性，能够被 module_name.public_method