Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/lib/Drupal/Core/Plugin/CategorizingPluginManagerTrait.php

@@ -76,7 +76,7 @@ trait CategorizingPluginManagerTrait {
   }
 
   /**
-   * Implements \Drupal\Component\Plugin\CategorizingPluginManagerInterface::getCategories().
+   * {@inheritdoc}
    */
   public function getCategories() {
     /** @var \Drupal\Core\Plugin\CategorizingPluginManagerTrait|\Drupal\Component\Plugin\PluginManagerInterface $this */
@@ -89,7 +89,7 @@ trait CategorizingPluginManagerTrait {
   }
 
   /**
-   * Implements \Drupal\Component\Plugin\CategorizingPluginManagerInterface::getSortedDefinitions().
+   * {@inheritdoc}
    */
   public function getSortedDefinitions(array $definitions = NULL, $label_key = 'label') {
     // Sort the plugins first by category, then by label.
@@ -105,7 +105,7 @@ trait CategorizingPluginManagerTrait {
   }
 
   /**
-   * Implements \Drupal\Component\Plugin\CategorizingPluginManagerInterface::getGroupedDefinitions().
+   * {@inheritdoc}
    */
   public function getGroupedDefinitions(array $definitions = NULL, $label_key = 'label') {
     /** @var \Drupal\Core\Plugin\CategorizingPluginManagerTrait|\Drupal\Component\Plugin\PluginManagerInterface $this */

core/lib/Drupal/Core/Plugin/Context/Context.php

@@ -121,11 +121,9 @@ class Context extends ComponentContext implements ContextInterface {
     if (!isset($this->contextData)) {
       $definition = $this->getContextDefinition();
       $default_value = $definition->getDefaultValue();
-      if (isset($default_value)) {
-        // Store the default value so that subsequent calls don't have to look
-        // it up again.
-        $this->contextData = $this->getTypedDataManager()->create($definition->getDataDefinition(), $default_value);
-      }
+      // Store the default value so that subsequent calls don't have to look
+      // it up again.
+      $this->contextData = $this->getTypedDataManager()->create($definition->getDataDefinition(), $default_value);
     }
     return $this->contextData;
   }

core/lib/Drupal/Core/Plugin/ContextAwarePluginAssignmentTrait.php

@@ -53,7 +53,7 @@ trait ContextAwarePluginAssignmentTrait {
         ];
       }
 
-      if (count($options) > 1) {
+      if (count($options) > 1 || !$definition->isRequired()) {
         $assignments = $plugin->getContextMapping();
         $element[$context_slot] = [
           '#title' => $definition->getLabel() ?: $this->t('Select a @context value:', ['@context' => $context_slot]),
@@ -63,6 +63,9 @@ trait ContextAwarePluginAssignmentTrait {
           '#default_value' => !empty($assignments[$context_slot]) ? $assignments[$context_slot] : '',
           '#description' => $definition->getDescription(),
         ];
+        if (!$definition->isRequired()) {
+          $element[$context_slot]['#empty_value'] = '';
+        }
       }
     }
     return $element;

core/lib/Drupal/Core/Plugin/ContextAwarePluginBase.php

@@ -93,7 +93,7 @@ abstract class ContextAwarePluginBase extends ComponentContextAwarePluginBase im
   public function setContextMapping(array $context_mapping) {
     if ($this instanceof ConfigurablePluginInterface) {
       $configuration = $this->getConfiguration();
-      $configuration['context_mapping'] = $context_mapping;
+      $configuration['context_mapping'] = array_filter($context_mapping);
       $this->setConfiguration($configuration);
     }
     else {

core/lib/Drupal/Core/Plugin/ContextAwarePluginInterface.php

@@ -31,11 +31,11 @@ interface ContextAwarePluginInterface extends ComponentContextAwarePluginInterfa
    * @param string $name
    *   The name of the context in the plugin definition.
    *
-   * @throws \Drupal\Component\Plugin\Exception\PluginException
-   *   If the requested context is not defined.
-   *
    * @return \Drupal\Core\Plugin\Context\ContextDefinitionInterface.
    *   The definition against which the context value must validate.
+   *
+   * @throws \Drupal\Component\Plugin\Exception\PluginException
+   *   If the requested context is not defined.
    */
   public function getContextDefinition($name);
 

core/lib/Drupal/Core/Plugin/DefaultPluginManager.php

@@ -8,13 +8,14 @@
 namespace Drupal\Core\Plugin;
 
 use Drupal\Component\Plugin\Discovery\CachedDiscoveryInterface;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Cache\UseCacheBackendTrait;
 use Drupal\Component\Plugin\Discovery\DiscoveryCachedTrait;
 use Drupal\Core\Plugin\Discovery\ContainerDerivativeDiscoveryDecorator;
 use Drupal\Component\Plugin\PluginManagerBase;
 use Drupal\Component\Plugin\PluginManagerInterface;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
-use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
 use Drupal\Core\Plugin\Factory\ContainerFactory;
@@ -27,13 +28,7 @@ use Drupal\Core\Plugin\Factory\ContainerFactory;
 class DefaultPluginManager extends PluginManagerBase implements PluginManagerInterface, CachedDiscoveryInterface {
 
   use DiscoveryCachedTrait;
-
-  /**
-   * Cache backend instance.
-   *
-   * @var \Drupal\Core\Cache\CacheBackendInterface
-   */
-  protected $cacheBackend;
+  use UseCacheBackendTrait;
 
   /**
    * The cache key.
@@ -80,13 +75,6 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
    */
   protected $defaults = array();
 
-  /**
-   * Flag whether persistent caches should be used.
-   *
-   * @var bool
-   */
-  protected $useCaches = TRUE;
-
   /**
    * The name of the annotation that contains the plugin definition.
    *
@@ -236,30 +224,6 @@ class DefaultPluginManager extends PluginManagerBase implements PluginManagerInt
     }
   }
 
-  /**
-   * Fetches from the cache backend, respecting the use caches flag.
-   *
-   * @see \Drupal\Core\Cache\CacheBackendInterface::get()
-   */
-  protected function cacheGet($cid) {
-    if ($this->useCaches && $this->cacheBackend) {
-      return $this->cacheBackend->get($cid);
-    }
-    return FALSE;
-  }
-
-  /**
-   * Stores data in the persistent cache, respecting the use caches flag.
-   *
-   * @see \Drupal\Core\Cache\CacheBackendInterface::set()
-   */
-  protected function cacheSet($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    if ($this->cacheBackend && $this->useCaches) {
-      $this->cacheBackend->set($cid, $data, $expire, $tags);
-    }
-  }
-
-
   /**
    * Performs extra processing on plugin definitions.
    *

core/lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php

@@ -47,7 +47,7 @@ class HookDiscovery implements DiscoveryInterface {
   }
 
   /**
-   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
+   * {@inheritdoc}
    */
   public function getDefinitions() {
     $definitions = array();

core/lib/Drupal/Core/Plugin/Discovery/InfoHookDecorator.php

@@ -45,7 +45,7 @@ class InfoHookDecorator implements DiscoveryInterface {
   }
 
   /**
-   * Implements Drupal\Component\Plugin\Discovery\DiscoveryInterface::getDefinitions().
+   * {@inheritdoc}
    */
   public function getDefinitions() {
     $definitions = $this->decorated->getDefinitions();