Issue #3171743 by chr.fritsch, paulocs, Berdir, phenaproxima: Not possible to overwrite the upload forms for media library

core/modules/media_library/media_library.module

@@ -84,12 +84,22 @@ function media_library_help($route_name, RouteMatchInterface $route_match) {
  * Implements hook_media_source_info_alter().
  */
 function media_library_media_source_info_alter(array &$sources) {
+  if (empty($sources['audio_file']['forms']['media_library_add'])) {
     $sources['audio_file']['forms']['media_library_add'] = FileUploadForm::class;
+  }
+  if (empty($sources['file']['forms']['media_library_add'])) {
     $sources['file']['forms']['media_library_add'] = FileUploadForm::class;
+  }
+  if (empty($sources['image']['forms']['media_library_add'])) {
     $sources['image']['forms']['media_library_add'] = FileUploadForm::class;
+  }
+  if (empty($sources['video_file']['forms']['media_library_add'])) {
     $sources['video_file']['forms']['media_library_add'] = FileUploadForm::class;
+  }
+  if (empty($sources['oembed:video']['forms']['media_library_add'])) {
     $sources['oembed:video']['forms']['media_library_add'] = OEmbedForm::class;
   }
+}
 
 /**
  * Implements hook_theme().

core/modules/media_library/tests/modules/media_library_form_overwrite_test/media_library_form_overwrite_test.info.yml

@@ -0,0 +1,10 @@
+name: 'Media Library Form Overwrite test'
+type: module
+description: 'Test module for Media Library.'
+package: Testing
+dependencies:
+  - drupal:image
+  - drupal:media_library
+  - drupal:menu_ui
+  - drupal:node
+  - drupal:path

core/modules/media_library/tests/modules/media_library_form_overwrite_test/media_library_form_overwrite_test.module

@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * @file
+ * Contains.
+ */
+
+use Drupal\media_library_form_overwrite_test\Form\TestAddForm;
+
+function media_library_form_overwrite_test_media_source_info_alter(array &$sources) {
+  $sources['image']['forms']['media_library_add'] = TestAddForm::class;
+}

core/modules/media_library/tests/modules/media_library_form_overwrite_test/src/Form/TestAddForm.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\media_library_form_overwrite_test\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\media_library\Form\AddFormBase;
+
+/**
+ * Test add form.
+ */
+class TestAddForm extends AddFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildInputElement(array $form, FormStateInterface $form_state) {
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'test_add_form';
+  }
+
+}

core/modules/media_library/tests/src/Kernel/MediaLibraryAddFormTest.php

@@ -7,6 +7,7 @@ use Drupal\KernelTests\KernelTestBase;
 use Drupal\media_library\Form\FileUploadForm;
 use Drupal\media_library\Form\OEmbedForm;
 use Drupal\media_library\MediaLibraryState;
+use Drupal\media_library_form_overwrite_test\Form\TestAddForm;
 use Drupal\Tests\media\Traits\MediaTypeCreationTrait;
 use Drupal\Tests\user\Traits\UserCreationTrait;
 
@@ -137,4 +138,19 @@ class MediaLibraryAddFormTest extends KernelTestBase {
     \Drupal::formBuilder()->buildForm(FileUploadForm::class, $form_state);
   }
 
+  /**
+   * Tests overwriting of the add form.
+   */
+  public function testDifferentAddForm() {
+    $this->enableModules(['media_library_form_overwrite_test']);
+
+    $entity_type_manager = \Drupal::entityTypeManager();
+    $image = $entity_type_manager->getStorage('media_type')->load('image');
+
+    $image_source_definition = $image->getSource()->getPluginDefinition();
+
+    // Assert the overwritten form class is set to the media source.
+    $this->assertSame(TestAddForm::class, $image_source_definition['forms']['media_library_add']);
+  }
+
 }