观察者模式
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转自:http://blog.csdn.net/jialinqiang/article/details/8878570
一、问题描述
Weather-O-Rama气象站计划建立下一代的Internet气象观察站,该气象站必须建立在WeatherData对象的基础上,WeatherData对象提供天气数据,有三种布告板,分别显示目前的状况、气象统计及简单的预报。并且以后可以方便地增加布告板进行扩展。
使用观察者模式进行设计,WeatherData对象即观察者模式中的主题对象,三个布告板即观察者。
二、类图
三、实现代码
1.抽象主题Subject
- public interface Subject {
- public void registerObserver(Observer o);
- public void removeObserver(Observer o);
- public void notifyObservers();
- }
2.抽象观察者Obsever
- public interface Observer {
- public void update(float temp, float humidity, float pressure);
- }
3.具体主题WeatherData
- public class WeatherData implements Subject {
- private ArrayList observers;
- //temperature、humidity、pressure就是WeatherData的“状态”
- private float temperature;
- private float humidity;
- private float pressure;
- public WeatherData() {
- observers = new ArrayList();
- }
- public void registerObserver(Observer o) {
- observers.add(o);
- }
- public void removeObserver(Observer o) {
- int i = observers.indexOf(o);
- if (i >= 0) {
- observers.remove(i);
- }
- }
- public void notifyObservers() {
- for (int i = 0; i < observers.size(); i++) {
- Observer observer = (Observer)observers.get(i);
- observer.update(temperature, humidity, pressure);
- }
- }
- public void measurementsChanged() {
- notifyObservers();
- }
- public void setMeasurements(float temperature, float humidity, float pressure) {
- this.temperature = temperature;
- this.humidity = humidity;
- this.pressure = pressure;
- measurementsChanged();
- }
- public float getTemperature() {
- return temperature;
- }
- public float getHumidity() {
- return humidity;
- }
- public float getPressure() {
- return pressure;
- }
- }
4.本系统的辅助接口DisplayElement
- public interface DisplayElement {
- public void display();
- }
5.具体观察者
(1)布告板StatisticsDisplay:显示最小、平均和最大的温度观测值
- public class StatisticsDisplay implements Observer, DisplayElement {
- private float maxTemp = 0.0f;
- private float minTemp = 200;
- private float tempSum= 0.0f;
- private int numReadings;//记录观测的次数以便计算平均温度值
- private Subject weatherData;
- public StatisticsDisplay(Subject weatherData) {
- this.weatherData = weatherData;
- weatherData.registerObserver(this);
- }
- //这属于“推”数据,本类只使用到了第一个参数
- public void update(float temp, float humidity, float pressure) {
- tempSum += temp;
- numReadings++;
- if (temp > maxTemp) {
- maxTemp = temp;
- }
- if (temp < minTemp) {
- minTemp = temp;
- }
- display();
- }
- public void display() {
- System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
- + "/" + maxTemp + "/" + minTemp);
- }
- }
(2)布告板CurrentConditionsDisplay:显示当前的温度、湿度
- public class CurrentConditionsDisplay implements Observer, DisplayElement {
- private float temperature;//温度
- private float humidity;//湿度
- private Subject weatherData;
- public CurrentConditionsDisplay(Subject weatherData) {
- this.weatherData = weatherData;
- weatherData.registerObserver(this);
- }
- //推数据,这里只使用到了前两个参数
- public void update(float temperature, float humidity, float pressure) {
- this.temperature = temperature;
- this.humidity = humidity;
- display();
- }
- public void display() {
- System.out.println("Current conditions: " + temperature
- + "F degrees and " + humidity + "% humidity");
- }
- }
(3)布告板ForecastDisplay:天气预报
- public class ForecastDisplay implements Observer, DisplayElement {
- private float currentPressure = 29.92f;
- private float lastPressure;
- private Subject weatherData;
- public ForecastDisplay(Subject weatherData) {
- this.weatherData = weatherData;
- weatherData.registerObserver(this);
- }
- public void update(float temp, float humidity, float pressure) {
- lastPressure = currentPressure;
- currentPressure = pressure;
- display();
- }
- public void display() {
- System.out.print("Forecast: ");
- if (currentPressure > lastPressure) {
- System.out.println("Improving weather on the way!");
- } else if (currentPressure == lastPressure) {
- System.out.println("More of the same");
- } else if (currentPressure < lastPressure) {
- System.out.println("Watch out for cooler, rainy weather");
- }
- }
- }
(4)增加新的布告板,增加一个“酷热指数”布告板HeatIndexDisplay
- public class HeatIndexDisplay implements Observer, DisplayElement {
- float heatIndex = 0.0f;
- private Subject weatherData;
- public HeatIndexDisplay(Subject weatherData) {
- this.weatherData = weatherData;
- weatherData.registerObserver(this);
- }
- public void update(float t, float rh, float pressure) {
- heatIndex = computeHeatIndex(t, rh);
- display();
- }
- private float computeHeatIndex(float t, float rh) {
- float index = (float)((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 index;
- }
- public void display() {
- System.out.println("Heat index is " + heatIndex);
- }
- }
6.测试
- public class WeatherStationHeatIndex {
- public static void main(String[] args) {
- WeatherData weatherData = new WeatherData();
- CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
- StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
- ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);
- HeatIndexDisplay heatIndexDisplay = new HeatIndexDisplay(weatherData);
- weatherData.setMeasurements(80, 65, 30.4f);
- weatherData.setMeasurements(82, 70, 29.2f);
- weatherData.setMeasurements(78, 90, 29.2f);
- }
- }
四、说明
1.通过上面的update()方法,我们可以看出主题是通过“推”数据将数据传给观察者的。
2.通过以上代码可以看出,每个布告板(即具体观察者)中都有一个Subject类型的引用,用于指向其所注册的具体主题。在以上的需求中,我们可能会发现保存的这个Subject类型的引用没有使用到,因为直接通过update(float temp, float humidity, float pressure)方法就将WeatherData的状态数据传送给了观察者。然而这个引用并不是没有用处的,比如我们以后产生了新的需求,让观察者自行解除对主题的注册或使用主题的方法时,这时这个引用就派上了用场。有时候我们不得不将引用的类型由Subject改为ConcreteSubject(具体主题的类型),因为我们使用到的可能是具体主题所独有的方法。但是不管这个引用是Subject类型的还是ConcreteSubject(如WeatherData)类型的,这并不是观察者模式所关心的,《设计模式》中的这个引用的类型就是ConcreteSubject的,但是能使用Subject就尽量使用Subject而不要使用ConcreteSubject,比如本例就是将该引用的类型声明成Subject的,毕竟这样会使主题和观察者的耦合度更低(观察者可以注册到实现Subject接口的所有具体主题)。
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