Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/lib/Drupal/Core/Display/ContextAwareVariantInterface.php

@@ -0,0 +1,33 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Display\ContextAwareVariantInterface.
+ */
+
+namespace Drupal\Core\Display;
+
+/**
+ * Provides an interface for variant plugins that are context-aware.
+ */
+interface ContextAwareVariantInterface extends VariantInterface {
+
+  /**
+   * Gets the values for all defined contexts.
+   *
+   * @return \Drupal\Component\Plugin\Context\ContextInterface[]
+   *   An array of set contexts, keyed by context name.
+   */
+  public function getContexts();
+
+  /**
+   * Sets the context values for this display variant.
+   *
+   * @param \Drupal\Component\Plugin\Context\ContextInterface[] $contexts
+   *   An array of contexts, keyed by context name.
+   *
+   * @return $this
+   */
+  public function setContexts(array $contexts);
+
+}

core/lib/Drupal/Core/Display/PageVariantInterface.php

@@ -36,4 +36,15 @@ interface PageVariantInterface extends VariantInterface {
    */
   public function setMainContent(array $main_content);
 
+  /**
+   * Sets the title for the page being rendered.
+   *
+   * @param string|array $title
+   *   The page title: either a string for plain titles or a render array for
+   *   formatted titles.
+   *
+   * @return $this
+   */
+  public function setTitle($title);
+
 }

core/lib/Drupal/Core/Display/VariantBase.php

@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Display;
 
+use Drupal\Core\Cache\RefinableCacheableDependencyTrait;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\Core\Plugin\PluginDependencyTrait;
@@ -23,6 +24,7 @@ use Drupal\Core\Session\AccountInterface;
 abstract class VariantBase extends PluginBase implements VariantInterface {
 
   use PluginDependencyTrait;
+  use RefinableCacheableDependencyTrait;
 
   /**
    * {@inheritdoc}

core/lib/Drupal/Core/Display/VariantInterface.php

@@ -9,6 +9,7 @@ namespace Drupal\Core\Display;
 
 use Drupal\Component\Plugin\ConfigurablePluginInterface;
 use Drupal\Component\Plugin\PluginInspectionInterface;
+use Drupal\Core\Cache\RefinableCacheableDependencyInterface;
 use Drupal\Core\Plugin\PluginFormInterface;
 use Drupal\Core\Session\AccountInterface;
 
@@ -20,7 +21,7 @@ use Drupal\Core\Session\AccountInterface;
  * @see \Drupal\Core\Display\VariantManager
  * @see plugin_api
  */
-interface VariantInterface extends PluginInspectionInterface, ConfigurablePluginInterface, PluginFormInterface {
+interface VariantInterface extends PluginInspectionInterface, ConfigurablePluginInterface, PluginFormInterface, RefinableCacheableDependencyInterface {
 
   /**
    * Returns the user-facing display variant label.
@@ -79,6 +80,10 @@ interface VariantInterface extends PluginInspectionInterface, ConfigurablePlugin
   /**
    * Builds and returns the renderable array for the display variant.
    *
+   * The variant can contain cacheability metadata for the configuration that
+   * was passed in setConfiguration(). In the build() method, this should be
+   * added to the render array that is returned.
+   *
    * @return array
    *   A render array for the display variant.
    */