CacheableDependencyInterface added to describe methods needed by Drupal Cache API

The CacheableDependencyInterface provides the methods necessary to do some calculation (build a render array, calculate a number, anything) that is invalidated automatically whenever the objects that the calculation depends on is modified. Objects that implement CacheableDependencyInterface can communicate their cacheability in a standardized way, so that the calculation can be easily cached using the Drupal Cache API. This will commonly be used to fill out the #cache property of a render array.

One of the key advantages to using this interface, is that it allows an object to provide all the necessary details about how it should be turned into a cacheable render array, without requiring that class to have knowledge of complex patterns like #pre_render.

Before CacheableDependencyInterface:

 
class SomeClass {
  protected function doBuild() {
    // something expensive
  }
 
  public function build() {
    return array(
      'content' => array(
        // SomeClass has intimate knowledge of render API semantics and has to
        // put its expensive work in a #pre_render callback.
        '#pre_render' => array(array($some_class, 'doBuild')),
      ),
      '#cache' => array(
        'keys' => array('some_class', $some_class->id()),
        'tags' => array('content' => TRUE),
        'expire' => REQUEST_TIME + $this->settings['max_age'],
      ),
    );
  }
}
 
class SomeClassViewBuilder {
  public function viewMultiple() {
    // View builder cannot get cache info without building the entire render 
    // array.
    $build[$entity_id] = $some_class->build();
  }
}

With CacheableDependencyInterface:

 
class SomeClass implements CacheableDependencyInterface {
  public function build() {
    // something expensive
  }
  
  public function getCacheTags() {
    return array('content' => TRUE);
  }
 
  public function getCacheMaxAge() {
    return $this->settings['max_age'];
  }
}
 
class SomeClassViewBuilder {
  public function viewMultiple() {
    $build[$entity_id] += array(
      'content' => array(
        // View Builder can now move the expensive work to a #pre_render
        // callback so that the cacheable object isn't responsible for getting
        // this right.
        '#pre_render' => array(array($some_class, 'build')),
      ),
      '#cache' => array(
        'keys' => ['some_class', $some_class->id()],
        'tags' => $some_class->cacheTags(),
        'expire' => REQUEST_TIME + $some_class->cacheMaxAge(),
      ),
    );
  }
}