Ruby - 使用class_eval定义方法

Question

我正在做SaaS Stanford课程,试图完成这项任务的第5部分

我很难掌握这个概念,这就是我试图做的事情:

class Class
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s
    attr_reader attr_name
    attr_reader attr_name + '_history'
    class_eval %Q'{def #{attr_name}(a);#{attr_name}_history.push(a) ; end;}'
  end
end

我可能会做各种各样的错误,阅读关于元编程的The Book of Ruby章节,我仍然没有得到它,有人能帮我理解这个吗？

Answer 1

这很有趣!!!

class Class
    def attr_accessor_with_history(attr_name)
        attr_name = attr_name.to_s # make sure it's a string
        attr_reader attr_name
        attr_reader attr_name+"_history"
        class_eval %Q"
            def #{attr_name}=(value)
                if !defined? @#{attr_name}_history
                    @#{attr_name}_history = [@#{attr_name}]
                end
                @#{attr_name} = value
                @#{attr_name}_history << value
            end
        "
    end
end

class Foo
    attr_accessor_with_history :bar
end

class Foo2
    attr_accessor_with_history :bar
    def initialize()
        @bar = 'init'
    end
end

f = Foo.new
f.bar = 1
f.bar = nil
f.bar = '2'
f.bar = [1,nil,'2',:three]
f.bar = :three
puts "First bar:", f.bar.inspect, f.bar_history.inspect
puts "Correct?", f.bar_history == [f.class.new.bar, 1, nil, '2', [1,nil,'2',:three], :three] ? "yes" : "no"
old_bar_history = f.bar_history.inspect

f2 = Foo2.new
f2.bar = 'baz'
f2.bar = f2
puts "\nSecond bar:", f2.bar.inspect, f2.bar_history.inspect
puts "Correct?", f2.bar_history == [f2.class.new.bar, 'baz', f2] ? "yes" : "no"

puts "\nIs the old f.bar intact?", f.bar_history.inspect == old_bar_history ? "yes" : "no"

请注意,您需要使用字符串与class_eval的唯一原因是,你可以参考价值的attr_name定义自定义设置方法时.否则通常会将一个块传递给class_eval.

Answer 2

关于你所做的事情,你实际上是在解决方案的尖端.它只是 #{attr_name}_history在您的代码中不存在.您将需要创建一个实例变量,如果它不存在则将其设置为nil.你已经应该处理的数据如果确实存在则推入数组.

有几种方法可以做到这一点.一种方法是 if defined? @#{attr_name}_history DoStuffHere