设计模式－观察者模式(Java)

观察者模式

当对象间存在一对多关系时，则使用观察者模式（Observer Pattern）
比如，当一个对象被修改时，则会自动通知它的依赖对象，观察者模式属于行为型模式。

使用

在java中实现观察者模式需要用到java.util包中提供的Observable类和Observer接口，java已经给我们提供好类使用。Observable可以查看java源码，下面是Observer接口：

/**
 * {@code Observer} is the interface to be implemented by objects that
 * receive notification of updates on an {@code Observable} object.
 *
 * @see Observable
 */
public interface Observer {

    /**
     * This method is called if the specified {@code Observable} object's
     * {@code notifyObservers} method is called (because the {@code Observable}
     * object has been updated.
     *
     * @param observable
     *            the {@link Observable} object.
     * @param data
     *            the data passed to {@link Observable#notifyObservers(Object)}.
     */
    void update(Observable observable, Object data);
}

实例

古董被观察，观察者为people用来观察古董价钱波动

• 被观察者需要继承Observable
• 维护内部数据即可

public class Antique extends Observable {
    private float mPrice;// 价钱

    public Antique(float price) {
        this.mPrice = price;
    }

    public float getPrice() {
        return this.mPrice;
    }

    public void setPrice(float price) {
        super.setChanged();
        super.notifyObservers(price);// 价格被改变
        this.mPrice = price;
    }

    public String toString() {
        return "古董价格为：" + this.mPrice;
    }
}

观察者实现Observer接口，重写update方法即可

public class People implements Observer{
    
    private String name;
    
    public People(String name) {
        this.name = name;
    }
    
    @Override
    public void update(Observable observable, Object data) {
        Log.e("","People update() -> update name:"+ this.name + ",price:"+ ((Float)data).floatValue());
    }

}

主函数调用

 Antique house = new Antique(1222f);
 People p1 = new People("p1");
 People p2 = new People("p2");
 People p3 = new People("p3");
 house.addObserver(p1);
 house.addObserver(p2);
 house.addObserver(p3);
 Log.e("", house+""); // 输出价格
 house.setPrice(111f);
 Log.e("", house+""); // 输出价格

参考

• 设计模式
• Java常用类库--观察者设计模式(Observable类Observer接口)