PHP设计模式之责任链模式的深入解析

  责任链模式,其目的是组织一个对象链处理一个如方法调用的请求。

  当ConcreteHandler(具体的处理程序)不知道如何满足来自Client的请求时,或它的目的不是这个时,它会委派给链中的下一个Handler(处理程序)来处理。

  这个设计模式通常和复合模式一起使用,其中有些叶子或容器对象默认委派操作给它们的父对象。另一个例子是,本地化通常是使用责任链处理的,当德语翻译适配器没有为翻译关键词找到合适的结果时,就返回到英语适配器或干脆直接显示关键词本身。

  耦合减少到最低限度:Client类不知道由哪个具体的类来处理请求;在创建对象图时配置了链;ConcreteHandlers不知道哪个对象是它们的继承者。行为在对象之间分配是成功的,链中最近的对象有优先权和责任满足请求。

  参与者:

  ◆Client(客户端):向Handler(处理程序)提交一个请求;

  ◆Handler(处理程序)抽象:接收一个请求,以某种方式满足它;

  ◆ConcreteHandlers(具体的处理程序):接收一个请求,设法满足它,如果不成功就委派给下一个处理程序。

  下面的代码实现了一个最著名的责任链示例:多级缓存。

  

复制代码 代码如下:

  /**

  * The Handler abstraction. Objects that want to be a part of the

  * ChainOfResponsibility must implement this interface directly or via

  * inheritance from an AbstractHandler.

  */

  interface KeyValueStore

  {

  /**

  * Obtain a value.

  * @param string $key

  * @return mixed

  */

  public function get($key);

  }

  /**

  * Basic no-op implementation which ConcreteHandlers not interested in

  * caching or in interfering with the retrieval inherit from.

  */

  abstract class AbstractKeyValueStore implements KeyValueStore

  {

  protected $_nextHandler;

  public function get($key)

  {

  return $this->_nextHandler->get($key);

  }

  }

  /**

  * Ideally the last ConcreteHandler in the chain. At least, if inserted in

  * a Chain it will be the last node to be called.

  */

  class SlowStore implements KeyValueStore

  {

  /**

  * This could be a somewhat slow store: a database or a flat file.

  */

  protected $_values;

  public function __construct(array $values = array())

  {

  $this->_values = $values;

  }

  public function get($key)

  {

  return $this->_values[$key];

  }

  }

  /**

  * A ConcreteHandler that handles the request for a key by looking for it in

  * its own cache. Forwards to the next handler in case of cache miss.

  */

  class InMemoryKeyValueStore implements KeyValueStore

  {

  protected $_nextHandler;

  protected $_cached = array();

  public function __construct(KeyValueStore $nextHandler)

  {

  $this->_nextHandler = $nextHandler;

  }

  protected function _load($key)

  {

  if (!isset($this->_cached[$key])) {

  $this->_cached[$key] = $this->_nextHandler->get($key);

  }

  }

  public function get($key)

  {

  $this->_load($key);

  return $this->_cached[$key];

  }

  }

  /**

  * A ConcreteHandler that delegates the request without trying to

  * understand it at all. It may be easier to use in the user interface

  * because it can specialize itself by defining methods that generates

  * html, or by addressing similar user interface concerns.

  * Some Clients see this object only as an instance of KeyValueStore

  * and do not care how it satisfy their requests, while other ones

  * may use it in its entirety (similar to a class-based adapter).

  * No client knows that a chain of Handlers exists.

  */

  class FrontEnd extends AbstractKeyValueStore

  {

  public function __construct(KeyValueStore $nextHandler)

  {

  $this->_nextHandler = $nextHandler;

  }

  public function getEscaped($key)

  {

  return htmlentities($this->get($key), ENT_NOQUOTES, 'UTF-8');

  }

  }

  // Client code

  $store = new SlowStore(array('pd' => 'Philip K. Dick',

  'ia' => 'Isaac Asimov',

  'ac' => 'Arthur C. Clarke',

  'hh' => 'Helmut Heißenbüttel'));

  // in development, we skip cache and pass $store directly to FrontEnd

  $cache = new InMemoryKeyValueStore($store);

  $frontEnd = new FrontEnd($cache);

  echo $frontEnd->get('ia'), "\n";

  echo $frontEnd->getEscaped('hh'), "\n";

  关于PHP责任链设计模式的一些实现说明:

  ◆责任链可能已经存在于对象图中,和复合模式的例子一样;

  ◆此外,Handler抽象可能存在,也可能不存在,最好的选择是一个分开的Handler接口只可以执行handleRequest()操作,不要强制一个链只在一个层次中,因为后面的已经存在了;

  ◆也可能引入一个抽象类,但由于请求处理是一个正交关注,因此具体的类可能已经继承了其它类;

  ◆通过constructor 或setter,Handler(或下一个Handler)被注入到Client或前一个Handler;

  ◆请求对象通常是一个ValueObject,也可能被实现为一个Flyweight,在PHP中,它可能是一个标量类型,如string,注意在某些语言中,一个string就是一个不变的ValueObject。

  简单的总结责任链模式,可以归纳为:用一系列类(classes)试图处理一个请求request,这些类之间是一个松散的耦合,唯一共同点是在他们之间传递request. 也就是说,来了一个请求,A类先处理,如果没有处理,就传递到B类处理,如果没有处理,就传递到C类处理,就这样象一个链条(chain)一样传递下去。