Question

我创建了以下类：

  class Decorator < OpenStruct
    def initialize(options={})
      define_predicate options
      super(options)
    end

    def define_predicate(options)
      options.each do |k,v|
        define_method "#{k}?" do
          v.present?
        end
      end
    end
  end

我希望能够做到以下几点：

decor = Decorator.new({ highlight: 'light', color: 'green' })
if decor.highlight?
  puts decor.highlight
end
if decor.color?
  puts decor.color
end

不幸的是，当我尝试初始化Decorator类时，我收到以下错误：

NoMethodError: undefined method `each' for nil:NilClass
    from /Users/donato/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/ostruct.rb:245:in `inspect'
    from /Users/donato/.rvm/rubies/ruby-2.1.2/lib/ruby/2.1.0/ostruct.rb:187:in `method_missing'
    from (irb):16:in `block in define_predicate'
    from (irb):15:in `each'
    from (irb):15:in `define_predicate'
    from (irb):10:in `initialize'
    from (irb):22:in `new'

为什么define_method会抛出此错误？

这有效：

class Decorator < OpenStruct
    def initialize(options={})
      Decorator.define_predicate options
      super(options)
    end

    def self.define_predicate(options)
      options.each do |k,v|
        define_method "#{k}?" do
          v.present?
        end
      end
    end
  end

Answer 1

define_method是一个类方法。在第一种情况下，在对象实例上触发define_method。

define_method返回未定义的方法`每个＆＃39;为零：NilClass

1 个答案: