1. 应用场景

某个实例的变化将影响其他多个对象。观察者模式多用于实现订阅功能的场景，例如微博的订阅，当我们订阅了某个人的微博账号，当这个人发布了新的消息，就会通知我们。

2.概念

定义对象之间的一对多依赖，当一个对象状态改变时，它的所有依赖都会收到通知并且自动更新状态。主题（Subject）是被观察的对象，而其所有依赖者（Observer）称为观察者。

Java设计模式(二)：观察者设计模式

3. Class Diagram

主题（Subject）具有注册和移除观察者、并通知所有观察者的功能，主题是通过维护一张观察者列表来实现这些操作的。

观察者（Observer）的注册功能需要调用主题的 registerObserver() 方法。

Java设计模式(二)：观察者设计模式

4. Implementation

天气数据布告板会在天气信息发生改变时更新其内容，布告板有多个，并且在将来会继续增加。

Java设计模式(二)：观察者设计模式

public interface Subject {
    void registerObserver(Observer o);

    void removeObserver(Observer o);

    void notifyObserver();
}

public class WeatherData implements Subject {
    private List<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;

    public WeatherData() {
        observers = new ArrayList<>();
    }

    public void setMeasurements(float temperature, float humidity, float pressure) {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        notifyObserver();
    }

    @Override
    public void registerObserver(Observer o) {
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) {
        int i = observers.indexOf(o);
        if (i >= 0) {
            observers.remove(i);
        }
    }

    @Override
    public void notifyObserver() {
        for (Observer o : observers) {
            o.update(temperature, humidity, pressure);
        }
    }
}

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

public interface DisplayElement {
    public void display();
}

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 CurrentConditaionsDisplay implements Observer,DisplayElement {
    private WeatherData weatherData;
    private float temperature;
    private float humidity;

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

    @Override
    public void display() {
        System.out.println("temperature: "+temperature+" humidity: "+ humidity);
    }

    @Override
    public void update(float temp, float humidity, float preesure) {
        this.humidity=humidity;
        this.temperature=temp;
        display();
    }
}

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");
		}
	}
}

public class WeatherStation {
    public static void main(String[] args) {
        WeatherData weatherData=new WeatherData();
        CurrentConditaionsDisplay conditaionsDisplay=new CurrentConditaionsDisplay(weatherData);
        StatisticsDisplay statisticsDisplay=new StatisticsDisplay(weatherData);
        ForecastDisplay forecastDisplay=new ForecastDisplay(weatherData);
        weatherData.setMeasurements(80,65,30.4f);
        weatherData.setMeasurements(82,75,35.4f);
        weatherData.setMeasurements(82,75,35.4f);
    }
}

运行结果：

temperature: 80.0 humidity: 65.0
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Improving weather on the way!
temperature: 82.0 humidity: 75.0
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Improving weather on the way!
temperature: 82.0 humidity: 75.0
Avg/Max/Min temperature = 81.333336/82.0/80.0
Forecast: More of the same

#5. JDK

Java设计模式(二)：观察者设计模式

1. 应用场景

2.概念

3. Class Diagram

4. Implementation

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