观察者模式

观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态上发生变化时，会通知所有观察者对象，使它们能够自动更新自己。Android中我们常用到的监听事件也属于观察者模式，以及RX系列都是这种模式。分为推模型和拉模型，推模型是以参数的方式将更新的部分数据或全部数据传递给观察者，一旦观察者需要的数据有变动，观察者和被观察者都要去改方法，不易于维护拓展；而拉模型是将被观察者本身提供查询数据的方法，更新时将自身传递给观察者，观察者按需要去取数据，比较灵活和便于拓展。

示例如下：
推模型

public class Observer {
    public static void main(String[] a){
        Observer observer=new Observer();
        Product product=observer.new Product("NIKE AIR",4000);
        IPurchaser 张三 = observer.new NIKEFans("张三");
        IPurchaser 李四 = observer.new NIKEFans("李四");
        IPurchaser 王五 = observer.new NIKEFans("王五");
        IPurchaser 麻子 = observer.new NIKEFans("麻子");
        IStore Nike = observer.new NIKE();
        Nike.registerPurchaser(张三);
        Nike.registerPurchaser(李四);
        Nike.registerPurchaser(王五);
        Nike.registerPurchaser(麻子);
        Nike.ArrivalNewProduct(product);
    }

    class Product{
        private String name;
        private double price;

        public Product(String name, double price) {
            this.name = name;
            this.price = price;
        }

        @Override
        public String toString() {
            return "Product{" +
                    "name='" + name + '\'' +
                    ", price=" + price +
                    '}';
        }
    }

    interface IStore{
        void registerPurchaser(IPurchaser iPurchaser);
        void unRegisterPurchaser(IPurchaser iPurchaser);
        void ArrivalNewProduct(Product product);
        void notifyNewProduct(Product product);
    }

    class NIKE implements IStore{
        private List<Product> products=new ArrayList<>();
        private List<IPurchaser> purchasers=new ArrayList<>();
        @Override
        public void ArrivalNewProduct(Product product){
            products.add(product);
            notifyNewProduct(product);
        }
        @Override
        public void registerPurchaser(IPurchaser iPurchaser) {
            purchasers.add(iPurchaser);
        }

        @Override
        public void unRegisterPurchaser(IPurchaser iPurchaser) {
            purchasers.remove(iPurchaser);
        }

        @Override
        public void notifyNewProduct(Product product) {
            for(IPurchaser iPurchaser:purchasers){
                iPurchaser.newProductArrivals(product);
            }
        }
    }

    interface IPurchaser{
        void newProductArrivals(Product product);
    }

    class NIKEFans implements IPurchaser{
        private String name;

        public NIKEFans(String name) {
            this.name = name;
        }

        @Override
        public void newProductArrivals(Product product) {
            System.out.println(name+"新货到了，快去买"+product.toString());
        }
    }
}

拉模型

public class Observer {
    public static void main(String[] a){
        Observer observer=new Observer();
        Product product=observer.new Product("NIKE AIR",4000);
        IPurchaser 张三 = observer.new NIKEFans("张三");
        IPurchaser 李四 = observer.new NIKEFans("李四");
        IPurchaser 王五 = observer.new NIKEFans("王五");
        IPurchaser 麻子 = observer.new NIKEFans("麻子");
        IStore Nike = observer.new NIKE();
        Nike.registerPurchaser(张三);
        Nike.registerPurchaser(李四);
        Nike.registerPurchaser(王五);
        Nike.registerPurchaser(麻子);
        Nike.ArrivalNewProduct(product);
    }

    class Product{
        private String name;
        private double price;

        public Product(String name, double price) {
            this.name = name;
            this.price = price;
        }

        @Override
        public String toString() {
            return "Product{" +
                    "name='" + name + '\'' +
                    ", price=" + price +
                    '}';
        }
    }

    interface IStore{
        void registerPurchaser(IPurchaser iPurchaser);
        void unRegisterPurchaser(IPurchaser iPurchaser);
        void ArrivalNewProduct(Product product);
        void notifyNewProduct();
    }

    class NIKE implements IStore{
        private Product newProduct;
        private List<IPurchaser> purchasers=new ArrayList<>();

        public Product getNewProduct() {
            return newProduct;
        }

        @Override
        public void ArrivalNewProduct(Product product){
            newProduct=product;
            notifyNewProduct();
        }
        @Override
        public void registerPurchaser(IPurchaser iPurchaser) {
            purchasers.add(iPurchaser);
        }

        @Override
        public void unRegisterPurchaser(IPurchaser iPurchaser) {
            purchasers.remove(iPurchaser);
        }

        @Override
        public void notifyNewProduct() {
            for(IPurchaser iPurchaser:purchasers){
                iPurchaser.newProductArrivals(this);
            }
        }
    }

    interface IPurchaser{
        void newProductArrivals(IStore iStore);
    }

    class NIKEFans implements IPurchaser{
        private String name;

        public NIKEFans(String name) {
            this.name = name;
        }

        @Override
        public void newProductArrivals(IStore iStore) {
            System.out.println(name+"新货到了，快去买"+((NIKE)iStore).getNewProduct().toString());
        }
    }
}