举例讲解Python设计模式编程中的访问者与观察者模式

# 轮子,引擎, 车身这些定义好了都不需要变动
class Wheel:
  def __init__(self, name):
    self.name = name
  def accept(self, visitor):
    # 每个visitor是同样的，但是其中的方法是不一样的，比如这里是visitWheel，
    # 然后传入了self，想想？他其实想做什么就能做什么
    visitor.visitWheel(self)

class Engine:
  def accept(self, visitor):
    visitor.visitEngine(self)

class Body:
  def accept(self, visitor):
    visitor.visitBody(self)

# 我们要组合成车
class Car:
  def __init__(self):
    self.engine = Engine()
    self.body  = Body()
    self.wheels = [ Wheel("front left"), Wheel("front right"),
            Wheel("back left") , Wheel("back right") ]

  # 这个也不需要在动，他只是上面部件的组合，只是做了属性的委托
  def accept(self,visitor):
    visitor.visitCar(self)
    self.engine.accept(visitor)
    self.body.accept(visitor)
    for wheel in self.wheels:
      wheel.accept(visitor)

# 这个才是我们的访问者，每次的修改都在这里面
class PrintVisitor:
  def visitWheel(self, wheel):
    print "Visiting "+wheel.name+" wheel"
  def visitEngine(self, engine):
    print "Visiting engine"
  def visitBody(self, body):
    print "Visiting body"
  def visitCar(self, car):
    print "Visiting car"

if __name__ == '__main__':
  car = Car()
  visitor = PrintVisitor()
  car.accept(visitor)

# 这个是观察者基类
class Subject(object):
  def __init__(self):
    self._observers = []

  # 添加依赖的对象
  def attach(self, observer):
    if not observer in self._observers:
      self._observers.append(observer)

  # 取消添加
  def detach(self, observer):
    try:
      self._observers.remove(observer)
    except ValueError:
      pass

  # 这里只是通知上面注册的依赖对象新的变化
  def notify(self, modifier=None):
    for observer in self._observers:
      # 可以设置过滤条件，对不符合过滤条件的更新
      if modifier != observer:
        observer.update(self)


# 观察者类
class Data(Subject):
  def __init__(self, name=''):
    super(Data, self).__init__()
    self.name = name
    self._data = 0

  # python2.6新增的写法,获取属性为property，设置属性为(假设属性名字为x)@x.setter,删除为@x.deleter
  @property
  def data(self):
    return self._data

  @data.setter
  def data(self, value):
    self._data = value
    self.notify()

# 这里有2个被观察者，也就是依赖的对象，每次Data有改变，这2个view都会变动
class HexViewer(object):
  def update(self, subject):
    print 'HexViewer: Subject %s has data 0x%x' % (subject.name, subject.data)

class DecimalViewer(object):
  def update(self, subject):
    print 'DecimalViewer: Subject %s has data %d' % (subject.name, subject.data)


if __name__ == '__main__':

  data1 = Data('Data 1')
  data2 = Data('Data 2')
  view1 = DecimalViewer()
  view2 = HexViewer()
  data1.attach(view1)
  data1.attach(view2)
  data2.attach(view2)
  data2.attach(view1)

  print "Setting Data 1 = 10"
  data1.data = 10
  print "Setting Data 2 = 15"
  data2.data = 15
  print "Setting Data 1 = 3"
  data1.data = 3
  print "Setting Data 2 = 5"
  data2.data = 5
  print "Update data1's view2 Because view1 is be filtered"
  data1.notify(modifier=view1) 
  print "Detach HexViewer from data1 and data2."
  data1.detach(view2)
  data2.detach(view2)
  print "Setting Data 1 = 10"
  data1.data = 10
  print "Setting Data 2 = 15"
  data2.data = 15