观察者模式(Observer Pattern)（二）：HeadFirst中的气象站的实现

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);
}

可以看出，使用的是推数据的方式。

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;
	}
}

WeatherData中的temperature、humidity、pressure这三个属性就是WeatherData的“状态”（即我们在以前提到的State状态）

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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