观察者模式的核心是把客户元素（观察者）从一个中心类（主体）中分离开来。当主体知道事件发生时，观察者需要被通知到。同时，我们并不希望将主体与观察者之间的关系进行硬编码。为了达到这个目的，我们可以允许观察者在主体上进行注册。
观察者模式实现了低耦合、非侵入式的通知与更新机制。
场景：当一个事件发生后，要执行一连串更新操作。
<span>EventGenerator</span>.php
<?<span>php
namespace Baobab;
</span><span>/*</span><span>*
* Class EventGenerator 基类，事件发生者
</span><span>*/</span>
<span>abstract</span> <span>class</span><span> EventGenerator{
</span><span>//</span><span>观察者对于事件发生者是不可见的，事件发生者不关心有哪些观察者关注了事件</span>
<span>private</span> <span>$observers</span> = <span>array</span><span>();
</span><span>/*</span><span>*
* 添加观察者
</span><span>*/</span>
<span>function</span> addObserver(Observer <span>$observer</span><span>){
</span><span>$this</span>->observers[] = <span>$observer</span><span>;
}
</span><span>/*</span><span>*
*逐个通知观察者进行更新的操作
</span><span>*/</span>
<span>function</span><span> notify(){
</span><span>foreach</span>(<span>$this</span>->observers <span>as</span> <span>$observer</span><span>){
</span><span>$observer</span>-><span>update();
}
}
}</span><span>Observer</span>.php
<?<span>php
namespace Baobab;
</span><span>/*</span><span>*
* Class Observer 观察者，观察的对象是事件发生者
* @package Baobab
</span><span>*/</span>
<span>interface</span><span> Observer{
</span><span>/*</span><span>*
* @param null $event_info 事件的信息
* 表示事件发生后进行更新的操作
</span><span>*/</span>
<span>function</span> update(<span>$event_info</span> = <span>null</span><span>);
}</span>index.php
<span>class</span> Event <span>extends</span><span> \Baobab\EventGenerator{
</span><span>/*</span><span>*
*触发一个新的事件
</span><span>*/</span>
<span>function</span><span> trigger(){
</span><span>echo</span> 'Event发生'<span>;
</span><span>$this</span>-><span>notify();
}
}
</span><span>/*</span><span>*
* 观察者1
</span><span>*/</span>
<span>class</span> Observer1 <span>implements</span><span> \Baobab\Observer{
</span><span>function</span> Update(<span>$event_info</span> = <span>null</span><span>){
</span><span>echo</span> '更新操作代码1'<span>;
}
}<br />
<span>/*<span>* <br /> * 观察者2 <br /><span> */</span></span></span>
</span><span>class</span> Observer2 <span>implements</span><span> \Baobab\Observer{
</span><span>function</span> Update(<span>$event_info</span> = <span>null</span><span>){
</span><span>echo</span> '更新操作代码2'<span>;
}
}
</span><span>$event</span> = <span>new</span><span> Event();
</span><span>$event</span>->addObserver(<span>new</span><span> Observer1());
</span><span>$event</span>->addObserver(<span>new</span><span> Observer2());
</span><span>$event</span>->trigger();
