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

core/lib/Drupal/Core/State/State.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\State\State.
+ */
+
+namespace Drupal\Core\State;
+
+use Drupal\Core\KeyValueStore\KeyValueFactoryInterface;
+
+/**
+ * Provides the state system using a key value store.
+ */
+class State implements StateInterface {
+
+  /**
+   * The key value store to use.
+   *
+   * @var \Drupal\Core\KeyValueStore\KeyValueStoreInterface
+   */
+  protected $keyValueStore;
+
+  /**
+   * Static state cache.
+   *
+   * @var array
+   */
+  protected $cache = array();
+
+  /**
+   * Constructs a State object.
+   *
+   * @param \Drupal\Core\KeyValueStore\KeyValueFactoryInterface $key_value_factory
+   *   The key value store to use.
+   */
+  function __construct(KeyValueFactoryInterface $key_value_factory) {
+    $this->keyValueStore = $key_value_factory->get('state');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get($key, $default = NULL) {
+    $values = $this->getMultiple(array($key));
+    return isset($values[$key]) ? $values[$key] : $default;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getMultiple(array $keys) {
+    $values = array();
+    $load = array();
+    foreach ($keys as $key) {
+      // Check if we have a value in the cache.
+      if (isset($this->cache[$key])) {
+        $values[$key] = $this->cache[$key];
+      }
+      // Load the value if we don't have an explicit NULL value.
+      elseif (!array_key_exists($key, $this->cache)) {
+        $load[] = $key;
+      }
+    }
+
+    if ($load) {
+      $loaded_values = $this->keyValueStore->getMultiple($load);
+      foreach ($load as $key) {
+        // If we find a value, even one that is NULL, add it to the cache and
+        // return it.
+        if (isset($loaded_values[$key]) || array_key_exists($key, $loaded_values)) {
+          $values[$key] = $loaded_values[$key];
+          $this->cache[$key] = $loaded_values[$key];
+        }
+        else {
+          $this->cache[$key] = NULL;
+        }
+      }
+    }
+
+    return $values;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function set($key, $value) {
+    $this->cache[$key] = $value;
+    $this->keyValueStore->set($key, $value);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setMultiple(array $data) {
+    foreach ($data as $key => $value) {
+      $this->cache[$key] = $value;
+    }
+    $this->keyValueStore->setMultiple($data);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete($key) {
+    $this->deleteMultiple(array($key));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteMultiple(array $keys) {
+    foreach ($keys as $key) {
+      unset($this->cache[$key]);
+    }
+    $this->keyValueStore->deleteMultiple($keys);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resetCache() {
+    $this->cache = array();
+  }
+
+}

core/lib/Drupal/Core/State/StateInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\State\StateInterface.
+ */
+
+namespace Drupal\Core\State;
+
+/**
+ * Defines the interface for the state system.
+ *
+ * @ingroup state_api
+ */
+interface StateInterface {
+
+  /**
+   * Returns the stored value for a given key.
+   *
+   * @param string $key
+   *   The key of the data to retrieve.
+   * @param mixed $default
+   *   The default value to use if the key is not found.
+   *
+   * @return mixed
+   *   The stored value, or NULL if no value exists.
+   */
+  public function get($key, $default = NULL);
+
+  /**
+   * Returns the stored key/value pairs for a given set of keys.
+   *
+   * @param array $keys
+   *   A list of keys to retrieve.
+   *
+   * @return array
+   *   An associative array of items successfully returned, indexed by key.
+   */
+  public function getMultiple(array $keys);
+
+  /**
+   * Saves a value for a given key.
+   *
+   * @param string $key
+   *   The key of the data to store.
+   * @param mixed $value
+   *   The data to store.
+   */
+  public function set($key, $value);
+
+  /**
+   * Saves key/value pairs.
+   *
+   * @param array $data
+   *   An associative array of key/value pairs.
+   */
+  public function setMultiple(array $data);
+
+  /**
+   * Deletes an item.
+   *
+   * @param string $key
+   *   The item name to delete.
+   */
+  public function delete($key);
+
+  /**
+   * Deletes multiple items.
+   *
+   * @param array $keys
+   *   A list of item names to delete.
+   */
+  public function deleteMultiple(array $keys);
+
+  /**
+   * Resets the static cache.
+   *
+   * This is mainly used in testing environments.
+   */
+  public function resetCache();
+
+}