Implementation of multi-level cache---chain of responsibility model

Multi-level caching chain of responsibility model.
* The client submits it to the handler, and the handler finds a function in the chain of responsibility that can handle the task and processes it; it can be summarized as: using a series of classes to try to handle a request. There is a loose coupling between these classes, and the only thing they have in common is The point is to pass the request between them. In other words, when a request comes, Class A will handle it first. If it is not processed, it will be passed to Class B for processing. If it is not processed, it will be passed to Class C for processing. It is like a chain. (chain) is passed on.

/**
 * Chain of Responsibility pattern, its purpose is to organize an object chain to handle a request such as a method call.
 *
 * The most famous example of chain of responsibility: multi-level caching.
 * The client submits it to the handler, and the handler finds a function in the chain of responsibility that can handle the task and processes it;
 * It can be summarized as: using a series of classes to try to handle a request, and there is a loose coupling between these classes.
 * The only thing they have in common is that requests are passed between them. In other words, when a request comes, Class A will handle it first. If it is not processed,
 * will be passed to Class B for processing. If it is not processed, it will be passed to Class C for processing. , and it is passed on like a chain.
*/


/**
* 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 or
* inherit an abstract processing class
*/
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.
* Receives a request, tries to satisfy it, and delegates to the next handler if unsuccessful.
*/
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.
* Ideally, the last concrete processing class on the responsibility chain is added to the chain and will is the last node 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.
* In case of cache miss, forward to the next processing object
*/
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.
* 不用关心调用的具体实现的外部具体具体处理程序；背后是责任链。
*/
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 Hei.enbttel'
    )
);
// 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";

/**
 * expect: ...
 * Isaac Asimov
 * Helmut Hei.enbttel
 *
 * Participants:

◆Client (client): Submit a request to Handler (handler);
◆Handler (handler) Abstract: Receive a request, satisfy it in some way;
◆ConcreteHandlers (concrete handlers): Receive a request, try to satisfy it, and delegate to the next handler if unsuccessful.
*/