【设计模式-观察者模式】

definition: One-to-many dependency O objects. when one object changes states, all of its dependents are notified and updated automatically.
观察者模式是基于对象的状态变化和观察者的通讯，以便他们作出相应的操作。简单的例子就是一个天气系统，当天气变化时必须在展示给公众的视图中进行反映。这个视图对象是一个主体，而不同的视图是观察者。

被观察者对象

public class AppleStore {
    private List customers=new ArrayList<>();
    private int state;
    public void setState(int state){
        this.state=state;
        notifyCustomers(state);
    }
   public void addCustomer(Customer customer){
        customers.add(customer);
   }
    public void notifyCustomers(int state){
        for (Customer customer : customers) {
            customer.receiveNotice(state);
        }
    }
}

观察者

public class Customer {
    private AppleStore store;
    public void receiveNotice(int state){
        System.out.println("收到通知"+state);
    }
    //初始化时订阅商店消息
    public  Customer(AppleStore store){
        this.store=store;
        this.store.addCustomer(this);
    }
}

测试类

public class ObserverPatternTest {
    public static void main(String[] args) {
        AppleStore appleStore=new AppleStore();
        Customer customer1=new Customer(appleStore);
        Customer customer2=new Customer(appleStore);
        Customer customer3=new Customer(appleStore);
        appleStore.setState(1);
    }
}