ruby - 用历史初始化类似 `attr_accessor` 的方法

Question

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}=(new_value)
        @#{attr_name}_history = [nil] if @#{attr_name}_history.nil?
        @#{attr_name}_history << @#{attr_name} = new_value
      end
    }
  end
end

class Example
  attr_accessor_with_history :foo
  attr_accessor_with_history :bar
end

有 Class.attr_accessor_with_history 方法提供相同的作为 attr_accessor 的功能，但也跟踪属性具有的每个值曾经有过。

> a = Example.new; a.foo = 2; a.foo = "test"; a.foo_history
=> [nil, 2, "test"]

但是，

> a = Example.new; a.foo_history
=> nil

它应该是[nil。

如何为 Example 类定义单个 initialize 方法，其中每个…_history 值将被初始化为[nil]?

Answer 1

我认为，您最好的选择是为历史定义一个自定义阅读器(以及您的自定义编写器)。

class Class
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s
    attr_reader attr_name
    class_eval %Q{
      def #{attr_name}_history
        @#{attr_name}_history || [nil] # give default value if not assigned
      end

      def #{attr_name}=(new_value)
        @#{attr_name}_history ||= [nil] # shortcut, compare to your line
        @#{attr_name}_history << @#{attr_name} = new_value
      end
    }
  end
end

class Example
  attr_accessor_with_history :foo
  attr_accessor_with_history :bar
end

a = Example.new; a.foo = 2; a.foo = "test"; 
a.foo_history # => [nil, 2, "test"]

a = Example.new
a.foo_history # => [nil]

编辑:

这里有一个稍微冗长的代码片段，但它没有使用 class_eval(在没有必要的情况下使用时不受欢迎)。

class Class
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s
    attr_reader attr_name

    define_method "#{attr_name}_history" do
      instance_variable_get("@#{attr_name}_history") || [nil]
    end

    define_method "#{attr_name}=" do |new_value|
      v = instance_variable_get("@#{attr_name}_history")
      v ||= [nil]
      v << new_value

      instance_variable_set("@#{attr_name}_history", v)
      instance_variable_set("@#{attr_name}", new_value)
    end
  end
end