Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/lib/Drupal/Component/Discovery/DiscoverableInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Discovery\DiscoverableInterface.
+ */
+
+namespace Drupal\Component\Discovery;
+
+/**
+ * Interface for classes providing a type of discovery.
+ */
+interface DiscoverableInterface {
+
+  /**
+   * Returns an array of discoverable items.
+   *
+   * @return array
+   *   An array of discovered data keyed by provider.
+   */
+  public function findAll();
+
+}

core/lib/Drupal/Component/Discovery/YamlDiscovery.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Discovery\YamlDiscovery.
+ */
+
+namespace Drupal\Component\Discovery;
+
+use Drupal\Component\Serialization\Yaml;
+use Drupal\Component\FileCache\FileCacheFactory;
+
+/**
+ * Provides discovery for YAML files within a given set of directories.
+ */
+class YamlDiscovery implements DiscoverableInterface {
+
+  /**
+   * The base filename to look for in each directory.
+   *
+   * @var string
+   */
+  protected $name;
+
+  /**
+   * An array of directories to scan, keyed by the provider.
+   *
+   * @var array
+   */
+  protected $directories = array();
+
+  /**
+   * Constructs a YamlDiscovery object.
+   *
+   * @param string $name
+   *   The base filename to look for in each directory. The format will be
+   *   $provider.$name.yml.
+   * @param array $directories
+   *   An array of directories to scan, keyed by the provider.
+   */
+  public function __construct($name, array $directories) {
+    $this->name = $name;
+    $this->directories = $directories;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function findAll() {
+    $all = array();
+
+    $files = $this->findFiles();
+    $provider_by_files = array_flip($files);
+
+    $file_cache = FileCacheFactory::get('yaml_discovery:' . $this->name);
+
+    // Try to load from the file cache first.
+    foreach ($file_cache->getMultiple($files) as $file => $data) {
+      $all[$provider_by_files[$file]] = $data;
+      unset($provider_by_files[$file]);
+    }
+
+    // If there are files left that were not returned from the cache, load and
+    // parse them now. This list was flipped above and is keyed by filename.
+    if ($provider_by_files) {
+      foreach ($provider_by_files as $file => $provider) {
+        // If a file is empty or its contents are commented out, return an empty
+        // array instead of NULL for type consistency.
+        $all[$provider] = Yaml::decode(file_get_contents($file)) ?: [];
+        $file_cache->set($file, $all[$provider]);
+      }
+    }
+
+    return $all;
+  }
+
+  /**
+   * Returns an array of file paths, keyed by provider.
+   *
+   * @return array
+   */
+  protected function findFiles() {
+    $files = array();
+    foreach ($this->directories as $provider => $directory) {
+      $file = $directory . '/' . $provider . '.' . $this->name . '.yml';
+      if (file_exists($file)) {
+        $files[$provider] = $file;
+      }
+    }
+    return $files;
+  }
+
+}
+