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                <?php  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                namespace HighWire\DrupalPSR16;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                class Cache implements \Psr\SimpleCache\CacheInterface { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * @var \Drupal\Core\Cache\CacheBackendInterface  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  protected $drupal_cache;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * @param \Drupal\Core\Cache\CacheBackendInterface $drupal_cache  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    *   The drupal cache backend to convert to PSR-16  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    *   | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * @example  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    *   $drupalcache = \Drupal::cache('mybin'); | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    *   $psr16cache = new \HighWire\DrupalPSR16\Cache($drupalcache);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function __construct(\Drupal\Core\Cache\CacheBackendInterface $drupal_cache) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $this->drupal_cache = $drupal_cache;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function get($key, $default = null) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $cache = $this->drupal_cache->get($key);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    if (empty($cache)) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      return $default;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    else { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      return $cache->data;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function set($key, $value, $ttl = NULL) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    if ($ttl === NULL) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      $this->drupal_cache->set($key, $value);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    else { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      $this->drupal_cache->set($key, $value, time() + $ttl);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    return true;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                    */  | 
            
            
                                                                        
                            
            
                                    
            
            
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                  public function delete($key) { | 
            
            
                                                                        
                            
            
                                    
            
            
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                    $this->drupal_cache->delete($key);  | 
            
            
                                                                        
                            
            
                                    
            
            
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                    return true;  | 
            
            
                                                                        
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function clear() { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $this->drupal_cache->deleteAll();  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    return true;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function getMultiple($keys, $default = null) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $result = array();  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $k = $keys;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $caches = $this->drupal_cache->getMultiple($k);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    foreach ($keys as $key) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      if (!empty($caches[$key])) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $result[$key] = $caches[$key]->data;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      else { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $result[$key] = $default;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    return $result;  | 
            
                            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function setMultiple($values, $ttl = null) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $items = array();  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    foreach ($values as $key => $value) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      if ($ttl === NULL) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $items[$key] = array('data' => $value); | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      else { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                        $items[$key] = array('data' => $value, 'expire' => time() + $ttl); | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                      }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $this->drupal_cache->setMultiple($items);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    return true;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function deleteMultiple($keys) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $this->drupal_cache->deleteMultiple($keys);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    return true;  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  /**  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    * {@inheritdoc} | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    */  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  public function has($key) { | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    $cache = $this->drupal_cache->get($key);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                    return !empty($cache);  | 
            
            
                                                                                                            
                            
            
                                    
            
            
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                  }  | 
            
            
                                                                                                            
                                                                
            
                                    
            
            
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                }  | 
            
            
                        
If you return a value from a function or method, it should be a sub-type of the type that is given by the parent type f.e. an interface, or abstract method. This is more formally defined by the Lizkov substitution principle, and guarantees that classes that depend on the parent type can use any instance of a child type interchangably. This principle also belongs to the SOLID principles for object oriented design.
Let’s take a look at an example:
Our function
my_functionexpects aPostobject, and outputs the author of the post. The base classPostreturns a simple string and outputting a simple string will work just fine. However, the child classBlogPostwhich is a sub-type ofPostinstead decided to return anobject, and is therefore violating the SOLID principles. If aBlogPostwere passed tomy_function, PHP would not complain, but ultimately fail when executing thestrtouppercall in its body.