01 定义

观察者模式：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收到通知并自动更新。

02 情景

观察者A和观察者B都订阅了天气预报（被观察者），当天气预报发生变化时，观察者A和观察者B都能收到通知。

03 UML

Observer Pattern.png

04 Interface

// 被观察者
public interface Observable {
    void addObserver(Observer o);
    void deleteObserver(Observer o);
    void notifyObservers();
}

// 观察者
public interface Observer {
    // 更新天气
    void update(int temperature);
}

05 Class

// 天气预报（被观察者）
public class WeatherObservable implements Observable {
    private ArrayList<Observer> observers;
    private int temperature;

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

    public void SetTemperature(int temperature){
        this.temperature = temperature;
        temperatureChanged();
    }
    private void temperatureChanged(){
        notifyObservers();
    }
    @Override
    public void addObserver(Observer o) {
        if(o != null && !observers.contains(o)){
            observers.add(o);
        }
    }
    @Override
    public void deleteObserver(Observer o) {
        observers.remove(o);
    }
    @Override
    public void notifyObservers() {
        for(Observer observer : observers){
            observer.update(temperature);
        }
    }
}

// 观察者A
public class ObserverA implements Observer {
    public void subscribe(Observable observable){
        observable.addObserver(this);
    }
    public void unSubscribe(Observable observable){
        observable.deleteObserver(this);
    }
    @Override
    public void update(int temperature) {
        display(temperature);
    }
    public void display(int temperature){
        System.out.println("This is from ObserverA : " + temperature);
    }
}

// 观察者B
public class ObserverB implements Observer {
    public void subscribe(Observable observable){
        observable.addObserver(this);
    }
    public void unSubscribe(Observable observable){
        observable.deleteObserver(this);
    }
    @Override
    public void update(int temperature) {
        display(temperature);
    }
    public void display(int temperature){
        System.out.println("This is from ObserverB : " + temperature);
    }
}

06 测试

WeatherObservable weatherObservable = new WeatherObservable();

ObserverA observerA = new ObserverA();
observerA.subscribe(weatherObservable);
ObserverB observerB = new ObserverB();
observerB.subscribe(weatherObservable);

weatherObservable.SetTemperature(10);
// 取消订阅
observerA.unSubscribe(weatherObservable);
weatherObservable.SetTemperature(20);

TestResult.png