设计模式-观察者

定义：定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。

UML图示：

代码示例：就拿之前做过的项目举例，博主要实现餐厅打印订单的功能，如下图示。

通过上图很明显我们发现订单中心与打印机存在一对多的关系，根据上面对于观察者定义，采用观察者模式，可以让订单中心的状态改变的同时，让所有打印机都接收到状态的改变。下面我们再来看一张UML图。

定义Subject接口

public interface Subject {

 

    public void registerObserver(Observer obs);

    

    public void removeObserver(Observer obs);

    

    public void notifyObervers();

}

定义订单中心，实现Subject接口

public class OrderCenter implements Subject{

 

    private ArrayList<Observer> mObservers = null;

    

    private String order = null;

    

    public OrderCenter() {

        // TODO Auto-generated constructor stub

        mObservers = new ArrayList<Observer>();

    }

    

    @Override

    public void registerObserver(Observer obs) {

        // TODO Auto-generated method stub

        if(obs != null){

            mObservers.add(obs);

        }

    }

 

    @Override

    public void removeObserver(Observer obs) {

        // TODO Auto-generated method stub

        if(obs != null){

            mObservers.remove(obs);

        }

        

    }

 

    @Override

    public void notifyObervers() {

        // TODO Auto-generated method stub

        if(mObservers != null && mObservers.size() > 0){

            for(Observer obs : mObservers)

                obs.update();

        }

    }

    

    public void setOrder(String order){

        this.order = order;

        notifyObervers();

    }

    

    public String getOrder(){

        return order;

    }

 

}

 

 

定义打印接口

public interface Print {

 

    public void print();

}

定义观察者接口

public interface Observer {

 

    public void update();

    

}

前台打印机，实现打印，观察者接口

public class ReceptionPrint implements Print,Observer{

 

    private OrderCenter mOrderCenter = null;

    

    public ReceptionPrint(OrderCenter oc) {

        // TODO Auto-generated constructor stub

        mOrderCenter = oc;

    }

 

    @Override

    public void update() {

        // TODO Auto-generated method stub

        print();

    }

 

    @Override

    public void print() {

        // TODO Auto-generated method stub

        System.out.println("reception print order : " + mOrderCenter.getOrder());

    }

      

    

}

大堂打印机，实现打印，观察者接口

public class HallPrint implements Observer,Print{

 

    private OrderCenter mOrderCenter = null;

    

    public HallPrint(OrderCenter oc) {

        // TODO Auto-generated constructor stub

        mOrderCenter = oc;

    }

    @Override

    public void print() {

        // TODO Auto-generated method stub

        System.out.println("hall print order : " + mOrderCenter.getOrder());

    }

 

    @Override

    public void update() {

        // TODO Auto-generated method stub

        print();

    }

 

}

测试代码

public class Main {

 

    public static void main(String[] args) {

        OrderCenter oc = new OrderCenter();

        Observer hallObser = new HallPrint(oc);

        Observer receptionObser = new ReceptionPrint(oc);

        oc.registerObserver(hallObser);

        oc.registerObserver(receptionObser);

        oc.setOrder("take out order!");

        oc.removeObserver(receptionObser);

        oc.setOrder("hall order");

    }

}

 

测试结果

hall print order : take out order!

reception print order : take out order!

hall print order : hall order

上面我们自己定义了观察者和主题的接口，其实在JDK类包java.util包含最基础的Observer接口和Observable类，和我们定义的接口几乎一样，你甚至可以通过notifyObservers()或notifyObservers(Object arg)方法，选择pull或者push的方式让观察者获取数据。注意调研notifyObservers前先调用setChanged()方法。