rails Forwardable def_delegators

  设计模式中常说，组合优于继承，可是组合带来的问题是，你可能要在类中添加wrap方法，委托别的类去做这个实现，举例如下：

class Robot
  def initialize
    @heat_sensor = HeatSensor.new
    @arm = RobotArm.new
  end

  def measure_heat(scale="c")
    @heat_sensor.measure(scale)
  end

  def stack(boxes_number=1)
    @arm.stack(boxes_number)
  end

  def package
    @arm.package
  end
end

class HeatSensor
  # Celsius or Fahrenheit scale
  def measure(scale="c")
    t = rand(100)
    t = scale=="c" ? t : t * (9/5)
    puts "Heat is #{t}° #{scale.upcase}"
  end
end

class RobotArm
  def stack(boxes_number=1)
    puts "Stacking #{boxes_number} box(es)"
  end

  def package
    puts "Packaging"
  end
end

　   Robot这个类就包装了stack, package 以及measure_heat三个方法，都是委托其他类实现，这样的写法很土，借助于ruby强大的语言特性，其实你完全可以不用写如此丑陋的代码，你只要写成如下方式就好了

class Robot
  # Extending provides class methods
  extend Forwardable
  # Use of  def_delegators
  def_delegators :@arm,:package,:stack
  # Use of  def_delegator
  def_delegator :@heat_sensor, :measure ,:measure_heat
  def initialize
    @heat_sensor = HeatSensor.new
    @arm = RobotArm.new
  end
end

　　def_delegator(obj, method, alias = method)

      def_delegators(obj, *methods)