comparison core/lib/Drupal/Core/Theme/Registry.php @ 0:c75dbcec494b

Initial commit from drush-created site

+<?php
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\DestructableInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Extension\ThemeHandlerInterface;
+use Drupal\Core\Lock\LockBackendInterface;
+use Drupal\Core\Utility\ThemeRegistry;
+
+/**
+ * Defines the theme registry service.
+ *
+ * @internal
+ *
+ * Theme registry is expected to be used only internally since every
+ * hook_theme() implementation depends on the way this class is built. This
+ * class may get new features in minor releases so this class should be
+ * considered internal.
+ *
+ * @todo Replace local $registry variables in methods with $this->registry.
+ */
+class Registry implements DestructableInterface {
+
+  /**
+   * The theme object representing the active theme for this registry.
+   *
+   * @var \Drupal\Core\Theme\ActiveTheme
+   */
+  protected $theme;
+
+  /**
+   * The lock backend that should be used.
+   *
+   * @var \Drupal\Core\Lock\LockBackendInterface
+   */
+  protected $lock;
+
+  /**
+   * The complete theme registry.
+   *
+   * @var array
+   *   An array of theme registries, keyed by the theme name. Each registry is
+   *   an associative array keyed by theme hook names, whose values are
+   *   associative arrays containing the aggregated hook definition:
+   *   - type: The type of the extension the original theme hook originates
+   *     from; e.g., 'module' for theme hook 'node' of Node module.
+   *   - name: The name of the extension the original theme hook originates
+   *     from; e.g., 'node' for theme hook 'node' of Node module.
+   *   - theme path: The effective \Drupal\Core\Theme\ActiveTheme::getPath()
+   *      during \Drupal\Core\Theme\ThemeManagerInterface::render(), available
+   *      as 'directory' variable in templates. For functions, it should point
+   *      to the respective theme. For templates, it should point to the
+   *      directory that contains the template.
+   *   - includes: (optional) An array of include files to load when the theme
+   *     hook is executed by \Drupal\Core\Theme\ThemeManagerInterface::render().
+   *   - file: (optional) A filename to add to 'includes', either prefixed with
+   *     the value of 'path', or the path of the extension implementing
+   *     hook_theme().
+   *   In case of a theme base hook, one of the following:
+   *   - variables: An associative array whose keys are variable names and whose
+   *     values are default values of the variables to use for this theme hook.
+   *   - render element: A string denoting the name of the variable name, in
+   *     which the render element for this theme hook is provided.
+   *   In case of a theme template file:
+   *   - path: The path to the template file to use. Defaults to the
+   *     subdirectory 'templates' of the path of the extension implementing
+   *     hook_theme(); e.g., 'core/modules/node/templates' for Node module.
+   *   - template: The basename of the template file to use, without extension
+   *     (as the extension is specific to the theme engine). The template file
+   *     is in the directory defined by 'path'.
+   *   - template_file: A full path and file name to a template file to use.
+   *     Allows any extension to override the effective template file.
+   *   - engine: The theme engine to use for the template file.
+   *   In case of a theme function:
+   *   - function: The function name to call to generate the output.
+   *   For any registered theme hook, including theme hook suggestions:
+   *   - preprocess: An array of theme variable preprocess callbacks to invoke
+   *     before invoking final theme variable processors.
+   *   - process: An array of theme variable process callbacks to invoke
+   *     before invoking the actual theme function or template.
+   */
+  protected $registry = [];
+
+  /**
+   * The cache backend to use for the complete theme registry data.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  protected $cache;
+
+  /**
+   * The module handler to use to load modules.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * An array of incomplete, runtime theme registries, keyed by theme name.
+   *
+   * @var \Drupal\Core\Utility\ThemeRegistry[]
+   */
+  protected $runtimeRegistry = [];
+
+  /**
+   * Stores whether the registry was already initialized.
+   *
+   * @var bool
+   */
+  protected $initialized = FALSE;
+
+  /**
+   * The name of the theme for which to construct the registry, if given.
+   *
+   * @var string|null
+   */
+  protected $themeName;
+
+  /**
+   * The app root.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
+  /**
+   * The theme manager.
+   *
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
+   */
+  protected $themeManager;
+
+  /**
+   * The runtime cache.
+   *
+   * @var \Drupal\Core\Cache\CacheBackendInterface
+   */
+  protected $runtimeCache;
+
+  /**
+   * Constructs a \Drupal\Core\Theme\Registry object.
+   *
+   * @param string $root
+   *   The app root.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
+   *   The cache backend interface to use for the complete theme registry data.
+   * @param \Drupal\Core\Lock\LockBackendInterface $lock
+   *   The lock backend.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to use to load modules.
+   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
+   *   The theme handler.
+   * @param \Drupal\Core\Theme\ThemeInitializationInterface $theme_initialization
+   *   The theme initialization.
+   * @param string $theme_name
+   *   (optional) The name of the theme for which to construct the registry.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $runtime_cache
+   *   The cache backend interface to use for the runtime theme registry data.
+   */
+  public function __construct($root, CacheBackendInterface $cache, LockBackendInterface $lock, ModuleHandlerInterface $module_handler, ThemeHandlerInterface $theme_handler, ThemeInitializationInterface $theme_initialization, $theme_name = NULL, CacheBackendInterface $runtime_cache = NULL) {
+    $this->root = $root;
+    $this->cache = $cache;
+    $this->lock = $lock;
+    $this->moduleHandler = $module_handler;
+    $this->themeName = $theme_name;
+    $this->themeHandler = $theme_handler;
+    $this->themeInitialization = $theme_initialization;
+    $this->runtimeCache = $runtime_cache;
+  }
+
+  /**
+   * Sets the theme manager.
+   *
+   * @param \Drupal\Core\Theme\ThemeManagerInterface $theme_manager
+   *   The theme manager.
+   */
+  public function setThemeManager(ThemeManagerInterface $theme_manager) {
+    $this->themeManager = $theme_manager;
+  }
+
+  /**
+   * Initializes a theme with a certain name.
+   *
+   * This function does to much magic, so it should be replaced by another
+   * services which holds the current active theme information.
+   *
+   * @param string $theme_name
+   *   (optional) The name of the theme for which to construct the registry.
+   */
+  protected function init($theme_name = NULL) {
+    if ($this->initialized) {
+      return;
+    }
+    // Unless instantiated for a specific theme, use globals.
+    if (!isset($theme_name)) {
+      $this->theme = $this->themeManager->getActiveTheme();
+    }
+    // Instead of the active theme, a specific theme was requested.
+    else {
+      $this->theme = $this->themeInitialization->getActiveThemeByName($theme_name);
+      $this->themeInitialization->loadActiveTheme($this->theme);
+    }
+  }
+
+  /**
+   * Returns the complete theme registry from cache or rebuilds it.
+   *
+   * @return array
+   *   The complete theme registry data array.
+   *
+   * @see Registry::$registry
+   */
+  public function get() {
+    $this->init($this->themeName);
+    if (isset($this->registry[$this->theme->getName()])) {
+      return $this->registry[$this->theme->getName()];
+    }
+    if ($cache = $this->cache->get('theme_registry:' . $this->theme->getName())) {
+      $this->registry[$this->theme->getName()] = $cache->data;
+    }
+    else {
+      $this->build();
+      // Only persist it if all modules are loaded to ensure it is complete.
+      if ($this->moduleHandler->isLoaded()) {
+        $this->setCache();
+      }
+    }
+    return $this->registry[$this->theme->getName()];
+  }
+
+  /**
+   * Returns the incomplete, runtime theme registry.
+   *
+   * @return \Drupal\Core\Utility\ThemeRegistry
+   *   A shared instance of the ThemeRegistry class, provides an ArrayObject
+   *   that allows it to be accessed with array syntax and isset(), and is more
+   *   lightweight than the full registry.
+   */
+  public function getRuntime() {
+    $this->init($this->themeName);
+    if (!isset($this->runtimeRegistry[$this->theme->getName()])) {
+      $this->runtimeRegistry[$this->theme->getName()] = new ThemeRegistry('theme_registry:runtime:' . $this->theme->getName(), $this->runtimeCache ?: $this->cache, $this->lock, ['theme_registry'], $this->moduleHandler->isLoaded());
+    }
+    return $this->runtimeRegistry[$this->theme->getName()];
+  }
+
+  /**
+   * Persists the theme registry in the cache backend.
+   */
+  protected function setCache() {
+    $this->cache->set('theme_registry:' . $this->theme->getName(), $this->registry[$this->theme->getName()], Cache::PERMANENT, ['theme_registry']);
+  }
+
+  /**
+   * Returns the base hook for a given hook suggestion.
+   *
+   * @param string $hook
+   *   The name of a theme hook whose base hook to find.
+   *
+   * @return string|false
+   *   The name of the base hook or FALSE.
+   */
+  public function getBaseHook($hook) {
+    $this->init($this->themeName);
+    $base_hook = $hook;
+    // Iteratively strip everything after the last '__' delimiter, until a
+    // base hook definition is found. Recursive base hooks of base hooks are
+    // not supported, so the base hook must be an original implementation that
+    // points to a theme function or template.
+    while ($pos = strrpos($base_hook, '__')) {
+      $base_hook = substr($base_hook, 0, $pos);
+      if (isset($this->registry[$base_hook]['exists'])) {
+        break;
+      }
+    }
+    if ($pos !== FALSE && $base_hook !== $hook) {
+      return $base_hook;
+    }
+    return FALSE;
+  }
+
+  /**
+   * Builds the theme registry cache.
+   *
+   * Theme hook definitions are collected in the following order:
+   * - Modules
+   * - Base theme engines
+   * - Base themes
+   * - Theme engine
+   * - Theme
+   *
+   * All theme hook definitions are essentially just collated and merged in the
+   * above order. However, various extension-specific default values and
+   * customizations are required; e.g., to record the effective file path for
+   * theme template. Therefore, this method first collects all extensions per
+   * type, and then dispatches the processing for each extension to
+   * processExtension().
+   *
+   * After completing the collection, modules are allowed to alter it. Lastly,
+   * any derived and incomplete theme hook definitions that are hook suggestions
+   * for base hooks (e.g., 'block__node' for the base hook 'block') need to be
+   * determined based on the full registry and classified as 'base hook'.
+   *
+   * See the @link themeable Default theme implementations topic @endlink for
+   * details.
+   *
+   * @return \Drupal\Core\Utility\ThemeRegistry
+   *   The build theme registry.
+   *
+   * @see hook_theme_registry_alter()
+   */
+  protected function build() {
+    $cache = [];
+    // First, preprocess the theme hooks advertised by modules. This will
+    // serve as the basic registry. Since the list of enabled modules is the
+    // same regardless of the theme used, this is cached in its own entry to
+    // save building it for every theme.
+    if ($cached = $this->cache->get('theme_registry:build:modules')) {
+      $cache = $cached->data;
+    }
+    else {
+      foreach ($this->moduleHandler->getImplementations('theme') as $module) {
+        $this->processExtension($cache, $module, 'module', $module, $this->getPath($module));
+      }
+      // Only cache this registry if all modules are loaded.
+      if ($this->moduleHandler->isLoaded()) {
+        $this->cache->set("theme_registry:build:modules", $cache, Cache::PERMANENT, ['theme_registry']);
+      }
+    }
+
+    // Process each base theme.
+    // Ensure that we start with the root of the parents, so that both CSS files
+    // and preprocess functions comes first.
+    foreach (array_reverse($this->theme->getBaseThemes()) as $base) {
+      // If the base theme uses a theme engine, process its hooks.
+      $base_path = $base->getPath();
+      if ($this->theme->getEngine()) {
+        $this->processExtension($cache, $this->theme->getEngine(), 'base_theme_engine', $base->getName(), $base_path);
+      }
+      $this->processExtension($cache, $base->getName(), 'base_theme', $base->getName(), $base_path);
+    }
+
+    // And then the same thing, but for the theme.
+    if ($this->theme->getEngine()) {
+      $this->processExtension($cache, $this->theme->getEngine(), 'theme_engine', $this->theme->getName(), $this->theme->getPath());
+    }
+
+    // Hooks provided by the theme itself.
+    $this->processExtension($cache, $this->theme->getName(), 'theme', $this->theme->getName(), $this->theme->getPath());
+
+    // Discover and add all preprocess functions for theme hook suggestions.
+    $this->postProcessExtension($cache, $this->theme);
+
+    // Let modules and themes alter the registry.
+    $this->moduleHandler->alter('theme_registry', $cache);
+    $this->themeManager->alterForTheme($this->theme, 'theme_registry', $cache);
+
+    // @todo Implement more reduction of the theme registry entry.
+    // Optimize the registry to not have empty arrays for functions.
+    foreach ($cache as $hook => $info) {
+      if (empty($info['preprocess functions'])) {
+        unset($cache[$hook]['preprocess functions']);
+      }
+    }
+    $this->registry[$this->theme->getName()] = $cache;
+
+    return $this->registry[$this->theme->getName()];
+  }
+
+  /**
+   * Process a single implementation of hook_theme().
+   *
+   * @param array $cache
+   *   The theme registry that will eventually be cached; It is an associative
+   *   array keyed by theme hooks, whose values are associative arrays
+   *   describing the hook:
+   *   - 'type': The passed-in $type.
+   *   - 'theme path': The passed-in $path.
+   *   - 'function': The name of the function generating output for this theme
+   *     hook. Either defined explicitly in hook_theme() or, if neither
+   *     'function' nor 'template' is defined, then the default theme function
+   *     name is used. The default theme function name is the theme hook
+   *     prefixed by either 'theme_' for modules or '$name_' for everything
+   *     else. If 'function' is defined, 'template' is not used.
+   *   - 'template': The filename of the template generating output for this
+   *     theme hook. The template is in the directory defined by the 'path' key
+   *     of hook_theme() or defaults to "$path/templates".
+   *   - 'variables': The variables for this theme hook as defined in
+   *     hook_theme(). If there is more than one implementation and 'variables'
+   *     is not specified in a later one, then the previous definition is kept.
+   *   - 'render element': The renderable element for this theme hook as defined
+   *     in hook_theme(). If there is more than one implementation and
+   *     'render element' is not specified in a later one, then the previous
+   *     definition is kept.
+   *   - See the @link themeable Theme system overview topic @endlink for
+   *     detailed documentation.
+   * @param string $name
+   *   The name of the module, theme engine, base theme engine, theme or base
+   *   theme implementing hook_theme().
+   * @param string $type
+   *   One of 'module', 'theme_engine', 'base_theme_engine', 'theme', or
+   *   'base_theme'. Unlike regular hooks that can only be implemented by
+   *   modules, each of these can implement hook_theme(). This function is
+   *   called in aforementioned order and new entries override older ones. For
+   *   example, if a theme hook is both defined by a module and a theme, then
+   *   the definition in the theme will be used.
+   * @param string $theme
+   *   The actual name of theme, module, etc. that is being processed.
+   * @param string $path
+   *   The directory where $name is. For example, modules/system or
+   *   themes/bartik.
+   *
+   * @see \Drupal\Core\Theme\ThemeManagerInterface::render()
+   * @see hook_theme()
+   * @see \Drupal\Core\Extension\ThemeHandler::listInfo()
+   * @see twig_render_template()
+   *
+   * @throws \BadFunctionCallException
+   */
+  protected function processExtension(array &$cache, $name, $type, $theme, $path) {
+    $result = [];
+
+    $hook_defaults = [
+      'variables' => TRUE,
+      'render element' => TRUE,
+      'pattern' => TRUE,
+      'base hook' => TRUE,
+    ];
+
+    $module_list = array_keys($this->moduleHandler->getModuleList());
+
+    // Invoke the hook_theme() implementation, preprocess what is returned, and
+    // merge it into $cache.
+    $function = $name . '_theme';
+    if (function_exists($function)) {
+      $result = $function($cache, $type, $theme, $path);
+      foreach ($result as $hook => $info) {
+        // When a theme or engine overrides a module's theme function
+        // $result[$hook] will only contain key/value pairs for information being
+        // overridden.  Pull the rest of the information from what was defined by
+        // an earlier hook.
+
+        // Fill in the type and path of the module, theme, or engine that
+        // implements this theme function.
+        $result[$hook]['type'] = $type;
+        $result[$hook]['theme path'] = $path;
+
+        // If a theme hook has a base hook, mark its preprocess functions always
+        // incomplete in order to inherit the base hook's preprocess functions.
+        if (!empty($result[$hook]['base hook'])) {
+          $result[$hook]['incomplete preprocess functions'] = TRUE;
+        }
+
+        if (isset($cache[$hook]['includes'])) {
+          $result[$hook]['includes'] = $cache[$hook]['includes'];
+        }
+
+        // Load the includes, as they may contain preprocess functions.
+        if (isset($info['includes'])) {
+          foreach ($info['includes'] as $include_file) {
+            include_once $this->root . '/' . $include_file;
+          }
+        }
+
+        // If the theme implementation defines a file, then also use the path
+        // that it defined. Otherwise use the default path. This allows
+        // system.module to declare theme functions on behalf of core .include
+        // files.
+        if (isset($info['file'])) {
+          $include_file = isset($info['path']) ? $info['path'] : $path;
+          $include_file .= '/' . $info['file'];
+          include_once $this->root . '/' . $include_file;
+          $result[$hook]['includes'][] = $include_file;
+        }
+
+        // A template file is the default implementation for a theme hook, but
+        // if the theme hook specifies a function callback instead, check to
+        // ensure the function actually exists.
+        if (isset($info['function'])) {
+          if (!function_exists($info['function'])) {
+            throw new \BadFunctionCallException(sprintf(
+              'Theme hook "%s" refers to a theme function callback that does not exist: "%s"',
+              $hook,
+              $info['function']
+            ));
+          }
+        }
+        // Provide a default naming convention for 'template' based on the
+        // hook used. If the template does not exist, the theme engine used
+        // should throw an exception at runtime when attempting to include
+        // the template file.
+        elseif (!isset($info['template'])) {
+          $info['template'] = strtr($hook, '_', '-');
+          $result[$hook]['template'] = $info['template'];
+        }
+
+        // Prepend the current theming path when none is set. This is required
+        // for the default theme engine to know where the template lives.
+        if (isset($result[$hook]['template']) && !isset($info['path'])) {
+          $result[$hook]['path'] = $path . '/templates';
+        }
+
+        // If the default keys are not set, use the default values registered
+        // by the module.
+        if (isset($cache[$hook])) {
+          $result[$hook] += array_intersect_key($cache[$hook], $hook_defaults);
+        }
+
+        // Preprocess variables for all theming hooks, whether the hook is
+        // implemented as a template or as a function. Ensure they are arrays.
+        if (!isset($info['preprocess functions']) || !is_array($info['preprocess functions'])) {
+          $info['preprocess functions'] = [];
+          $prefixes = [];
+          if ($type == 'module') {
+            // Default variable preprocessor prefix.
+            $prefixes[] = 'template';
+            // Add all modules so they can intervene with their own variable
+            // preprocessors. This allows them to provide variable preprocessors
+            // even if they are not the owner of the current hook.
+            $prefixes = array_merge($prefixes, $module_list);
+          }
+          elseif ($type == 'theme_engine' || $type == 'base_theme_engine') {
+            // Theme engines get an extra set that come before the normally
+            // named variable preprocessors.
+            $prefixes[] = $name . '_engine';
+            // The theme engine registers on behalf of the theme using the
+            // theme's name.
+            $prefixes[] = $theme;
+          }
+          else {
+            // This applies when the theme manually registers their own variable
+            // preprocessors.
+            $prefixes[] = $name;
+          }
+          foreach ($prefixes as $prefix) {
+            // Only use non-hook-specific variable preprocessors for theming
+            // hooks implemented as templates. See the @defgroup themeable
+            // topic.
+            if (isset($info['template']) && function_exists($prefix . '_preprocess')) {
+              $info['preprocess functions'][] = $prefix . '_preprocess';
+            }
+            if (function_exists($prefix . '_preprocess_' . $hook)) {
+              $info['preprocess functions'][] = $prefix . '_preprocess_' . $hook;
+            }
+          }
+        }
+        // Check for the override flag and prevent the cached variable
+        // preprocessors from being used. This allows themes or theme engines
+        // to remove variable preprocessors set earlier in the registry build.
+        if (!empty($info['override preprocess functions'])) {
+          // Flag not needed inside the registry.
+          unset($result[$hook]['override preprocess functions']);
+        }
+        elseif (isset($cache[$hook]['preprocess functions']) && is_array($cache[$hook]['preprocess functions'])) {
+          $info['preprocess functions'] = array_merge($cache[$hook]['preprocess functions'], $info['preprocess functions']);
+        }
+        $result[$hook]['preprocess functions'] = $info['preprocess functions'];
+
+        // If a theme implementation definition provides both 'template' and
+        // 'function', the 'function' will be used. In this case, if the new
+        // result provides a 'template' value, any existing 'function' value
+        // must be removed for the override to be called.
+        if (isset($result[$hook]['template'])) {
+          unset($cache[$hook]['function']);
+        }
+      }
+
+      // Merge the newly created theme hooks into the existing cache.
+      $cache = NestedArray::mergeDeep($cache, $result);
+    }
+
+    // Let themes have variable preprocessors even if they didn't register a
+    // template.
+    if ($type == 'theme' || $type == 'base_theme') {
+      foreach ($cache as $hook => $info) {
+        // Check only if not registered by the theme or engine.
+        if (empty($result[$hook])) {
+          if (!isset($info['preprocess functions'])) {
+            $cache[$hook]['preprocess functions'] = [];
+          }
+          // Only use non-hook-specific variable preprocessors for theme hooks
+          // implemented as templates. See the @defgroup themeable topic.
+          if (isset($info['template']) && function_exists($name . '_preprocess')) {
+            $cache[$hook]['preprocess functions'][] = $name . '_preprocess';
+          }
+          if (function_exists($name . '_preprocess_' . $hook)) {
+            $cache[$hook]['preprocess functions'][] = $name . '_preprocess_' . $hook;
+            $cache[$hook]['theme path'] = $path;
+          }
+        }
+      }
+    }
+  }
+
+  /**
+   * Completes the definition of the requested suggestion hook.
+   *
+   * @param string $hook
+   *   The name of the suggestion hook to complete.
+   * @param array $cache
+   *   The theme registry, as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   */
+  protected function completeSuggestion($hook, array &$cache) {
+    $previous_hook = $hook;
+    $incomplete_previous_hook = [];
+    // Continue looping if the candidate hook doesn't exist or if the candidate
+    // hook has incomplete preprocess functions, and if the candidate hook is a
+    // suggestion (has a double underscore).
+    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
+      && $pos = strrpos($previous_hook, '__')) {
+      // Find the first existing candidate hook that has incomplete preprocess
+      // functions.
+      if (isset($cache[$previous_hook]) && !$incomplete_previous_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $incomplete_previous_hook = $cache[$previous_hook];
+        unset($incomplete_previous_hook['incomplete preprocess functions']);
+      }
+      $previous_hook = substr($previous_hook, 0, $pos);
+      $this->mergePreprocessFunctions($hook, $previous_hook, $incomplete_previous_hook, $cache);
+    }
+
+    // In addition to processing suggestions, include base hooks.
+    if (isset($cache[$hook]['base hook'])) {
+      // In order to retain the additions from above, pass in the current hook
+      // as the parent hook, otherwise it will be overwritten.
+      $this->mergePreprocessFunctions($hook, $cache[$hook]['base hook'], $cache[$hook], $cache);
+    }
+  }
+
+  /**
+   * Merges the source hook's preprocess functions into the destination hook's.
+   *
+   * @param string $destination_hook_name
+   *   The name of the hook to merge preprocess functions to.
+   * @param string $source_hook_name
+   *   The name of the hook to merge preprocess functions from.
+   * @param array $parent_hook
+   *   The parent hook if it exists. Either an incomplete hook from suggestions
+   *   or a base hook.
+   * @param array $cache
+   *   The theme registry, as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   */
+  protected function mergePreprocessFunctions($destination_hook_name, $source_hook_name, $parent_hook, array &$cache) {
+    // If base hook exists clone of it for the preprocess function
+    // without a template.
+    // @see https://www.drupal.org/node/2457295
+    if (isset($cache[$source_hook_name]) && (!isset($cache[$source_hook_name]['incomplete preprocess functions']) || !isset($cache[$destination_hook_name]['incomplete preprocess functions']))) {
+      $cache[$destination_hook_name] = $parent_hook + $cache[$source_hook_name];
+      if (isset($parent_hook['preprocess functions'])) {
+        $diff = array_diff($parent_hook['preprocess functions'], $cache[$source_hook_name]['preprocess functions']);
+        $cache[$destination_hook_name]['preprocess functions'] = array_merge($cache[$source_hook_name]['preprocess functions'], $diff);
+      }
+      // If a base hook isn't set, this is the actual base hook.
+      if (!isset($cache[$source_hook_name]['base hook'])) {
+        $cache[$destination_hook_name]['base hook'] = $source_hook_name;
+      }
+    }
+  }
+
+  /**
+   * Completes the theme registry adding discovered functions and hooks.
+   *
+   * @param array $cache
+   *   The theme registry as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   * @param \Drupal\Core\Theme\ActiveTheme $theme
+   *   Current active theme.
+   *
+   * @see ::processExtension()
+   */
+  protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
+    // Gather prefixes. This will be used to limit the found functions to the
+    // expected naming conventions.
+    $prefixes = array_keys((array) $this->moduleHandler->getModuleList());
+    foreach (array_reverse($theme->getBaseThemes()) as $base) {
+      $prefixes[] = $base->getName();
+    }
+    if ($theme->getEngine()) {
+      $prefixes[] = $theme->getEngine() . '_engine';
+    }
+    $prefixes[] = $theme->getName();
+
+    $grouped_functions = $this->getPrefixGroupedUserFunctions($prefixes);
+
+    // Collect all variable preprocess functions in the correct order.
+    $suggestion_level = [];
+    $matches = [];
+    // Look for functions named according to the pattern and add them if they
+    // have matching hooks in the registry.
+    foreach ($prefixes as $prefix) {
+      // Grep only the functions which are within the prefix group.
+      list($first_prefix,) = explode('_', $prefix, 2);
+      if (!isset($grouped_functions[$first_prefix])) {
+        continue;
+      }
+      // Add the function and the name of the associated theme hook to the list
+      // of preprocess functions grouped by suggestion specificity if a matching
+      // base hook is found.
+      foreach ($grouped_functions[$first_prefix] as $candidate) {
+        if (preg_match("/^{$prefix}_preprocess_(((?:[^_]++|_(?!_))+)__.*)/", $candidate, $matches)) {
+          if (isset($cache[$matches[2]])) {
+            $level = substr_count($matches[1], '__');
+            $suggestion_level[$level][$candidate] = $matches[1];
+          }
+        }
+      }
+    }
+
+    // Add missing variable preprocessors. This is needed for modules that do
+    // not explicitly register the hook. For example, when a theme contains a
+    // variable preprocess function but it does not implement a template, it
+    // will go missing. This will add the expected function. It also allows
+    // modules or themes to have a variable process function based on a pattern
+    // even if the hook does not exist.
+    ksort($suggestion_level);
+    foreach ($suggestion_level as $level => $item) {
+      foreach ($item as $preprocessor => $hook) {
+        if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
+          // Add missing preprocessor to existing hook.
+          $cache[$hook]['preprocess functions'][] = $preprocessor;
+        }
+        elseif (!isset($cache[$hook]) && strpos($hook, '__')) {
+          // Process non-existing hook and register it.
+          // Look for a previously defined hook that is either a less specific
+          // suggestion hook or the base hook.
+          $this->completeSuggestion($hook, $cache);
+          $cache[$hook]['preprocess functions'][] = $preprocessor;
+        }
+      }
+    }
+    // Inherit all base hook variable preprocess functions into suggestion
+    // hooks. This ensures that derivative hooks have a complete set of variable
+    // preprocess functions.
+    foreach ($cache as $hook => $info) {
+      // The 'base hook' is only applied to derivative hooks already registered
+      // from a pattern. This is typically set from
+      // drupal_find_theme_functions() and drupal_find_theme_templates().
+      if (isset($info['incomplete preprocess functions'])) {
+        $this->completeSuggestion($hook, $cache);
+        unset($cache[$hook]['incomplete preprocess functions']);
+      }
+
+      // Optimize the registry.
+      if (isset($cache[$hook]['preprocess functions']) && empty($cache[$hook]['preprocess functions'])) {
+        unset($cache[$hook]['preprocess functions']);
+      }
+      // Ensure uniqueness.
+      if (isset($cache[$hook]['preprocess functions'])) {
+        $cache[$hook]['preprocess functions'] = array_unique($cache[$hook]['preprocess functions']);
+      }
+    }
+  }
+
+  /**
+   * Invalidates theme registry caches.
+   *
+   * To be called when the list of enabled extensions is changed.
+   */
+  public function reset() {
+    // Reset the runtime registry.
+    foreach ($this->runtimeRegistry as $runtime_registry) {
+      $runtime_registry->clear();
+    }
+    $this->runtimeRegistry = [];
+
+    $this->registry = [];
+    Cache::invalidateTags(['theme_registry']);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function destruct() {
+    foreach ($this->runtimeRegistry as $runtime_registry) {
+      $runtime_registry->destruct();
+    }
+  }
+
+  /**
+   * Gets all user functions grouped by the word before the first underscore.
+   *
+   * @param $prefixes
+   *   An array of function prefixes by which the list can be limited.
+   * @return array
+   *   Functions grouped by the first prefix.
+   */
+  public function getPrefixGroupedUserFunctions($prefixes = []) {
+    $functions = get_defined_functions();
+
+    // If a list of prefixes is supplied, trim down the list to those items
+    // only as efficiently as possible.
+    if ($prefixes) {
+      $theme_functions = preg_grep('/^(' . implode(')|(', $prefixes) . ')_/', $functions['user']);
+    }
+    else {
+      $theme_functions = $functions['user'];
+    }
+
+    $grouped_functions = [];
+    // Splitting user defined functions into groups by the first prefix.
+    foreach ($theme_functions as $function) {
+      list($first_prefix,) = explode('_', $function, 2);
+      $grouped_functions[$first_prefix][] = $function;
+    }
+
+    return $grouped_functions;
+  }
+
+  /**
+   * Wraps drupal_get_path().
+   *
+   * @param string $module
+   *   The name of the item for which the path is requested.
+   *
+   * @return string
+   */
+  protected function getPath($module) {
+    return drupal_get_path('module', $module);
+  }
+
+}