设计模式学习－观察者模式

1.定义定义对象间一种一对多的依赖关系，使得当每一个对象改变状态，则所有依赖于它的对象都会得到通知并自动更新。2.类图

public void notifyObservers(Object arg) {
/*
* a temporary array buffer, used as a snapshot of the state of
* current Observers.
*/
Object[] arrLocal;

synchronized (this) {
/* We don't want the Observer doing callbacks into
* arbitrary code while holding its own Monitor.
* The code where we extract each Observable from
* the Vector and store the state of the Observer
* needs synchronization, but notifying observers
* does not (should not).  The worst result of any
* potential race-condition here is that:
* 1) a newly-added Observer will miss a
*   notification in progress
* 2) a recently unregistered Observer will be
*   wrongly notified when it doesn't care
*/

//看这里
if (!changed)
return;
arrLocal = obs.toArray();
clearChanged();
}

for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}

View Code6.思考思考如下两个问题性能问题　　如果注册的观察者较多，或者观察者处理能力弱、耗时长，那么很可能出现性能问题，毕竟只是简单的遍历调用。订阅/发布　　当然，对于订阅/发布模型的支持有更好的开源框架，各种mq实现了这种模型，并且是异步架构，性能也有保障。推或拉　　大家都喜欢说推模式和拉模式。其实本质上来说都是推模式，这也是使用观察者模式带来的好处。拉模式只不过推了一个引用，可以通过这个引用拿到更多的信息而已。7.参考

1.《JAVA与模式》之观察者模式