每天学一点设计模式-观察者模式

观察者模式

英文名

Observer pattern

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，它的所有依赖者都会受到通知并自动更新。

设计原则

1.为了交互对象之间的松耦合设计而努力

理解

对象和观察者抽象为接口，对象接口中定义三个方法：添加观察者、删除观察者和通知观察者。观察者接口中只有一个方法，更新数据方法。实现对象接口的类中要有保存观察者的数组。继承与观察者接口的子类的构造方法的参数为对象。

代码

对象类（被观察者）

public interface Subject {
	
	public void registerObserver(Observer o);

	public void removeObserver(Observer o);

	public void notifyObservers();
}

观察者

public interface Observer {
	public void update(float temp, float humidity, float pressure);
}

表现行为接口

public interface DisplayElement {
	public void display();
}

对象的子类

public class WeatherData implements Subject {
	private ArrayList observers;
	private float temperature;
	private float humidity;
	private float pressure;

	public WeatherData() {
		// TODO Auto-generated constructor stub
		observers = new ArrayList();
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see
	 * com.sprd.pattern.observer.impl.Subject#registerObserver(com.sprd.pattern
	 * .observer.impl.Observer)
	 */
	@Override
	public void registerObserver(Observer o) {
		// TODO Auto-generated method stub
		observers.add(o);
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see
	 * com.sprd.pattern.observer.impl.Subject#removeObserver(com.sprd.pattern
	 * .observer.impl.Observer)
	 */
	@Override
	public void removeObserver(Observer o) {
		int i = observers.indexOf(o);
		if (i >= 0)
			observers.remove(o);
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see com.sprd.pattern.observer.impl.Subject#notifyObservers()
	 */
	@Override
	public void notifyObservers() {
		// TODO Auto-generated method stub
		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();
	}
}

3个观察者

public class CurrentConditionsDisplay implements Observer, DisplayElement {
	private float temperature;
	private float humidity;
	private Subject weatherData;

	public CurrentConditionsDisplay(Subject weatherData) {
		this.weatherData = weatherData;
		weatherData.registerObserver(this);
	}

	/*
	 * (non-Javadoc)
	 */
	@Override
	public void display() {
		System.out.println(String.format(
				"Current conditions : %f F degrees and %.0f %% humidity",
				temperature, humidity));
	}

	/*
	 * (non-Javadoc)
	 */
	@Override
	public void update(float temp, float humidity, float pressure) {
		this.temperature = temp;
		this.humidity = humidity;
		display();
	}

}

public class ForecastDisplay implements DisplayElement, Observer {
	private float temperature;
	private float humidity;
	private Subject weatherData;

	public ForecastDisplay(Subject weatherData) {
		this.weatherData = weatherData;
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see com.sprd.pattern.observer.impl.Observer#update(float, float, float)
	 */
	@Override
	public void update(float temp, float humidity, float pressure) {
		// TODO Auto-generated method stub
		this.temperature = temp;
		this.humidity = humidity;
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see com.sprd.pattern.observer.impl.DisplayElement#display()
	 */
	@Override
	public void display() {
		// TODO Auto-generated method stub

	}

}

public class StatisticsDisplay implements DisplayElement, Observer {
	
	public StatisticsDisplay(Subject weatherData){
		
	}

	/* (non-Javadoc)
	 * @see com.sprd.pattern.observer.impl.Observer#update(float, float, float)
	 */
	@Override
	public void update(float temp, float humidity, float pressure) {
		// TODO Auto-generated method stub

	}

	/* (non-Javadoc)
	 * @see com.sprd.pattern.observer.impl.DisplayElement#display()
	 */
	@Override
	public void display() {
		// TODO Auto-generated method stub

	}

}

测试程序

public class WeatherStation {

	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);
		weatherData.setMeasurements(80, 65, 30.4f);
		weatherData.setMeasurements(82, 70, 29.2f);
		weatherData.setMeasurements(78, 90, 29.2f);
		
	}

}

显示结果

Current conditions : 80.000000 F degrees and 65 % humidity
Current conditions : 82.000000 F degrees and 70 % humidity
Current conditions : 78.000000 F degrees and 90 % humidity