Add submission handler

compose.module

@@ -266,6 +266,7 @@ function compose_inline_entity_form_reference_form_alter(&$reference_form, &$for
         '#type' => 'submit',
         '#value' => t('Select'),
         '#name' => 'ief-reference-add-existing-' . $entity_id,
+        '#ief_row_delta' => $i,
         '#limit_validation_errors' => [$reference_form['#parents']],
         '#id' => 'entity-id-'. $entity_id,
         '#attributes' => [
@@ -282,8 +283,8 @@ function compose_inline_entity_form_reference_form_alter(&$reference_form, &$for
           ],
         ],
       ];
+      $reference_form['entity_table'][$i]['button']['#submit'][] = 'Drupal\compose\Plugin\Field\FieldWidget\ComposeWidget::addExistingSubmit';
       $reference_form['entity_table'][$i]['button']['#submit'][] = 'Drupal\inline_entity_form\Plugin\Field\FieldWidget\InlineEntityFormComplex::submitCloseRow';
-      InlineEntityFormComplex::addSubmitCallbacks($reference_form['entity_table'][$i]['button']);
 
       $i++;
     }

src/Plugin/Field/FieldWidget/ComposeWidget.php

@@ -86,20 +86,13 @@ class ComposeWidget extends InlineEntityFormComplex implements ContainerFactoryP
     if ($entity_id !== NULL && $entity_type !== NULL) {
       $storage = \Drupal::entityTypeManager()->getStorage($entity_type);
       $entity = $storage->load($entity_id);
-      $entities = &$form_state->get(['inline_entity_form', $ief_id, 'entities']);
+      $entities = $form_state->get(['inline_entity_form', $ief_id, 'entities']);
       // Determine the correct weight of the new element.
       $weight = 0;
       if ($entities) {
         $weight = max(array_keys($entities)) + 1;
       }
 
-      $entities[] = [
-        'entity' => $entity,
-        'weight' => $weight,
-        'form' => NULL,
-        'needs_save' => FALSE,
-      ];
-
       $element['entities'][] = [
         '#entity' => $entity,
         '#label' => $entity->label(),
@@ -113,9 +106,6 @@ class ComposeWidget extends InlineEntityFormComplex implements ContainerFactoryP
 
       // Pass along the IEF form id, field name and entity type.
       $element['entities']['#form_id'] = $ief_id;
-
-      $form_state->set(['inline_entity_form', $ief_id, 'entities'], $entities);
-      $form_state->setCached(TRUE);
     }
 
     return $element;
@@ -165,4 +155,46 @@ class ComposeWidget extends InlineEntityFormComplex implements ContainerFactoryP
       }
     }
   }
+
+  /**
+   * Button #submit callback: Add existing Compose buttons.
+   *
+   * @param $form
+   *   The complete parent form.
+   * @param $form_state
+   *   The form state of the parent form.
+   */
+  public static function addExistingSubmit($form, FormStateInterface $form_state) {
+    $element = inline_entity_form_get_element($form, $form_state);
+    $ief_id = $element['#ief_id'];
+    $triggering_element = $form_state->getTriggeringElement();
+    $button_name = $triggering_element['#name'];
+    $instance = $form_state->get(['inline_entity_form', $ief_id, 'instance']);
+    $entity_type = $instance->getSetting('target_type');
+
+    if (strpos($button_name, 'ief-reference-add-existing-') !== FALSE) {
+      $entity_id = str_replace('ief-reference-add-existing-', '', $button_name);
+    }
+
+    if ($entity_id !== NULL && $entity_type !== NULL) {
+      $storage = \Drupal::entityTypeManager()->getStorage($entity_type);
+      $entity = $storage->load($entity_id);
+      $entities = &$form_state->get(['inline_entity_form', $ief_id, 'entities']);
+      // Determine the correct weight of the new element.
+      $weight = 0;
+      if ($entities) {
+        $weight = max(array_keys($entities)) + 1;
+      }
+
+      $entities[] = [
+        'entity' => $entity,
+        'weight' => $weight,
+        'form' => NULL,
+        'needs_save' => FALSE,
+      ];
+
+      $form_state->set(['inline_entity_form', $ief_id, 'entities'], $entities);
+      $form_state->setRebuild();
+    }
+  }
 }