Head First 设计模式——观察者模式（Observer Pattern）——Python实现（1）

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作者：liuyuan_jq

2011-06-13

观察者模式定义

定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖都会收到通知并自动更新。观察者模式提供了一种对象设计，让主题和观察者之间松耦合。

设计原则

　　为了交互对象之间的松耦合设计而努力。松耦合的设计之所以能让我们建立有弹性的系统，能够应对变化，是因为对象之间的互相依赖降到了最低。
问题
　　建立下一代Internet气象观察站，该气象站必须建立在我们专利申请中的WeatherData对象上，由WeatherData对象负责追踪目前的天气状况（温度，湿度，气压）。

• 建立一个应用，有三种布告板，分别显示目前的状态、气象统计及简单的预报。
  
• 当WeatherObject对象获得最新的测量数据时，三种布告板必须实时更新。
  
• 公布一组API，让其他开发人员可以写出自己的气象布告板。
  

设计气象站
　　WeatherData实现Subject接口
　　class Subject(object):"""主题接口类"""def registerObserver(observer):raise NotImplementedError("abstract Subject")def removeObserver(observer):raise NotImplementedError("abstract Subject")def notifyObservers(self):raise NotImplementedError("abstract Subject")
　　气象组件实现观察者接口
　　class Observer(object):"""观察者接口类"""def update(temp, humidity, pressure):raise NotImplementedError("abstract Observer")
　　布告栏显示接口
　　class DisplayElement(object):def display(self):raise NotImplementedError("abstract DisplayElement")
源代码具体实现
　　subject.py
　　#!/usr/bin/env python# -*- coding:utf-8 -*-class Subject(object):"""主题接口类"""def registerObserver(observer):raise NotImplementedError("abstract Subject")def removeObserver(observer):raise NotImplementedError("abstract Subject")def notifyObservers(self):raise NotImplementedError("abstract Subject")
　　observer.py
　　#!/usr/bin/env python# -*- coding:utf-8 -*-class Observer(object):"""观察者接口类"""def update(temp, humidity, pressure):raise NotImplementedError("abstract Observer")
　　weatherData.py
　　#!/usr/bin/env python# -*- coding:utf-8 -*-from subject import Subjectclass WeatherData(Subject):"""天气数据类，是一个主题类"""# 类静态成员observers = []def __init__(self):self.temperature = 0.0self.humidity    = 0.0self.pressure    = 0.0def registerObserver(self, observer):"""注册观察者对象"""self.observers.append(observer)def removeObserver(self, observer):"""注销观察者对象"""self.observers.remove(observer)def notifyObservers(self):"""通知所有的观察者对象"""for observer in self.observers:observer.update(self.temperature, self.humidity, self.pressure)def measurementsChanged(self):self.notifyObservers()def setMeasurements(self, temperature, humidity, pressure):self.temperature = temperatureself.humidity    = humidityself.pressure    = pressureself.measurementsChanged()def getTemperature(self):return self.temperaturedef getHumidity(self):return self.humiditydef getPressure(self):return self.pressure
　　displayElement.py
　　#!/usr/bin/env python# -*- coding:utf-8 -*-from observer import Observerfrom weatherData import WeatherDataclass DisplayElement(object):def display(self):raise NotImplementedError("abstract DisplayElement")class CurrentConditionsDisplay(Observer, DisplayElement):"""目前状况布告板"""def __init__(self, weatherData):self.weatherData = weatherDataself.weatherData.registerObserver(self)def update(self, temperature, humidity, pressure):self.temperature = temperatureself.humidity    = humidityself.display()def display(self):print("Current conditions: " + str(self.temperature) + "F degrees and " + str(self.humidity) + "% humidity")class ForecastDisplay(Observer, DisplayElement):"""天气预报布告板"""def __init__(self, weatherData):self.weatherData = weatherDataself.currentPressure = 29.92self.weatherData.registerObserver(self)def update(self, temp, humidity, pressure):self.lastPressure    = self.currentPressureself.currentPressure = pressureself.display()def display(self):print("Forecast: ");if self.currentPressure > self.lastPressure:print("Improving weather on the way!")elif self.currentPressure == self.lastPressure:print("More of the same")elif self.currentPressure < self.lastPressure:print("Watch out for cooler, rainy weather")class StatisticsDisplay(Observer, DisplayElement):"""气象统计布告板"""def __init__(self, weatherData):self.maxTemp = 0.0self.minTemp = 200self.tempSum = 0.0self.numReadings = 0self.weatherData = weatherDataself.weatherData.registerObserver(self)def update(self, temp, humidity, pressure):self.tempSum += tempself.numReadings += 1if temp > self.maxTemp:self.maxTemp = tempif temp < self.minTemp:self.minTemp = tempself.display()def display(self):print("Avg/Max/Min temperature = " + str(self.tempSum / self.numReadings)+ "/" + str(self.maxTemp) + "/" + str(self.minTemp))class HeatIndexDisplay(Observer, DisplayElement):"""酷热指数布告板"""def __init__(self, weatherData):self.weatherData = weatherDataself.weatherData.registerObserver(self)self.heatIndex = 0.0def update(self, t, rh, pressure):self.heatIndex = self._computeHeatIndex(t, rh)self.display()def _computeHeatIndex(self, t, rh):index = ((16.923 + (0.185212 * t) + (5.37941 * rh) - (0.100254 * t * rh) + (0.00941695 * (t * t)) + (0.00728898 * (rh * rh)) + (0.000345372 * (t * t * rh)) - (0.000814971 * (t * rh * rh)) +(0.0000102102 * (t * t * rh * rh)) - (0.000038646 * (t * t * t)) + (0.0000291583 * (rh * rh * rh)) + (0.00000142721 * (t * t * t * rh)) + (0.000000197483 * (t * rh * rh * rh)) - (0.0000000218429 * (t * t * t * rh * rh)) +0.000000000843296 * (t * t * rh * rh * rh)) -(0.0000000000481975 * (t * t * t * rh * rh * rh)))return float(index)def display(self):print("Heat index is " + str(self.heatIndex))
　　测试
　　weatherStation.py
　　#!/usr/bin/env python# -*- coding:utf-8 -*-from weatherData import WeatherDatafrom displayElement import *weatherData = WeatherData()currentDisplay    = CurrentConditionsDisplay(weatherData)statisticsDisplay = StatisticsDisplay(weatherData)forecastDisplay   = ForecastDisplay(weatherData)weatherData.setMeasurements(80, 65, 30.4)weatherData.setMeasurements(82, 70, 29.2)weatherData.setMeasurements(78, 90, 29.2)
　　Current conditions: 80F degrees and 65% humidityAvg/Max/Min temperature = 80.0/80/80Forecast: Improving weather on the way!Current conditions: 82F degrees and 70% humidityAvg/Max/Min temperature = 81.0/82/80Forecast: Watch out for cooler, rainy weatherCurrent conditions: 78F degrees and 90% humidityAvg/Max/Min temperature = 80.0/82/78Forecast: More of the same
　　weatherStationHeatIndex.py
　　#!/usr/bin/env python# -*- coding:utf-8 -*-from weatherData import WeatherDatafrom displayElement import *weatherData = WeatherData()currentDisplay    = CurrentConditionsDisplay(weatherData)statisticsDisplay = StatisticsDisplay(weatherData)forecastDisplay   = ForecastDisplay(weatherData)heatIndexDisplay  = HeatIndexDisplay(weatherData)weatherData.setMeasurements(80, 65, 30.4)weatherData.setMeasurements(82, 70, 29.2)weatherData.setMeasurements(78, 90, 29.2)
　　Current conditions: 80F degrees and 65% humidityAvg/Max/Min temperature = 80.0/80/80Forecast: Improving weather on the way!Heat index is 82.9553506371Current conditions: 82F degrees and 70% humidityAvg/Max/Min temperature = 81.0/82/80Forecast: Watch out for cooler, rainy weatherHeat index is 86.9012330639Current conditions: 78F degrees and 90% humidityAvg/Max/Min temperature = 80.0/82/78Forecast: More of the sameHeat index is 83.6496713956