Custom element type for mapping available webform fields to other data

@damienmckenna Yep the WebformMapping element could be extended to create a WebformElementsMapping.

My long-term dream is to have a WebformHandler that maps a webform's submission data to any entity's fields. There is even a prototype of the UI.

+++ b/src/Element/WebformValueMapping.php
@@ -0,0 +1,77 @@
+    // Get all of the custom fields on this Webform.
+    $webform = $this->getWebform();
+    $elements = $webform->getElementsInitializedFlattenedAndHasValue('view');
+    foreach ($elements as $key => $element) {
+      $fields[$key] = $this->t("@title (%type)", [
+          '@title' => ($element['#admin_title'] ?: $element['#title'] ?: $key),
+          '%type' => (isset($element['#type']) ? $element['#type'] : ''),
+        ]);
+    }
+
+    // List all of the fields on the base entity.
+    $submission_storage = \Drupal::entityTypeManager()
+      ->getStorage('webform_submission');
+    $field_definitions = $submission_storage->getFieldDefinitions();
+    $field_definitions = $submission_storage->checkFieldDefinitionAccess($webform, $field_definitions);
+    foreach ($field_definitions as $key => $field_definition) {
+      $fields[$key] = $this->t("@title (%type)", [
+          '@title' => $field_definition['title'],
+          '%type' => $field_definition['type'],
+        ]);
+    }

If a field definition key matches an element key, it will overwrite the element's definition. The existing WebformExcludedColumns element has the same problem.

The solution I have used in other places is to prefix element keys with 'element__' in \Drupal\webform\Plugin\WebformElementBase::getTableColumn.

We probably need to dedicated ticket to fix the excluded columns issues.

Maybe the new element's name should WebformSubmissionMapping?

This would allow the WebformSubmissionMapping element to be extended to create a WebformSubmissionToMyCrmMapping element.

@DamienMcKenna I am not sure this type of element is immediately needed in the Webform (core) module maybe this code can be developed as an experimental module.