cache and make fieldblocks available using the render cache and #post_render_cache

fieldblock.module

@@ -64,29 +64,37 @@ function fieldblock_field_display_submit($form, FormStateInterface $form_state)
     }
   }
   $view_mode_config->save();
+
+  // Invalidate the block cache to update fielblock derivatives.
+  if (\Drupal::moduleHandler()->moduleExists('block')) {
+    \Drupal::service('plugin.manager.block')->clearCachedDefinitions();
+  }
 }
 
 /**
  * Implements hook_entity_view_alter().
- * Stores specific fields on the current entity, to be retrieved later when the
- * fieldblock is built, and hides the field from the renderable array.
  *
- * @todo: This should be in a post_render_cache callback. See setFieldBlock();
+ * Takes fields out of the current entity and cached them in a post render cache
+ * context. The #post_render_cache callback makes this data available to the
+ * fieldblock when it is built, We also hide the field from the render array.
+ * @see \Drupal\fieldblock\Plugin\Block\FieldBlock::fieldBlockPostRenderCache
+ * @see https://www.drupal.org/node/2151609
  */
 function fieldblock_entity_view_alter(array &$build, Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display) {
   $fieldblock_settings = $display->getThirdPartySettings('fieldblock');
   $display_id = $display->get('id');
 
-  $cache_tags = array();
-  if (isset($build['#cache']['tags'])) {
-    $cache_tags = $build['#cache']['tags'];
-  }
-
   foreach ($fieldblock_settings as $field_name => $field_label) {
     $fieldblock_id = $display_id . ':' . $field_name;
 
     if (count(\Drupal\Core\Render\Element::children($build[$field_name]))) {
-      FieldBlock::setFieldBlock($build[$field_name], $fieldblock_id, $cache_tags);
+      // This is where we take out the field and cache it in a post render
+      // cache context.
+      //
+      $build['#post_render_cache']['Drupal\fieldblock\Plugin\Block\FieldBlock::fieldBlockPostRenderCache'][] = array(
+        'build' => $build[$field_name],
+        'fieldblock_id' => $fieldblock_id,
+      );
       hide($build[$field_name]);
     }
     else {
@@ -94,12 +102,3 @@ function fieldblock_entity_view_alter(array &$build, Drupal\Core\Entity\EntityIn
     }
   }
 }
-
-///**
-// * Give fieldblocks a meaningful html id in spite of the hashed block deltas.
-// */
-//function fieldblock_preprocess_block(&$variables) {
-//  if ($variables['block']->module == 'fieldblock') {
-//    $variables['block_html_id'] = drupal_html_id('block-' . $variables['block']->fieldblock_name);
-//  }
-//}

src/Plugin/Block/FieldBlock.php

@@ -30,33 +30,48 @@ class FieldBlock extends BlockBase {
   }
 
   /**
-   * Stores a the render array of a field for use as fieldblock.
-   *
-   * @param array $build
-   * @param string $fieldblock_id
-   * @param array $cache_tags
-   *
-   * @todo: The current storage approach does not work because its too hard to control the visibility of the block.
-   *   Do we have to switch to storing in a static property, called from a post_render_cache callback?
+   * @var array Static storage for fields that are grabbed from the entity's
+   *   render array, to be retrieved when fieldblocks are built.
    */
-  public static function setFieldBlock($build, $fieldblock_id, $cache_tags) {
-    $cid = 'fieldblock:' . $fieldblock_id;
-    \Drupal::cache()->set($cid, $build, CacheBackendInterface::CACHE_PERMANENT, $cache_tags);
-  }
+  protected static $fieldBlocks;
 
+  /**
+   * @param string $block_id
+   *   The identifier of the fieldblock.
+   * @return array | false
+   *   The render array of the field that is published as block or false if the
+   *   field is not available.
+   */
   public static function getFieldBlock($block_id) {
-    $cid = 'fieldblock:' . $block_id;
-    if ($cache = \Drupal::cache()->get($cid)) {
-      return $cache->data;
+    if (isset(self::$fieldBlocks[$block_id])) {
+      return self::$fieldBlocks[$block_id];
     }
     else {
       return FALSE;
     }
   }
 
-  public static function unsetFieldBlock($fieldblock_id) {
-    $cid = 'fieldblock:' . $fieldblock_id;
-    \Drupal::cache()->delete($cid);
+  /**
+   * #post_render_cache callback, temporarily stores a field's render array in a
+   * static variable and returns the original element as post render cache
+   * callbacks are supposed to do.
+   *
+   * Note that this is an atypical way to use the post render cache mechanism.
+   * Post render cache is meant to allow modules to dynamically alter pieces of
+   * cached content. Here we use it as some kind of context-aware cache, because
+   * the cached field will only be retrieved and displayed as a block when the
+   * entity is viewed.
+   *
+   * @param array $element
+   *   The element (entity) being rendered.
+   * @param array $context
+   *   Array containing the fieldblock ID and the field's render array.
+   * @return array
+   *   The render array of the entity.
+   */
+  public static function fieldBlockPostRenderCache(array $element, array $context) {
+    self::$fieldBlocks[$context['fieldblock_id']] = $context['build'];
+    return $element;
   }
 
 }