PHP 高级编程(3/5) - 使用SPL(标准PHP库)实现观察者模式

SPL（标准PHP库 - Standard PHP Library）是PHP5面向对象功能中重要的部分。原文解释是这样的“The Standard PHP Library (SPL) is a collection of interfaces and classes that are meant to solve common problems”。

SplSubject 和 SplObserver 接口

The SplSubject interface is used alongside SplObserver to implement the Observer Design Pattern.

观察者模式是一种简单的事件系统，包含了两个或更多的互相交互的类。这一模式允许某个类观察另一个类的状态，当被观察类的状态发生变化时，这个模式会得到通知。被观察的类叫subject，负责观察的类叫做Observer 。PHP 提供的 SplSubject 和 SplObserver接口可用来表达这些内容。

SplSubject {
/* 方法 */
abstract public void attach ( SplObserver $observer )
abstract public void detach ( SplObserver $observer )
abstract public void notify ( void )
}

SplObserver {
/* 方法 */
abstract public void update ( SplSubject $subject )
}

这里，splsubject类维护了一个特定状态，当这个状态发生变化时，他就会调用notify方法，所以之前使用attach注册的splobserver实例的update就会被调用。这里我们实现一个简单地观察者模式的例子

<?php
/**
 * Subject,that who makes news
 */
class Newspaper implements \SplSubject{
    private $name;
    private $observers = array();
    private $content;

    public function __construct($name) {
        $this->name = $name;
    }

    //add observer
    public function attach(\SplObserver $observer) {
        $this->observers[] = $observer;
    }

    //remove observer
    public function detach(\SplObserver $observer) {

        $key = array_search($observer,$this->observers, true);
        if($key){
            unset($this->observers[$key]);
        }
    }

    //set breakouts news
    public function breakOutNews($content) {
        $this->content = $content;
        $this->notify();
    }

    public function getContent() {
        return $this->content." ({$this->name})";
    }

    //notify observers(or some of them)
    public function notify() {
        foreach ($this->observers as $value) {
            $value->update($this);
        }
    }
}

/**
 * Observer,that who recieves news
 */
class Reader implements SplObserver{
    private $name;

    public function __construct($name) {
        $this->name = $name;
    }

    public function update(\SplSubject $subject) {
        echo $this->name.' is reading breakout news <b>'.$subject->getContent().'</b><br>';
    }
}

$newspaper = new Newspaper('Newyork Times');

$allen = new Reader('Allen');
$jim = new Reader('Jim');
$linda = new Reader('Linda');

//add reader
$newspaper->attach($allen);
$newspaper->attach($jim);
$newspaper->attach($linda);

//remove reader
$newspaper->detach($linda);

//set break outs
$newspaper->breakOutNews('USA break down!');

//=====output======
//Allen is reading breakout news USA break down! (Newyork Times)
//Jim is reading breakout news USA break down! (Newyork Times)

上例中我们通过数组存储 observer对象，使用数组及可能会出现两个问题:

1、同一个observer可能会被加载多次，导致多次调用同一个对象的update方法。

2、detach中需要通过迭代或者搜索数组来找到要删除的observer对象，导致运行效率降低。

SplObjectStorage类

PHP5提供了SplObjectStorage类，在这里我们可以用来存储observer对象，SplObjectStoraged的实例就像一个数组，但是他所存放的对象是唯一的。SplObjectStorage还可以使用detach直接从集合中删除指定的对象而不用遍历或搜索整个集合。下面看一个SplObjectStorage的例子：

<?php

 $ObjectStorage = new SplObjectStorage();

 class classa
 {
     #code...
 }

 class classb
 {
    #code...
 }

$a = new classa();
$b = new classb();

$ObjectStorage->attach($a);
$ObjectStorage->attach($b);
$ObjectStorage->attach($a);

foreach ($ObjectStorage as $key => $item) {
    echo ($key+).'、'.(new ReflectionClass($item))->getName()."\n";
}

//output

1、classa
  2、classb
  [Finished in 0.1s]

?>

上例中我们可以看到，在ObjectStorage这个集合中只有1个calssa，尽管我们添加了两次。并且冲集合中删除一个元素也变得极为简单，拿上面的代码来说，只需使用$ObjectStorage->attach($a);即可轻松的将$a从集合中移除。

结合 SplObjectStorage 我们再来修改最上面那个观察者模式的例子

<?php
/**
 * Subject,that who makes news
 */
class Newspaper implements \SplSubject{
    private $name;
    private $observers;
    private $content;

    public function __construct($name) {
        $this->name = $name;
        $this->observers = new SplObjectStorage();
    }

    //add observer
    public function attach(\SplObserver $observer) {
        $this->observers -> attach($observer);
    }

    //remove observer
    public function detach(\SplObserver $observer) {
        $this->observers -> detach($observer);
    }

    //set breakouts news
    public function breakOutNews($content) {
        $this->content = $content;
        $this->notify();
    }

    public function getContent() {
        return $this->content." ({$this->name})";
    }

    //notify observers(or some of them)
    public function notify() {
        foreach ($this->observers as $value) {
            $value->update($this);
        }
    }
}

/**
 * Observer,that who recieves news
 */
class Reader implements SplObserver{
    private $name;

    public function __construct($name) {
        $this->name = $name;
    }

    public function update(\SplSubject $subject) {
        echo $this->name.' is reading breakout news '.$subject->getContent()."\n";
    }
}

$newspaper = new Newspaper('Newyork Times');

$allen = new Reader('Allen');
$jim = new Reader('Jim');
$linda = new Reader('Linda');

//add reader
$newspaper->attach($allen);
$newspaper->attach($jim);
$newspaper->attach($linda);

//remove reader
$newspaper->detach($linda);

//set break outs
$newspaper->breakOutNews('USA break down!');

//=====output======
//Allen is reading breakout news USA break down! (Newyork Times)
//Jim is reading breakout news USA break down! (Newyork Times)