观察者模式

观察者模式（有时又被称为发布（publish-订阅（Subscribe）模式、模型-视图（View）模式、源-收听者(Listener)模式或从属者模式）是软件设计模式的一种。在此种模式中，一个目标物件管理所有相依于它的观察者物件，并且在它本身的状态改变时主动发出通知。这通常透过呼叫各观察者所提供的方法来实现。此种模式通常被用来实现事件处理系统。

观察者模式中包含以下几种对象类型：

• 观察者(Observer)：观察者将自己注册到被观察对象中，被观察对象将观察者存放在一个容器（Container）里。
• 被观察对象(Subject)：被观察对象发生了某种变化，从容器中得到所有注册过的观察者，将变化通知观察者。

示例

# coding:utf-8
import weakref

class Subject(object):
    """被观察对象，使用列表存储所有观察者"""
    def __init__(self, name):
        """此处使用弱引用，当删除observer是，WeakSet也会自动删除对应对象"""
        self.name = name
        self._observers = weakref.WeakSet()


    def register_observer(self, observer):
        """订阅事件，相当于将观察者添加到列表中"""
        self._observers.add(observer)
        print "observer {0} now listening on {1}".format(observer.name, self.name)


    def notify_observers(self, msg):
        """遍历列表，发送消息给每一个观察者"""
        print("subject notifying observers about {}".format(msg,))
        for observer in self._observers:
            observer.notify(self, msg)


class Observer(object):
    def __init__(self, name):
        self.name = name

    def start_observing(self, subject):
        """订阅事件"""
        subject.register_observer(self)


    def notify(self, subject, msg):
        """通知观察者"""
        print "{0} get msg from {1} that {2}".format(self.name, subject.name, msg)

class_homework = Subject("class homework")
student1 = Observer("student 1")
student2 = Observer("student 2")

student1.start_observing(class_homework)
student2.start_observing(class_homework)

class_homework.notify_observers("result if out")
del student1
class_homework.notify_observers("20/20 passed this exam")

输出结果为：

observer student 1 now listening on class homework
observer student 2 now listening on class homework
subject notifying observers about result if out
student 1 get msg from class homework that result if out
student 2 get msg from class homework that result if out
subject notifying observers about 20/20 passed this exam
student 2 get msg from class homework that 20/20 passed this exam