设计模式:灵活编程(观察者模式)

系统中的每个类应将重点放在某一个功能上,而不是其他方面。一个对象只做一件事情,并且将他做好。

定义对象间的一种一对多的依赖关系,当一个对象的状态发生改变时,有可能导致其它依赖对象的修改更新,那么开发任务会很快变成一个产生bug和消除bug的恶性循环。当我们创建一个对象的时候,一个对象的创建应当尽可能减少和其它对象间的耦合!一个对象的改变尽可能的不会引起代码库其它地方的修改。使用观察者模式能有效的解决此问题,一个对象(目标对象)的状态发生改变,所有的依赖对象(观察者对象)都将得到通知并被自动更新。

观察者模式(有时又被称为模型-视图(View)模式、源-收听者(Listener)模式或从属者模式)是软件设计模式的一种。在此种模式中,一个目标物件管理所有相依于它的观察者物件,并且在它本身的状态改变时主动发出通知。这通常通过呼叫各观察者所提供的方法来实现。此种模式通常被用来实现事件处理系统。

问题

假设一个负责处理用户登录的类:

class Login
{
    const LOGIN_USER_UNKNOWN = 1;
    const LOGIN_WRONG_PASS   = 2;
    const LOGIN_ACCESS       = 3;
    private $_status = array();

    public function handleLogin($user, $pass, $ip)
    {
        switch (rand(1,3))
        {
            case self::LOGIN_ACCESS:
                $this->setStatus(self::LOGIN_ACCESS, $user, $ip);
                $ret = true; break;
            case self::LOGIN_WRONG_PASS:
                $this->setStatus(self::LOGIN_WRONG_PASS, $user, $ip);
                $ret = false; break;
            case self::LOGIN_USER_UNKNOWN:
            default:
                $this->setStatus(self::LOGIN_USER_UNKNOWN, $user, $ip);
                $ret = false; break;
        }
        return $ret;
    }

    private function setStatus($status, $user, $ip)
    {
        $this->_status = array($status, $user, $ip);
    }

    public function getStatus()
    {
        return $this->_status;
    }
}

$login = new Login();
$login->handleLogin('BNDong', '123456', '127.0.0.1');
var_dump($login->getStatus());

当然这个类并没有实际功能, handleLogin 方法会存储验证用户数据,该方法有3个潜在的结果。状态标签会被设置为 LOGIN_USER_UNKNOWN 、 LOGIN_WRONG_PASS 或 LOGIN_ACCESS 。

现在看上去还可以,但是一个登录组件可不可能只有这面点东西,我们试着增加需求:(代码的腐败就是不断的迭代出来的)

记录登录IP地址

public function handleLogin($user, $pass, $ip)
{
    ...
    Logger::logIp($user, $ip, $this->getStatus());
    ...
}

登录失败发送邮件通知管理员

public function handleLogin($user, $pass, $ip)
{
    ...
    !$ret && Notifier::mailWarning($user, $ip, $this->getStatus());
    ...
}

当然这些都是简单的功能,但是依这种方式来处理 Login 类,会发现该类和系统的依赖越来越深,代码的扩展和复用性越来越差! handleLogin 处理的东西越来越多。

实现

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,我们允许观察者在主体上进行注册。

interface Observable
{
    public function attach(Observer $observer);
    public function detach(Observer $observer);
    public function notify();
}

class Login implements Observable
{
    const LOGIN_USER_UNKNOWN = 1;
    const LOGIN_WRONG_PASS   = 2;
    const LOGIN_ACCESS       = 3;
    private $_status = array();
    private $_observers;

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

    public function handleLogin($user, $pass, $ip)
    {
        switch (rand(1,3))
        {
            case self::LOGIN_ACCESS:
                $this->setStatus(self::LOGIN_ACCESS, $user, $ip);
                $ret = true; break;
            case self::LOGIN_WRONG_PASS:
                $this->setStatus(self::LOGIN_WRONG_PASS, $user, $ip);
                $ret = false; break;
            case self::LOGIN_USER_UNKNOWN:
            default:
                $this->setStatus(self::LOGIN_USER_UNKNOWN, $user, $ip);
                $ret = false; break;
        }
        $this->notify();
        return $ret;
    }

    private function setStatus($status, $user, $ip)
    {
        $this->_status = array($status, $user, $ip);
    }

    public function getStatus()
    {
        return $this->_status;
    }

    public function attach(Observer $observer)
    {
        $this->_observers[] = $observer;
    }

    public function detach(Observer $observer)
    {
        $newobservers = array();
        foreach ($this->_observers as $obs) {
            if ($obs !== $observer) {
                $newobservers[] = $obs;
            }
        }
        $this->_observers = $newobservers;
    }

    public function notify()
    {
        foreach ($this->_observers as $obs) {
            $obs->update($this);
        }
    }
}

现在 Login 类管理着一系列观察者对象。这些观察者可以由第三方通过 attach 方法添加进 Login 类,也可以通过 detach 方法来移除。 notify 方法用来告诉观察者一些相关事情发生了。 notify 方法会遍历观察者列表,调用每个观察者的 update 方法。

 Login 类在它的 handleLogin 方法中调用 notify 方法。然后定义 Observer 接口,任何实现这个接口的对象都可以通过 attach 方法加入 Login 类中。

interface Observer
{
    public function update(Observable $observable);
}

class SecurityMonitor implements Observer
{
    public function update(Observable $observable)
    {
        $status = $observable->getStatus();
        if ($status[0] == Login::LOGIN_WRONG_PASS) {
            // 发送邮件给系统管理员
            print __CLASS__.":发送邮件给系统给管理员<br>";
        }
    }
}

$login = new Login();
$login->attach(new SecurityMonitor());
$login->handleLogin('BNDong', '123456', '127.0.0.1');

至此实现了一个观察者模式,减少了各个对象之间的耦合。

优化

这里还存在一个问题,获取主体类状态是通过 getStatus 方法来获取的,而并不能判断调用的  getStatus 方法是存在并且可用的,所以要解决这个问题。

第一种方法:修改接口 Observer 中 update 方法参数 $observable 类型约束为 Login ,但是这样整个结构就被一个类限制了,多个登录类不能兼容,所以不推荐!!

第二种方法:在接口 Observable 中添加 getStatus 方法,但是这样会失去接口的通用性!!

第三种方法:继续保持 Observable 接口的通用性,将会添加 Observer 类型的对象来执行一些它们共有的任务。

下面针对第三种方法来优化上面的代码:

使用自建类优化

 创建一个抽象超类:

abstract class LoginObserver implements Observer
{
    private $_login;

    public function __construct(Login $login)
    {
        $this->_login = $login;
        $login->attach($this);
    }

    public function update(Observable $observable)
    {
        if ($observable == $this->_login) {
            $this->doUpdate($observable);
        }
    }

    abstract protected function doUpdate(Login $login);
}

  LoginObserver 类的构造函数需要一个 Login 对象作为参数。 LoginObserver 保存对 Login 对象的引用,并且调用 Login::attach() 方法。当 update 方法被调用时, LoginObserver 会检查参数传入的 Observable 对象是否是正确的引用,然后 LoginObserver 会调用模板方法 doUpdate 。现在可以创建一批 LoginObserver 对象,它们能够判断使用的是 Login 对象,而不是任意 Observable 对象:

class SecurityMonitor extends LoginObserver
{
    public function doUpdate(Login $login)
    {
        $status = $login->getStatus();
        if ($status[0] == Login::LOGIN_WRONG_PASS) {
            // 发送邮件给系统管理员
            print __CLASS__.":发送邮件给系统给管理员<br>";
        }
    }
}

class GeneralLogger extends LoginObserver
{
    public function doUpdate(Login $login)
    {
        $status = $login->getStatus();
        // 记录登录数据到日志
        print __CLASS__.":记录登录数据到日志<br>";
    }
}

$login = new Login();
new SecurityMonitor($login);
new GeneralLogger($login);
$login->handleLogin('BNDong', '123456', '127.0.0.1');

 因此在主体类和观察者之间创建了一个很灵活的关系。

使用PHP内置SPL优化

PHP通过内置的SPL(Standard PHP Library,PHP标准类)扩展提供了对观察者模式的原生支持。其中的观察者(Observer)由3个元素组成:SplObserver、SplSubject 和 SplObjectStorage。SplObserver 和 SplSubject 都是接口,与之前示例中的 Observer 和 Observable 接口完全相同。SplObjectStorage 是一个工具类,用于更好的存储对象和删除对象。

/**
 * The <b>SplSubject</b> interface is used alongside
 * <b>SplObserver</b> to implement the Observer Design Pattern.
 * @link http://php.net/manual/en/class.splsubject.php
 */
interface SplSubject  {

        /**
         * Attach an SplObserver
         * @link http://php.net/manual/en/splsubject.attach.php
         * @param SplObserver $observer <p>
     * The <b>SplObserver</b> to attach.
         * </p>
         * @return void 
         * @since 5.1.0
         */
        public function attach (SplObserver $observer);

        /**
         * Detach an observer
         * @link http://php.net/manual/en/splsubject.detach.php
         * @param SplObserver $observer <p>
     * The <b>SplObserver</b> to detach.
         * </p>
         * @return void 
         * @since 5.1.0
         */
        public function detach (SplObserver $observer);

        /**
         * Notify an observer
         * @link http://php.net/manual/en/splsubject.notify.php
         * @return void 
         * @since 5.1.0
         */
        public function notify ();

}
SplSubject
/**
 * The <b>SplObserver</b> interface is used alongside
 * <b>SplSubject</b> to implement the Observer Design Pattern.
 * @link http://php.net/manual/en/class.splobserver.php
 */
interface SplObserver  {

        /**
         * Receive update from subject
         * @link http://php.net/manual/en/splobserver.update.php
         * @param SplSubject $subject <p>
     * The <b>SplSubject</b> notifying the observer of an update.
         * </p>
         * @return void 
         * @since 5.1.0
         */
        public function update (SplSubject $subject);

}
SplObserver
/**
 * The SplObjectStorage class provides a map from objects to data or, by
 * ignoring data, an object set. This dual purpose can be useful in many
 * cases involving the need to uniquely identify objects.
 * @link http://php.net/manual/en/class.splobjectstorage.php
 */
class SplObjectStorage implements Countable, Iterator, Traversable, Serializable, ArrayAccess {

        /**
         * Adds an object in the storage
         * @link http://php.net/manual/en/splobjectstorage.attach.php
         * @param object $object <p>
         * The object to add.
         * </p>
         * @param mixed $data [optional] <p>
         * The data to associate with the object.
         * </p>
         * @return void 
         * @since 5.1.0
         */
        public function attach ($object, $data = null) {}

        /**
     * Removes an object from the storage
         * @link http://php.net/manual/en/splobjectstorage.detach.php
         * @param object $object <p>
         * The object to remove.
         * </p>
         * @return void 
         * @since 5.1.0
         */
        public function detach ($object) {}

        /**
         * Checks if the storage contains a specific object
         * @link http://php.net/manual/en/splobjectstorage.contains.php
         * @param object $object <p>
         * The object to look for.
         * </p>
     * @return bool true if the object is in the storage, false otherwise.
         * @since 5.1.0
         */
        public function contains ($object) {}

        /**
         * Adds all objects from another storage
         * @link http://php.net/manual/en/splobjectstorage.addall.php
         * @param SplObjectStorage $storage <p>
         * The storage you want to import.
         * </p>
         * @return void 
         * @since 5.3.0
         */
    public function addAll ($storage) {}

        /**
         * Removes objects contained in another storage from the current storage
         * @link http://php.net/manual/en/splobjectstorage.removeall.php
         * @param SplObjectStorage $storage <p>
         * The storage containing the elements to remove.
         * </p>
         * @return void 
         * @since 5.3.0
         */
    public function removeAll ($storage) {}

        /**
     * Removes all objects except for those contained in another storage from the current storage
     * @link http://php.net/manual/en/splobjectstorage.removeallexcept.php
     * @param SplObjectStorage $storage <p>
     * The storage containing the elements to retain in the current storage.
     * </p>
     * @return void
     * @since 5.3.6
     */
    public function removeAllExcept ($storage) {}

    /**
         * Returns the data associated with the current iterator entry
         * @link http://php.net/manual/en/splobjectstorage.getinfo.php
         * @return mixed The data associated with the current iterator position.
         * @since 5.3.0
         */
        public function getInfo () {}

        /**
         * Sets the data associated with the current iterator entry
         * @link http://php.net/manual/en/splobjectstorage.setinfo.php
         * @param mixed $data <p>
         * The data to associate with the current iterator entry.
         * </p>
         * @return void 
         * @since 5.3.0
         */
        public function setInfo ($data) {}

        /**
         * Returns the number of objects in the storage
         * @link http://php.net/manual/en/splobjectstorage.count.php
         * @return int The number of objects in the storage.
         * @since 5.1.0
         */
        public function count () {}

        /**
         * Rewind the iterator to the first storage element
         * @link http://php.net/manual/en/splobjectstorage.rewind.php
         * @return void 
         * @since 5.1.0
         */
        public function rewind () {}

        /**
         * Returns if the current iterator entry is valid
         * @link http://php.net/manual/en/splobjectstorage.valid.php
     * @return bool true if the iterator entry is valid, false otherwise.
         * @since 5.1.0
         */
        public function valid () {}

        /**
         * Returns the index at which the iterator currently is
         * @link http://php.net/manual/en/splobjectstorage.key.php
         * @return int The index corresponding to the position of the iterator.
         * @since 5.1.0
         */
        public function key () {}

        /**
         * Returns the current storage entry
         * @link http://php.net/manual/en/splobjectstorage.current.php
         * @return object The object at the current iterator position.
         * @since 5.1.0
         */
        public function current () {}

        /**
         * Move to the next entry
         * @link http://php.net/manual/en/splobjectstorage.next.php
         * @return void 
         * @since 5.1.0
         */
        public function next () {}

        /**
         * Unserializes a storage from its string representation
         * @link http://php.net/manual/en/splobjectstorage.unserialize.php
         * @param string $serialized <p>
         * The serialized representation of a storage.
         * </p>
         * @return void 
         * @since 5.2.2
         */
        public function unserialize ($serialized) {}

        /**
         * Serializes the storage
         * @link http://php.net/manual/en/splobjectstorage.serialize.php
         * @return string A string representing the storage.
         * @since 5.2.2
         */
        public function serialize () {}

        /**
         * Checks whether an object exists in the storage
         * @link http://php.net/manual/en/splobjectstorage.offsetexists.php
         * @param object $object <p>
         * The object to look for.
         * </p>
     * @return bool true if the object exists in the storage,
         * and false otherwise.
         * @since 5.3.0
         */
        public function offsetExists ($object) {}

        /**
         * Associates data to an object in the storage
         * @link http://php.net/manual/en/splobjectstorage.offsetset.php
         * @param object $object <p>
         * The object to associate data with.
         * </p>
     * @param mixed $data [optional] <p>
         * The data to associate with the object.
         * </p>
         * @return void 
         * @since 5.3.0
         */
    public function offsetSet ($object, $data = null) {}

        /**
         * Removes an object from the storage
         * @link http://php.net/manual/en/splobjectstorage.offsetunset.php
         * @param object $object <p>
         * The object to remove.
         * </p>
         * @return void 
         * @since 5.3.0
         */
        public function offsetUnset ($object) {}

        /**
         * Returns the data associated with an <type>object</type>
         * @link http://php.net/manual/en/splobjectstorage.offsetget.php
         * @param object $object <p>
         * The object to look for.
         * </p>
         * @return mixed The data previously associated with the object in the storage.
         * @since 5.3.0
         */
        public function offsetGet ($object) {}

        /**
         * Calculate a unique identifier for the contained objects
         * @link http://php.net/manual/en/splobjectstorage.gethash.php
         * @param $object  <p>
         * object whose identifier is to be calculated.
         * @return string A string with the calculated identifier.
         * @since 5.4.0
        */
        public function getHash($object) {}

}
SplObjectStorage

下面是改进过的示例代码:

class Login implements SplSubject
{
    const LOGIN_USER_UNKNOWN = 1;
    const LOGIN_WRONG_PASS   = 2;
    const LOGIN_ACCESS       = 3;
    private $_status = array();
    private $_storage;

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

    public function handleLogin($user, $pass, $ip)
    {
        switch (rand(1,3))
        {
            case self::LOGIN_ACCESS:
                $this->setStatus(self::LOGIN_ACCESS, $user, $ip);
                $ret = true; break;
            case self::LOGIN_WRONG_PASS:
                $this->setStatus(self::LOGIN_WRONG_PASS, $user, $ip);
                $ret = false; break;
            case self::LOGIN_USER_UNKNOWN:
            default:
                $this->setStatus(self::LOGIN_USER_UNKNOWN, $user, $ip);
                $ret = false; break;
        }
        $this->notify();
        return $ret;
    }

    private function setStatus($status, $user, $ip)
    {
        $this->_status = array($status, $user, $ip);
    }

    public function getStatus()
    {
        return $this->_status;
    }

    public function attach(SplObserver $observer)
    {
        $this->_storage->attach($observer);
    }

    public function detach(SplObserver $observer)
    {
        $this->_storage->detach($observer);

    }

    public function notify()
    {
        foreach ($this->_storage as $obs) {
            $obs->update($this);
        }
    }
}

abstract class LoginObserver implements SplObserver
{
    private $_login;

    public function __construct(Login $login)
    {
        $this->_login = $login;
        $login->attach($this);
    }

    public function update(SplSubject $subject)
    {
        if ($subject == $this->_login) {
            $this->doUpdate($subject);
        }
    }

    abstract protected function doUpdate(Login $login);
}

class SecurityMonitor extends LoginObserver
{
    public function doUpdate(Login $login)
    {
        $status = $login->getStatus();
        if ($status[0] == Login::LOGIN_WRONG_PASS) {
            // 发送邮件给系统管理员
            print __CLASS__.":发送邮件给系统给管理员<br>";
        }
    }
}

class GeneralLogger extends LoginObserver
{
    public function doUpdate(Login $login)
    {
        $status = $login->getStatus();
        // 记录登录数据到日志
        print __CLASS__.":记录登录数据到日志<br>";
    }
}

$login = new Login();
new SecurityMonitor($login);
new GeneralLogger($login);
$login->handleLogin('BNDong', '123456', '127.0.0.1');

参考资料

《深入PHP面向对象、模式与实践》(第三版)

https://baike.baidu.com/item/%E8%A7%82%E5%AF%9F%E8%80%85%E6%A8%A1%E5%BC%8F/5881786?fr=aladdin

http://www.runoob.com/design-pattern/observer-pattern.html

posted @ 2018-08-10 15:02  BNDong  阅读(...)  评论(...编辑  收藏