Observer Pattern

发表于 | 分类于
Observer Pattern

Observer Pattern

  1. 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，让他们能够自动更新自己。
  2. 组成：
  • 抽象主题角色：把所有对观察者对象的引用保存在一个集合中，每个抽象主题角色都可以有任意数量的观察者。抽象主题提供一个接口，可以增加和删除观察者角色。
  • 抽象观察者角色：为所有具体的观察者定义一个接口，在得到主题的通知时更新自己。
  • 具体主题角色：在具体主题内部状态改变时，给所有登记过的观察者发出通知。
  • 具体观察者角色：该角色实现抽象观察者橘色所要求的更新接口，以便使本身的状态与主题的状态相协调。如果需要，具体观察者角色可以保存一个指向具体主题角色的引用。
1
2
3
4
5
6
7
8
9
10
11
/**
 * 抽象的主题
 */
public interface Watched {
    public void addWatcher(Watcher watcher);
    public void removeWatcher(Watcher watcher);
    public void notifyWatchers(String str);
}
1
2
3
4
5
6
7
8
/**
 * 抽象观察者
 */
public interface Watcher {
    void update(String str);
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
/**
 * 具体主题
 */
public class ConcreteWatched implements Watched {
    private List<Watcher> watchers = new ArrayList<>();
    @Override
    public void addWatcher(Watcher watcher) {
        watchers.add(watcher);
    }
    @Override
    public void removeWatcher(Watcher watcher) {
        watchers.remove(watcher);
    }
    @Override
    public void notifyWatchers(String str) {
        for (Watcher watcher : this.watchers) {
            watcher.update(str);
        }
    }
}
1
2
3
4
5
6
7
8
9
/**
 * 具体观察者
 */
public class ConcreteWatcher implements Watcher {
    @Override
    public void update(String str) {
        System.out.println(str);
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public class Test {
    public static void main(String[] args) {
        Watched girl = new ConcreteWatched();
        Watcher watcher = new ConcreteWatcher();
        Watcher watcher1 = new ConcreteWatcher();
        Watcher watcher2 = new ConcreteWatcher();
        girl.addWatcher(watcher);
        girl.addWatcher(watcher1);
        girl.addWatcher(watcher2);
        girl.notifyWatchers("I am happy!");
        girl.removeWatcher(watcher1);
        girl.notifyWatchers("I am sad!");
    }
}

jdk对于观察者模式的实现

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
class BeingWatched extends Observable {
    void counter(int number) {
        for (; number >= 0; number--) {
            this.setChanged();
            this.notifyObservers(number);
        }
    }
}
class Watcher1 implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        System.out.println("Watcher1's count is " + arg);
    }
}
class Watcher2 implements Observer {
    @Override
    public void update(Observable o, Object arg) {
        if (((Integer) arg).intValue() <= 5) {
            System.out.println("Watcher2's count is " + arg);
        }
    }
}
public class TwoObservers {
    public static void main(String[] args) {
        Watcher1 watcher1 = new Watcher1();
        Watcher2 watcher2 = new Watcher2();
        BeingWatched beingWatched = new BeingWatched();
        beingWatched.addObserver(watcher1);
        beingWatched.addObserver(watcher2);
        beingWatched.counter(10);
    }
}