Observer pattern

观察者(Observer) : 又称 发布-订阅模式(publish-subscribe：Pub/Sub)：He is a notification mechanism,让发送通知的一方(被观察方) 和接收通知的一方(观察者) 能彼此分离,互不影响

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,所有依赖于他的对象都得到通知并自动更新

假设一个电商网站,有多种Product（商品）,同时,Customer（消费者）和Admin（管理员）对商品上架、价格改变都感兴趣,希望能第一时间获得通知.于是,Store（商场）可以这么写：
StoreHope to send notifications to those 关心 Product的人,And don't want to know who these people are,,Observer pattern is to separate the observed,和观察者之间的耦合关系
Implement an interface for everyone you want to observe,,在Store When modifying product information,and push to these observers

/** * 要观察 这个产品 需要实现的接口 */
public interface ProductObserver {
    
    void onPublished(Product product);

    void onPriceChange(Product product);
}

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Product {
    
    private String name;
    private double price;
}

public class Store {
    

    private List<ProductObserver> observers = new ArrayList<>();
    private Map<String,Product> map = new HashMap<>();

    // 注册观察者
    public void addObserver(ProductObserver productObserver){
    
        this.observers.add(productObserver);
    }

    // 取消注册
    public void removeObserver(ProductObserver productObserver){
    
        this.observers.remove(productObserver);
    }

    public void addNewProduct(String name,double price){
    
        Product product = new Product(name, price);
        map.put(name,product);
        // 通知观察者
        observers.forEach(observer->{
    observer.onPublished(product);});
    }

    public void setProductPrice(String name,double price){
    
        Product product = map.get(name);
        product.setPrice(price);
        // 通知观察者
        observers.forEach(observer->{
    observer.onPriceChange(product);});
    }
}

测试：

public class Main {
    
    public static void main(String[] args) {
    
        Store store = new Store();
        store.addObserver(new Observer01());
        store.addObserver(new Observer02());

        store.addNewProduct("gift",2d);
        store.setProductPrice("gift",3d);

    }

    static class Observer01 implements ProductObserver{
    
        @Override
        public void onPublished(Product product) {
    
            System.out.println("observer01 publish product "+product.getName());
        }

        @Override
        public void onPriceChange(Product product) {
    
            System.out.println("observer01 price change : "+product.getPrice());
        }
    }
   static class Observer02 implements ProductObserver{
    
        @Override
        public void onPublished(Product product) {
    
            System.out.println("observer02 publish product");
        }

        @Override
        public void onPriceChange(Product product) {
    
            System.out.println("observer02 price change");
        }
    }
}

引用：https://blog.csdn.net/u013087513/article/details/51839986
https://www.jianshu.com/p/2bb48cde23c9
https://www.liaoxuefeng.com/wiki/1252599548343744/1281319577321505