ruby模块如何在包含它的类上强制执行条件? [英] How can a ruby module enforce conditions on classes in which it is included?

问题描述

我如何编写一个ruby模块，对包含它的类强加一些条件，必须在当前打开的类定义的末尾满足该条件?

How can I write a ruby module that imposes some conditions on classes in which it is included, which must be met the end of currently opened class definition?

具体来说，假设条件是类变量@@foo应该定义为> 0"

To be concrete, suppose the condition is "a class variable @@foo should be defined to be >0 "

我想编写一个看起来像这样的模块:

I'd like to write a module that looks something like this:

module NeedPositiveFoo
  module ClassMethods
    def validate_positive_foo
      raise unless defined?(@@foo) && @@foo > 0
    end
  end

  def included(other)
    other.extend(ClassMethods)
  end
end

然后该类定义将有效:

class ValidClass
  include NeedPositiveFoo
  @@foo = 3
end

但是这些类定义将在它们的end结束后引发:

But these class definitions would raise after their closing end's:

class InvalidClass1
  include NeedPositiveFoo
  # @@foo is not defined
end

class InvalidClass2
  include NeedPositiveFoo
  @@foo = -2
end

推荐答案

当您允许在类定义的末尾添加include时，Uri Agassi的回答非常有效，尽管include被放置.

While the answer by Uri Agassi perfectly works when you are allowed to put includes in the very end of class definition, the code below will work despite where include was placed.

def included(other)
  TracePoint.new(:end) do |tp|
    if tp.self == other
      tp.disable
      raise unless defined?(other.class_variable_get(:@@foo)) # checks
    end
  end.enable
  other.extend(ClassMethods)
end

TracePoint文档.

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