Ruby模块，带有来自includer类的静态方法调用 [英] Ruby module with a static method call from includer class

问题描述

我需要在模块中定义使用包含此模块的类中的方法的常量：

I need to define the constant in the module that use the method from the class that includes this module:

module B 
  def self.included(base)
    class << base
  CONST = self.find
    end
  end
end 

class A
  def self.find
    "AAA"
  end
  include B 
end

puts A::CONST

但编译器会在第4行给出错误。

But the compiler gives the error on the 4th line.

还有其他方法可以定义常量吗？

Is there any other way to define the constant?

推荐答案

在Ruby中实现这一目标的更为惯用的方法是：

The more idiomatic way to achieve this in Ruby is:

module B 
  def self.included(klass)
    klass.class_eval <<-ruby_eval
      CONST = find
    ruby_eval

    # note that the block form of class_eval won't work
    # because you can't assign a constant inside a method
  end
end

class A
  def self.find
    "AAA"
  end
  include B 
end

puts A::CONST

你在做什么（class<< base）实际上会让你进入A的元类的上下文，而不是A本身。 find 方法是在A本身，而不是它的元类。要记住的是，类本身就是对象，因此有自己的元类。

What you were doing (class << base) actually puts you into the context of A's metaclass, not A itself. The find method is on A itself, not its metaclass. The thing to keep in mind is that classes are themselves objects, and so have their own metaclasses.

试图让它更清晰：

class Human
  def parent
    # this method is on the Human class and available
    # to all instances of Human.
  end

  class << self
    def build
      # this method is on the Human metaclass, and
      # available to its instance, Human itself.
    end

    # the "self" here is Human's metaclass, so build
    # cannot be called.
  end

  def self.build
    # exactly the same as the above
  end

  build # the "self" here is Human itself, so build can
        # be called
end

不确定是否有帮助，但是如果你不理解它，你仍然可以使用上面的class_eval成语。

Not sure if that helps, but if you don't understand it, you can still use the class_eval idiom above.

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