为什么模块的“self"方法不能成为类的单例方法?

问题描述

module Test
  def self.model_method
    puts "this is a module method"
  end
end

class A
  include Test
end

A.model_method

这将是错误的:

未定义的方法`model_method' for A:Class (NoMethodError)

但是当我使用 A 的元类时，它可以工作:

But when I use metaclass of A. it works:

module Test
  def model_method
    puts "this is a module method"
  end
end

class A
  class << self
    include Test
  end
end

A.model_method

有人能解释一下吗?

推荐答案

如果你想在包含一个模块时将类方法和实例方法混合到一个类中，你可以遵循以下模式:

If you want to have both class methods and instance methods mixed into a class when including a module, you may follow the pattern:

module YourModule
  module ClassMethods
    def a_class_method
      puts "I'm a class method"
    end
  end

  def an_instance_method
    puts "I'm an instance method"
  end

  def self.included(base)
    base.extend ClassMethods
  end
end

class Whatever
  include YourModule
end

Whatever.a_class_method
# => I'm a class method

Whatever.new.an_instance_method
# => I'm an instance method

基本上是为了过度简化它，你extend 来添加类方法，你include 来添加实例方法.当一个模块被包含时，它的 #included 方法被调用，使用它被包含的实际类.从这里你可以使用另一个模块的一些类方法extend.这是一种很常见的模式.

Basically to over-simplify it, you extend to add class methods and you include to add instance methods. When a module is included, it's #included method is invoked, with the actual class it was included in. From here you can extend the class with some class methods from another module. This is quite a common pattern.

另见:http://api.rubyonrails.org/classes/ActiveSupport/Concern.html

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