观察者模式

当一个对象状态发生改变时，依赖它的对象全部会受到通知，并自动更新

观察者：能够更便利地创建查看目标对象状态的对象，并且提供与核心对象非耦合的指定功能性

在创建其核心功能可能包含可观察状态变化的对象时候，最佳的做法是基于观察者设计模式创建于目标对象进行交互的其他类

常见的观察者设计模式示例有：插件系统，RSS源缓存器构建

被观察者： 基础CD类

<?php 
/**
 *被观察者： 基础CD类
 */
class CD {

    public $title = "";
    public $band  = "";
    protected $_observers = array();   // 观察者对象数组

    public function __construct($title, $band) {
        $this->title = $title;
        $this->band  = $band;
    }

    public function attachObserver($type, $observer) {
        $this->_observers[$type][] = $observer;
    }

    public function notifyObserver($type) {
        if (isset($this->_observers[$type])) {
            foreach ($this->_observers[$type] as $observer) {
                $observer->update($this);
            }
        }
    }

    public function buy() {
        $this->notifyObserver("purchased");
    }
}

观察者类

<?php
//观察者类 后台处理
class buyCDNotifyStreamObserver {
    public function update(CD $cd) {
        $activity  = "The CD named {$cd->title} by ";
        $activity .= "{$cd->band} was just purchased.";
        activityStream::addNewItem($activity);
    }
}

消息同事类

<?php
//消息同事类 前台输出
class activityStream {
    static public function addNewItem($item) {
        print $item;
    }
}

测试实例

<?php
//测试实例
$title = "Waste of a Rib";
$band  = "Never Again";

$cd    = new CD($title, $band);
$observer = new buyCDNotifyStreamObserver();

$cd->attachObserver("purchased", $observer);
$cd->buy();