Python 设计模式 观察者模式

  观察者模式 ：定义了内部的对象之间是1：n的关系，当一个对象的状态发生了变化，与这个对象相关联的数据都会同时发生改变。

类似报纸订阅，当订阅报纸后，一有更新就会自动收到，除非退订；这里我们将出版者称为“主题”（Subject），订阅者成为“观察者”（Observer）。

  类图：其中的ConcreteSubject就是相关的报纸。

案例：现在要实现一个气象站，观测四个数据，有气压，温度，湿度，可见度，但是有多个地点的公告板，需要使用到其中的数据。当其中的一个数据发生改变，其他的公告板，会马上得到更新。

基本实现如下：

定义基本模型： Subject,Obverser

class Subject:  

    def register(self, observer):  
        pass  
  
    def deregister(self, observer):  
        pass  
  
    def notify_observers(self):  
        pass  
  
class Observer:  
    def update(self, temperature, humidity, pressure):  
        pass  
  
class DisplayElement:  
    def display(self):  
        pass  

其中具体主题有WeatherData, 具体的订阅者有AverageDisplay,CurrentDisplay

class WeatherData(Subject):  
    def __init__(self):  
        self.observers = []  
  
    def register(self, observer):  
        if observer not in self.observers:  
            self.observers.append(observer)  
  
    def deregister(self, observer):  
        if observer in self.observers:  
            self.observers.remove(observer)  
  
    def notify_observers(self):  
        for o in self.observers:  
            o.update(self.temperature, self.humidity, self.pressure)  
  
    def data_changed(self):  
        self.notify_observers()  
  
    def set_data(self, temperature, humidity, pressure):  
        self.temperature = temperature  
        self.humidity = humidity  
        self.pressure = pressure  
        self.data_changed()  
  
  
class CurrentDisplay(Observer, DisplayElement):  
    def __init__(self, weather_data=None):  
        self.weather_data = weather_data  
        self.weather_data.register(self)  
  
    def update(self, temperature, humidity, pressure):  
        self.temperature = temperature   
        self.humidity = humidity  
        self.pressure = pressure  
        self.display()  
  
    def display(self):  
        print "Current Data: temperature=%s, humidity=%s, pressure=%s" % \  
            (self.temperature, self.humidity, self.pressure)  
  
class AverageDisplay(Observer, DisplayElement):  
    def __init__(self, weather_data=None):  
        self.temperature = []  
        self.humidity = []  
        self.pressure = []  
        self.weather_data = weather_data  
        self.weather_data.register(self)  
  
    def update(self, temperature, humidity, pressure):  
        self.temperature.append(temperature)  
        self.humidity.append(humidity)  
        self.pressure.append(pressure)  
        self.display()  
  
    def average(self,lst):  
        n = 0  
        for x in lst:  
            n += x  
        return n/len(lst)  
  
    def display(self):  
        print "Average Data: temperature=%s, humidity=%s, pressure=%s" % \  
            (self.average(self.temperature), \  
            self.average(self.humidity), \  
            self.average(self.pressure))  

测试

if __name__ == '__main__':  
    weather_data = WeatherData()  
  
    current = CurrentDisplay(weather_data)  
    average = AverageDisplay(weather_data)  
  
    weather_data.set_data(18,70,100); print  
    weather_data.set_data(20,70,120); print  
    weather_data.set_data(22,70,80); print  
    weather_data.set_data(24,70,40); print  
  
    weather_data.deregister(average)  
    weather_data.set_data(30,70,100); print  
    weather_data.set_data(40,70,120); print 

OK ，Game Over