设计模式：观察者模式（Python）
观察者模式（Observer）：
定义了对象间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。
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）。
原则：
为交互对象之间的松耦合而努力
案例：
现在要实现一个气象站，观测三个数据：温度，湿度和气压；气象站有两个布告板：分别显示当前最新值和平均值；当气象数据更新后，气象站要能自动跟新所有布告板。
代码：
#!/usr/bin/python
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
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))
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
输出：
Current Data: temperature=18, humidity=70, pressure=100 Average Data: temperature=18, humidity=70, pressure=100 Current Data: temperature=20, humidity=70, pressure=120 Average Data: temperature=19, humidity=70, pressure=110 Current Data: temperature=22, humidity=70, pressure=80 Average Data: temperature=20, humidity=70, pressure=100 Current Data: temperature=24, humidity=70, pressure=40 Average Data: temperature=21, humidity=70, pressure=85 Current Data: temperature=30, humidity=70, pressure=100 Current Data: temperature=40, humidity=70, pressure=120