Issue #3412075 by erik_petra: File size for Media transform

src/Plugin/Transform/Field/FileUrlSizeTransform.php

+<?php
+
+namespace Drupal\transform_api\Plugin\Transform\Field;
+
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\File\FileUrlGenerator;
+use Drupal\file\Entity\File;
+use Drupal\transform_api\FieldTransformBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Transform field plugin for file field types as Urls.
+ *
+ * @FieldTransform(
+ *  id = "file_url_size",
+ *  label = @Translation("File URL and Size"),
+ *  field_types = {
+ *    "file"
+ *  }
+ * )
+ */
+class FileUrlSizeTransform extends FieldTransformBase {
+
+  /**
+   * File url generator.
+   *
+   * @var \Drupal\Core\File\FileUrlGenerator
+   */
+  protected $fileUrlGenerator;
+
+  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $transform_mode, array $third_party_settings, FileUrlGenerator $file_url_generator) {
+    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $transform_mode, $third_party_settings);
+
+    $this->fileUrlGenerator = $file_url_generator;
+  }
+
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['transform_mode'], $configuration['third_party_settings'], $container->get('file_url_generator'));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transformElements(FieldItemListInterface $items, $langcode) {
+    $values = [];
+    /** @var \Drupal\Core\Field\FieldItemInterface $item */
+    foreach ($items as $item) {
+      if (!empty($item->getValue()['target_id'])) {
+        // File ID.
+        $fid = $item->getValue()['target_id'];
+        // Load file.
+        $file = File::load($fid);
+        $file_uri = $file->getFileUri();
+
+        $values[] = [
+          'url' => $this->fileUrlGenerator->generateString($file_uri),
+          'filesize' => $file->getSize(),
+        ];
+      }
+      else {
+        $values[] = NULL;
+      }
+    }
+    return $values;
+  }
+
+}