设计模式 - 行为型 - 观察者模式

介绍

意图：定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。

主要解决：一个对象状态改变给其他对象通知的问题，而且要考虑到易用和低耦合，保证高度的协作。

何时使用：一个对象（目标对象）的状态发生改变，所有的依赖对象（观察者对象）都将得到通知，进行广播通知。

如何解决：使用面向对象技术，可以将这种依赖关系弱化。

关键代码：在抽象类里有一个 ArrayList 存放观察者们。

应用实例：订阅报纸

实现

public interface Subject {
	public void registerObserver(Observer o);
	public void removeObserver(Observer o);
	public void notifyObservers();
}

---

public class WeatherData implements Subject {
	private ArrayList<Observer> observers;
	private float temperature;
	private float humidity;
	private float pressure;
	
	public WeatherData() {
		observers = new ArrayList<Observer>();
	}
	
	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 (Observer observer : observers) {
			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;
	}
}

---

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

---

public interface DisplayElement {
	public void display();
}

---

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);
	}
	
	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");
	}
}

---

public class StatisticsDisplay implements Observer, DisplayElement {
	private float maxTemp = 0.0f;
	private float minTemp = 200;
	private float tempSum= 0.0f;
	private int numReadings;
	private WeatherData weatherData;

	public StatisticsDisplay(WeatherData weatherData) {
		this.weatherData = weatherData;
		weatherData.registerObserver(this);
	}

	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);
	}
}

---

public class ForecastDisplay implements Observer, DisplayElement {
	private float currentPressure = 29.92f;  
	private float lastPressure;
	private WeatherData weatherData;

	public ForecastDisplay(WeatherData 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");
		}
	}
}

---

运行类

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);
	}
}

运行结果

Current conditions: 80.0F degrees and 65.0% humidity
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Improving weather on the way!
--------
Current conditions: 82.0F degrees and 70.0% humidity
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Watch out for cooler, rainy weather
--------
Current conditions: 78.0F degrees and 90.0% humidity
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same

Java内置的观察者模式

java.util.Observer

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java.util.Observable

public class WeatherData extends Observable {
	private float temperature;
	private float humidity;
	private float pressure;
	
	public WeatherData() { }
	
	public void measurementsChanged() {
		//关键
		setChanged();
		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;
	}
}

---

public class CurrentConditionsDisplay implements Observer, DisplayElement {
	Observable observable;
	private float temperature;
	private float humidity;
	
	//关键
	public CurrentConditionsDisplay(Observable observable) {
		this.observable = observable;
		observable.addObserver(this);
	}
	
	//关键
	public void update(Observable obs, Object arg) {
		if (obs instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)obs;
			this.temperature = weatherData.getTemperature();
			this.humidity = weatherData.getHumidity();
			display();
		}
	}
	
	public void display() {
		System.out.println("Current conditions: " + temperature 
			+ "F degrees and " + humidity + "% humidity");
	}
}

---

更多源码

注意：java.util.Observer是一个类

参考及引用

1.《Head First 设计模式》Eric Freeman 等 著

2.观察者模式 | 菜鸟教程