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Ruby 中的 module_function 和 extend self异同

时间：2022-11-14 07:59 编辑：来源：阅读：
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摘要：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？ [b]module_function[/b] 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 [b]extend self[/b] 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 [b]总结[/b] 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

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