Added a drupal alter to allow for other modules to modify the datastream edit registry after it has been built.

includes/datastream.inc

@@ -295,6 +295,12 @@ function islandora_datastream_get_url(AbstractDatastream $datastream, $type = 'd
  */
 function islandora_edit_datastream(AbstractDatastream $datastream) {
   $edit_registry = module_invoke_all('islandora_edit_datastream_registry', $datastream->parent, $datastream);
+  $context = array(
+    'datastream_parent' => $datastream->parent,
+    'datastream' => $datastream,
+    'original_edit_registry' => $edit_registry,
+  );
+  drupal_alter(ISLANDORA_EDIT_DATASTREAM_MODIFY_REGISTRY_HOOK, $edit_registry, $context);
   $edit_count = count($edit_registry);
   switch ($edit_count) {
     case 0:
@@ -305,7 +311,8 @@ function islandora_edit_datastream(AbstractDatastream $datastream) {
 
     case 1:
       // One registry implementation, go there.
-      drupal_goto($edit_registry[0]['url']);
+      $edit_registry = reset($edit_registry);
+      drupal_goto($edit_registry['url']);
       break;
 
     default:

islandora.api.php

@@ -849,3 +849,28 @@ function hook_islandora_get_breadcrumb_query_predicates() {
     'someotherpredicate',
   );
 }
+
+/**
+ * Use alter hook to modify registry paths before the paths are rendered.
+ *
+ * @param array $edit_registry
+ *   The array of registry paths.
+ * @param array $context
+ *   An associative array containing:
+ *   - datastream_parent: The datastream parent.
+ *   - datastream: The datastream being edited.
+ *   - original_edit_registry: The original edit_registry prior to any
+ *     modifications.
+ */
+function hook_islandora_edit_datastream_modify_registry_alter(&$edit_registry, $context) {
+  // Example: Remove xml form builder edit registry.
+  if (isset($edit_registry['xml_form_builder_edit_form_registry'])) {
+    unset($edit_registry['xml_form_builder_edit_form_registry']);
+  }
+  // Add custom form to replace the removed form builder edit_form.
+  $edit_registry['somemodule_custom_form'] =  array(
+    'name' => t('Somemodule Custom Form'),
+    'url' => "islandora/custom_form/{$context['datastream_parent']->id}/{$context['datastream']->id}",
+    'weight' => 1,
+  );
+}

islandora.module

@@ -52,6 +52,7 @@ define('ISLANDORA_UPDATE_RELATED_OBJECTS_PROPERTIES_HOOK', 'islandora_update_rel
 define('ISLANDORA_METADATA_OBJECT_ALTER', 'islandora_metadata_object');
 define('ISLANDORA_METADATA_OBJECT_DESCRIPTION_ALTER', 'islandora_metadata_object_description');
 define('ISLANDORA_BREADCRUMB_FILTER_PREDICATE_HOOK', 'islandora_get_breadcrumb_query_predicates');
+define('ISLANDORA_EDIT_DATASTREAM_MODIFY_REGISTRY_HOOK', 'islandora_edit_datastream_modify_registry');
 
 // @todo Add Documentation.
 define('ISLANDORA_OBJECT_INGESTED_HOOK', 'islandora_object_ingested');