Ruby.Metaprogramming。 class_eval

Question

我的代码似乎有误。但是我无法找到它。

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

  attr_reader attr_name
  attr_writer attr_name

  attr_reader attr_name + "_history"
  class_eval %Q{
   @#{attr_name}_history=[1,2,3]
  }

end
end

class Foo
 attr_accessor_with_history :bar
end

f = Foo.new
f.bar = 1
f.bar = 2
puts f.bar_history.to_s

我希望它返回一个数组[1,2,3]。但是，它不会返回任何内容。

Answer 1

您不应该打开Class来添加新方法。这就是模块的用途。

module History
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s

    attr_accessor attr_name

    class_eval %Q{
      def #{attr_name}_history
        [1, 2, 3]
      end
    }

  end
end

class Foo
  extend History
  attr_accessor_with_history :bar
end

f = Foo.new
f.bar = 1
f.bar = 2
puts f.bar_history.inspect
# [1, 2, 3]

这是你可能想写的代码（从名字来看）。

module History
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s

    class_eval %Q{
      def #{attr_name}
        @#{attr_name}
      end

      def #{attr_name}= val
        @#{attr_name}_history ||= []
        @#{attr_name}_history << #{attr_name}

        @#{attr_name} = val
      end

      def #{attr_name}_history
        @#{attr_name}_history
      end
    }

  end
end

class Foo
  extend History
  attr_accessor_with_history :bar
end

f = Foo.new
f.bar = 1
f.bar = 2
puts f.bar_history.inspect
# [nil, 1]

Answer 2

解决方案：

class Class
  def attr_accessor_with_history(attr_name)
    ivar         = "@#{attr_name}"
    history_meth = "#{attr_name}_history"
    history_ivar = "@#{history_meth}"

    define_method(attr_name) { instance_variable_get ivar }

    define_method "#{attr_name}=" do |value|
      instance_variable_set ivar, value
      instance_variable_set history_ivar, send(history_meth) << value
    end

    define_method history_meth do
      value = instance_variable_get(history_ivar) || []
      value.dup
    end
  end
end

试验：

describe 'Class#attr_accessor_with_history' do
  let(:klass)     { Class.new { attr_accessor_with_history :bar } }
  let(:instance)  { instance = klass.new }

  it 'acs as attr_accessor' do
    instance.bar.should be_nil
    instance.bar = 1
    instance.bar.should == 1
    instance.bar = 2
    instance.bar.should == 2
  end

  it 'remembers history of setting' do
    instance.bar_history.should == []
    instance.bar = 1
    instance.bar_history.should == [1]
    instance.bar = 2
    instance.bar_history.should == [1, 2]
  end

  it 'is not affected by mutating the history array' do
    instance.bar_history << 1
    instance.bar_history.should == []
    instance.bar = 1
    instance.bar_history << 2
    instance.bar_history.should == [1]
  end
end

Answer 3

您可以在Sergios answer找到问题的解决方案。这里有一个解释，你的代码出了什么问题。

用

class_eval %Q{
 @#{attr_name}_history=[1,2,3]
}

你执行

 @bar_history = [1,2,3]

您在类级别执行此操作，而不是在对象级别执行此操作。 变量@bar_history在Foo对象中不可用，但在Foo类中不可用。

用

puts f.bar_history.to_s

您可以在对象级别定义属性@bar_history上访问-never。

在课程级别定义阅读器时，您可以访问变量：

class << Foo 
  attr_reader :bar_history
end
p Foo.bar_history  #-> [1, 2, 3]

Answer 4

@Sergio Tulentsev的回答是有效的，但它促进了使用字符串评估的问题，这种做法通常充满了安全风险和其他意外，而输入并不是您所期望的。例如，如果有人打电话给塞尔吉奥的版本会发生什么（不要试试）：

attr_accessor_with_history %q{foo; end; system "rm -rf /"; def foo}

通常可以在没有字符串评估的情况下更仔细地进行ruby元编程。在这种情况下，使用带有instance_variable_ [get | set]的闭包的简单插值和define_method，并发送：

module History

  def attr_accessor_with_history(attr_name)
    getter_sym  = :"#{attr_name}"
    setter_sym  = :"#{attr_name}="
    history_sym = :"#{attr_name}_history"
    iv_sym      = :"@#{attr_name}"
    iv_hist     = :"@#{attr_name}_history"

    define_method getter_sym do
      instance_variable_get(iv_sym)
    end

    define_method setter_sym do |val|
      instance_variable_set( iv_hist, [] ) unless send(history_sym)
      send(history_sym).send( :'<<', send(getter_sym) )
      instance_variable_set( iv_sym, val @)
    end

    define_method history_sym do
      instance_variable_get(iv_hist)
    end

  end
end

Answer 5

这是应该做的。需要使用class_eval在Class中定义attr_writer。

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

    attr_reader attr_name
    #attr_writer attr_name  ## moved into class_eval

    attr_reader attr_name + "_history"

    class_eval %Q{
      def #{attr_name}=(value)
        @#{attr_name}_history=[1,2,3]
      end
    }

end
end