append_features和ActiveSupport::Concern的实现

module EggConcern
  def append_features base
    super
    base.instance_eval(&@_class_methods)
    base.class_eval(&@_class_eval)
  end

  def included base = nil, &block
    super
    @_class_eval = block
  end

  def class_methods &block
    @_class_methods = block
  end

end

module ActsAsField
  extend EggConcern

  included do    
    @@acts_as_fields = []
  end

  class_methods do
    def field name, path
      result = class_variable_get(:@@acts_as_fields)
      result << name.to_sym
      class_variable_set(:@@acts_as_fields, result)

      define_method(name) do
        case path
        when String
          path.split(".").inject(self.latest_data) { |data, key| data[key] }
        when Proc
          path.call(self)
        end
      end
    end
  end

  def acts_as_fields
    self.class.class_variable_get :@@acts_as_fields
  end

end

class Device
  include ActsAsField

  field :device_type, "device_type"
  field :battery, "data.battery"
  field :node_info, "data.node_info"

  field :battery_to_text, proc { |device|
    "#{device.battery}%"
  }

  def latest_data
    {
      "data" => {
        "node_info" => "this is a sensor",
        "battery" => 90
      },
      "device_type" => "Sensor"
    }
  end

end

d = Device.new
p d.node_info
p d.battery_to_text
p d.acts_as_fields

# 扩展
# active_support
require 'active_support/all'
class A
  class_attribute :title
  cattr_accessor :title
end

A.title = "A"
A.title

module B
  mattr_accessor :title
end