观察者模式

UML
Java
CLOS

当一个对象变化时，其它依赖该对象的对象都会收到通知，并且随着变化，对象之间是一种一对多的关系!

UML

Java

定义观察者接口Observer，当被观察的对象发生变化时，会调用这个接口的update方法：
```
public interface Observer {
    void update();
}
```

实现2个具体的观察者类：

public class Observer1 implements Observer {
    @Override
    public void update() {
        System.out.println("observer1 has received!");
    }
}

public class Observer1 implements Observer {
    @Override
    public void update() {
        System.out.println("observer1 has received!");
    }
}

定义被观察者的接口Subject：

public interface Subject {
    /*增加观察者*/
    public void add(Observer observer);

    /*删除观察者*/
    public void del(Observer observer);

    /*通知所有的观察者*/
    public void notifyObservers();

    /*自身的操作*/
    public void operation();
}

实现对观察者Observer的基本操作：

public abstract class AbstractSubject implements Subject {
    private Vector<Observer> observers = new Vector<>();

    @Override
    public void add(Observer observer) {
        observers.add(observer);
    }

    @Override
    public void del(Observer observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers() {
        for (Observer observer : observers)
            observer.update();
    }
}

实现被观察者真正的业务逻辑：

public class MySubject extends AbstractSubject {
    @Override
    public void operation() {
        System.out.println("update self!");
        notifyObservers();
    }
}

测试类：

public class ObserverTest {
    public static void main(String[] args) {
        Subject sub = new MySubject();
        sub.add(new Observer1());
        sub.add(new Observer2());

        sub.operation();
    }
}

测试结果：

update self!
observer1 has received!
observer2 has received!

CLOS

定义观察者的通用方法：

(defclass observer () ())
(defclass observer1 (observer) ())
(defclass observer2 (observer) ())

(defmethod update ((o observer1))
  (format t "observer1 has received! ~C" #\linefeed))
(defmethod update ((o observer2))
  (format t "observer2 has received! ~C" #\linefeed))

使用辅助方法:after实现更新：

(defclass subject ()
  ((observers :accessor subject-observers
              :initarg :observers
              :initform '())))

(defmethod operation ((s subject))
  (format t "update self! ~C" #\linefeed))
;;依次调用观察者列表中每个元素的update方法
(defmethod operation :after ((s subject))
  (mapc #'update (subject-observers s)))

测试：

(setf s (make-instance 'subject
                       :observers (list (make-instance 'observer1) (make-instance 'observer2))))

(operation s)
;; => update self! 
;;    observer1 has received! 
;;    observer2 has received!

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