Ruby.Metaprogramming。class_eval

Question

我的代码中似乎有一个错误。然而，我就是找不到。

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

您将在Sergios answer中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。

使用

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

你可以执行

 @bar_history = [1,2,3]

这是在类级别执行的，而不是在对象级别执行的。变量@bar_history在Foo-object中不可用，但在Foo-class中可用。

使用

puts f.bar_history.to_s

您可以在对象级别定义的属性@bar_history上访问-never。

当您在类级别上定义读取器时，您可以访问变量：

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

Answer 2

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]

下面是您可能想要编写的代码(从名称判断)。

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 3

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

测试：

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