module_function & extend self & inheritances & extend & include in Ruby

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 的模块 (module) 是方法和常量的集合。模块中的方法又可分为实例方法和模块方法, 当一个模块被 include 到一个 Ruby 类时 ,模块中的方法(注：没有被 module_function 关键字标记的方法)就是类中的实例方法，需要所在的类被实例化之后才能被调用；被 module_function 关键字标记的方法称为模块方法 (module method)，被标记之后模块中实例方法也会变成私有方法。调用模块方法的方式为：module_name.method_name，而在模块中没有被 module_function 关键字标记的方法不能这样被调用。

模块中的 module_function 关键字会把对应的方法编程模块方法且对于被 include 引入到所在的类来说不可见，所以不能被实例化对象调用。

如果要让模块方法也能以 module_name.method_name 的方式调用，可以考虑 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 本质是给类或者模块添加类方法。

extend self 让模块中的实例方法能够被 module_name.method_name 调用，同时保留模块中原方法的 public/private 属性，但又不像 module_function 一样把被标记的方法变成私有方法。

#!/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

inheritances & extend & include 的区别

inheritance :子类继承了父类的属性，包括访问这些属性的权限 (public/protected/private)，与其他编程语言不同的是，Ruby 能够访问父类的私有方法，甚至还可以把父类的私有方法转化为公有方法。
include :在指定的类中 mixed in 模块 (module) 中定义的方法作为该类的实例方法。include model 等同于把模块中的代码复制到所在类中，减少代码重复。通过这种方式，突破了 Ruby 只允许单继承的限制。
extend :把模块中的方法引入到当前类，并作为类方法。

总结

module_function改变模块内原方法的 public/private 属性并设置成模块方法,能够通过 module_name.method_name 的方法调用。

extend self就是在模块内自继承，不改变模块中方法的 public/private 属性，同样也能通过 module_name.public_method 的方法调用。

另外，rubocop 推荐使用 module_functionStyle/ModuleFunction: Use module_function instead of extend self. (https://github.com/bbatsov/ruby-style-guide#module-function)

module_function & extend self & inheritances & extend & include in Ruby

module_function

extend self

inheritances & extend & include 的区别

总结

参考链接