2.7.观察者模式

观察者模式又叫发布-订阅(Publish/Subscribe)模式、模型-视图(Model/View)模式、源-监听器(Source/Listener)模式或从属者(Dependents)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。

JDK中的使用

java.util.Observer
java.util.Observable

代码实现

水果店员没卖出一个订单的水果老板会关心每种水果卖出了多少老板娘会关心一共卖出了多少钱

@Getter
public enum Fruit {
    APPLE("苹果", new BigDecimal("2")),
    ORANGE("桔子", new BigDecimal("1.5")),
    GRAPE("葡萄", new BigDecimal("3")),
    PLUM("李子", new BigDecimal("6.5"));

    private String name;
    private BigDecimal unitPrice;
    Fruit(String name, BigDecimal unitPrice){
        this.name = name;
        this.unitPrice = unitPrice;
    }
}
@Data
@AllArgsConstructor
public class Order {
    private Fruit type;
    private BigDecimal weight;
}
public class FruitStoreAssistant extends Observable {
    public FruitStoreAssistant(Observer ...observers){
        super();
        if(observers != null && observers.length > 0){
            for(Observer observer : observers){
                super.addObserver(observer);
            }
        }
    }

    /**
     * 贩卖水果
     * @param order 水果订单
     */
    public void sell(Order order){
        super.setChanged();
        super.notifyObservers(order);
    }
}
@Getter
public class FruitsStoreBoss implements Observer {
    private Map<Fruit, BigDecimal> statistics = new HashMap<>();
    private BigDecimal total = new BigDecimal("0");

    public FruitsStoreBoss(){
        Fruit []fruits = Fruit.values();
        for(Fruit fruit : fruits){
            statistics.put(fruit, new BigDecimal("0"));
        }
    }

    @Override
    public void update(Observable o, Object arg) {
        if(arg instanceof Order){
            this.account((Order) arg);
        }
    }

    private void account(Order order){
        Fruit fruit = order.getType();
        BigDecimal current = this.statistics.get(fruit);
        this.statistics.put(fruit, current.add(order.getWeight()));
        this.total = this.total.add(order.getWeight());
    }
}
@Getter
public class FruitsStoreLandlady implements Observer {
    private Map<Fruit, BigDecimal> statistics = new HashMap<>();
    private BigDecimal total = new BigDecimal("0");

    public FruitsStoreLandlady(){
        Fruit []fruits = Fruit.values();
        for(Fruit fruit : fruits){
            statistics.put(fruit, new BigDecimal("0"));
        }
    }

    @Override
    public void update(Observable o, Object arg) {
        if(arg instanceof Order){
            this.account((Order) arg);
        }
    }

    private void account(Order order){
        Fruit fruit = order.getType();
        BigDecimal current = this.statistics.get(fruit);
        BigDecimal orderTotal = fruit.getUnitPrice().multiply(order.getWeight());
        this.statistics.put(fruit, current.add(orderTotal));
        this.total = this.total.add(orderTotal);
    }
}

单元测试

class FruitStoreSpec extends Specification {
    def fruitStoreEach() {
        given:
        def boss = new FruitsStoreBoss()
        def landlady = new FruitsStoreLandlady()
        def assistant = new FruitStoreAssistant(boss, landlady)

        when:
        assistant.sell(new Order(fruit, weight))
        then:
        landlady.getTotal() == totalAmount
        boss.getTotal() == totalWeight

        where:
        fruit        | weight || totalAmount                  || totalWeight
        Fruit.PLUM   | 8.7    || Fruit.PLUM.unitPrice * 8.7   || 8.7
        Fruit.APPLE  | 1      || Fruit.APPLE.unitPrice * 1    || 1
        Fruit.GRAPE  | 2.3    || Fruit.GRAPE.unitPrice * 2.3  || 2.3
        Fruit.ORANGE | 2.5    || Fruit.ORANGE.unitPrice * 2.5 || 2.5
    }
}