Batch API now calls static method on every iteration which returns a Drupal service

src/Batch/Batch.php

@@ -43,10 +43,12 @@ class Batch {
     switch ($this->batchInfo['from']) {
 
       case 'form':
+        // Start batch process.
         batch_set($this->batch);
         break;
 
       case 'drush':
+        // Start drush batch process.
         batch_set($this->batch);
         $this->batch =& batch_get();
         $this->batch['progressive'] = FALSE;
@@ -55,6 +57,7 @@ class Batch {
         break;
 
       case 'backend':
+        // Start backend batch process.
         batch_set($this->batch);
         $this->batch =& batch_get();
         $this->batch['progressive'] = FALSE;
@@ -62,6 +65,9 @@ class Batch {
         break;
 
       case 'nobatch':
+        // Call each batch operation the way the Drupal batch API would do, but
+        // within one process (so in fact not using batch API here, just
+        // mimicking it to avoid code duplication.
         $context = [];
         foreach($this->batch['operations'] as $i => $operation) {
           $operation[1][] = &$context;
@@ -92,7 +98,8 @@ class Batch {
    * @param array &$context
    */
   public static function generateBundleUrls($entity_info, $batch_info, &$context) {
-    \Drupal::service('simple_sitemap.batch_url_generator')->generateBundleUrls($entity_info, $batch_info, $context);
+    BatchUrlGenerator::service()
+      ->generateBundleUrls($entity_info, $batch_info, $context);
   }
 
   /**
@@ -103,7 +110,8 @@ class Batch {
    * @param array &$context
    */
   public static function generateCustomUrls($custom_paths, $batch_info, &$context) {
-    \Drupal::service('simple_sitemap.batch_url_generator')->generateCustomUrls($custom_paths, $batch_info, $context);
+    BatchUrlGenerator::service()
+      ->generateCustomUrls($custom_paths, $batch_info, $context);
   }
 
   /**
@@ -114,6 +122,7 @@ class Batch {
    * @param $operations
    */
   public static function finishGeneration($success, $results, $operations) {
-    \Drupal::service('simple_sitemap.batch_url_generator')->finishGeneration($success, $results, $operations);
+    BatchUrlGenerator::service()
+        ->finishGeneration($success, $results, $operations);
   }
 }

src/Batch/BatchUrlGenerator.php

@@ -25,6 +25,15 @@ class BatchUrlGenerator {
   protected $entityQuery;
   protected $anonUser;
 
+  /**
+   * BatchUrlGenerator constructor.
+   *
+   * @param $sitemap_generator
+   * @param $language_manager
+   * @param $entity_type_manager
+   * @param $path_validator
+   * @param $entity_query
+   */
   public function __construct(
     $sitemap_generator,
     $language_manager,
@@ -40,6 +49,22 @@ class BatchUrlGenerator {
     $this->anonUser = $this->entityTypeManager->getStorage('user')->load(self::ANONYMOUS_USER_ID);
   }
 
+  /**
+   * The Drupal batch API can only call procedural functions or static methods.
+   * To circumvent exclusively procedural code, on every batch iteration this
+   * static method is called by the batch API and returns a freshly created
+   * Drupal service object of this class. All following calls can be made on
+   * the returned service the OOP way. This is is obviously trading performance
+   * for cleanness. The service is created within its own class to improve
+   * testability.
+   *
+   * @return object
+   *   Symfony service object of this class
+   */
+  public static function service() {
+    return \Drupal::service('simple_sitemap.batch_url_generator');
+  }
+
   /**
    * @param $batch_info
    * @return bool

src/Simplesitemap.php

@@ -351,8 +351,9 @@ class Simplesitemap {
    *
    * @return string|false
    *  If no sitemap id provided, either a sitemap index is returned, or the
-   *  whole sitemap, if the amount of links does not exceed the max links setting.
-   *  If a sitemap id is provided, a sitemap chunk is returned.
+   *  whole sitemap, if the amount of links does not exceed the max links
+   *  setting. If a sitemap id is provided, a sitemap chunk is returned. False
+   *  if sitemap is not retrievable from the database.
    */
   public function getSitemap($chunk_id = NULL) {
     $chunks = $this->fetchSitemapChunks();