class Product

{

    private $observers = array();

    private $pid = 0;

    public function __construct($pid)

    {

        $this->pid = $pid;

    }

    // 注册观察者

    public function attachObserver($observer)

    {

        $this->observers[] = $observer;

    }

    public function buy()

    {

        //.....

        $this->notify();

    }

    // 通知观察者

    public function notify()

    {

        foreach ($this->observers as $obj) {

            $obj->receiveBuy($this);

        }

    }

    public function getPid()

    {

        return $this->pid;

    }

}

interface ProductObserver

{

    public function receiveBuy(Product $pro);

}

class Repertory implements ProductObserver

{

    public function receiveBuy(Product $pro)

    {

        echo $pro->getPid() . "被购买了,库存可以减去1了";

    }

}

// 调用示例

$product = new Product('s100');

$product->attachObserver(new Repertory());

$product->buy();

// 产品一旦被购买,我们就通知库存.