设计模式之观察者 | 郭旭升博客主页

意图

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

气象站和数据显示器 demo

public interface WeatherSubject {
    void registerObserver(WeatherObserver o);
    void removeObserver(WeatherObserver o);
    void notifyObserver();
}

public class WeatherData implements WeatherSubject {

    private List<WeatherObserver> 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;
// 状态更新之后通知所有观察者
        notifyObserver();
    }
    @Override
    public void registerObserver(WeatherObserver o) {
        observers.add(o);
    }

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

    @Override
    public void notifyObserver() {
    //更新所有观察者状态。
        for (WeatherObserver o : observers) {
            o.update(temperature, humidity, pressure);
        }
    }
}

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

public class StatisticsDisplay implements WeatherObserver {
// 构造函数执行时，将观察者注册到主题中去。
    public StatisticsDisplay(WeatherSubject weatherData) {
        weatherData.registerObserver(this);
    }

    @Override
    public void update(float temp, float humidity, float pressure) {
        System.out.println("StatisticsDisplay.update " + temp + " " + humidity + " " + pressure);
    }
}

public class CurrentConditionDisplay implements WeatherObserver {
// 构造函数执行时，将观察者注册到主题中去。
    public CurrentConditionDisplay(WeatherSubject weatherData) {
        weatherData.registerObserver(this);
    }

    @Override
    public void update(float temp, float humidity, float pressure) {
        System.out.println("CurrentConditionDisplay.update " + temp + " " + humidity + " " + pressure);
    }
}

public class WeatherStation {
// 气象站数据发生改变时，显示器收到通知自动更新状态。
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        CurrentConditionDisplay currentConditionDisplay = new CurrentConditionDisplay(weatherData);
        StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);

        weatherData.setMeasurements(0, 0, 1);
        weatherData.setMeasurements(1, 1, 2);
    }
}