API Move subsite dropdown logic for folders into FolderFormFactoryExtension

_config/extensions.yml

@@ -28,6 +28,10 @@ SilverStripe\Assets\File:
   extensions:
     - SilverStripe\Subsites\Extensions\FileSubsites
 
+SilverStripe\AssetAdmin\Forms\FolderFormFactory:
+  extensions:
+    - SilverStripe\Subsites\Extensions\FolderFormFactoryExtension
+
 SilverStripe\ErrorPage\ErrorPage:
   extensions:
     - SilverStripe\Subsites\Extensions\ErrorPageSubsite

css/LeftAndMain_Subsites.css

@@ -76,7 +76,7 @@ body.SubsiteAdmin .right form #URL .fieldgroup * {
     display: none;
 }
 
-#Root_DetailsView .subsites-move-dropdown,
+.editor__details .subsites-move-dropdown,
-#Form_ItemEditForm .subsites-move-dropdown {
+#Form_fileEditorForm.subsites-move-dropdown {
     display: block;
 }

src/Extension/FileSubsites.php

@@ -2,10 +2,6 @@
 
 namespace SilverStripe\Subsites\Extensions;
 
-use SilverStripe\Assets\Folder;
-use SilverStripe\Forms\DropdownField;
-use SilverStripe\Forms\FieldList;
-use SilverStripe\Forms\LiteralField;
 use SilverStripe\ORM\DataExtension;
 use SilverStripe\ORM\DataQuery;
 use SilverStripe\ORM\Queries\SQLSelect;
@@ -41,43 +37,6 @@ class FileSubsites extends DataExtension
         return $this->owner->Title;
     }
 
-    /**
-     * Add subsites-specific fields to the folder editor.
-     * @param FieldList $fields
-     */
-    public function updateCMSFields(FieldList $fields)
-    {
-        if ($this->owner instanceof Folder) {
-            $sites = Subsite::accessible_sites('CMS_ACCESS_AssetAdmin');
-            $values = [];
-            $values[0] = _t(__CLASS__ . '.AllSitesDropdownOpt', 'All sites');
-            foreach ($sites as $site) {
-                $values[$site->ID] = $site->Title;
-            }
-            ksort($values);
-            if ($sites) {
-                //Dropdown needed to move folders between subsites
-                /** @var @skipUpgrade */
-                $dropdown = new DropdownField(
-                    'SubsiteID',
-                    _t(__CLASS__ . '.SubsiteFieldLabel', 'Subsite'),
-                    $values
-                );
-                $dropdown->addExtraClass('subsites-move-dropdown');
-                $fields->push($dropdown);
-                $fields->push(new LiteralField(
-                    'Message',
-                    '<p class="message notice">' .
-                    _t(
-                        'ASSETADMIN.SUBSITENOTICE',
-                        'Folders and files created in the main site are accessible by all subsites.'
-                    )
-                    . '</p>'
-                ));
-            }
-        }
-    }
-
     /**
      * Update any requests to limit the results to the current site
      * @param SQLSelect $query
@@ -90,7 +49,8 @@ class FileSubsites extends DataExtension
         }
 
         // If you're querying by ID, ignore the sub-site - this is a bit ugly... (but it was WAYYYYYYYYY worse)
-        //@TODO I don't think excluding if SiteTree_ImageTracking is a good idea however because of the SS 3.0 api and ManyManyList::removeAll() changing the from table after this function is called there isn't much of a choice
+        // @TODO I don't think excluding if SiteTree_ImageTracking is a good idea however because of the SS 3.0 api and
+        // ManyManyList::removeAll() changing the from table after this function is called there isn't much of a choice
 
         $from = $query->getFrom();
         if (isset($from['SiteTree_ImageTracking']) || $query->filtersOnID()) {

src/Extension/FolderFormFactoryExtension.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace SilverStripe\Subsites\Extensions;
+
+use SilverStripe\Core\Extension;
+use SilverStripe\Forms\DropdownField;
+use SilverStripe\Forms\FieldList;
+use SilverStripe\Forms\LiteralField;
+use SilverStripe\Subsites\Model\Subsite;
+
+class FolderFormFactoryExtension extends Extension
+{
+    /**
+     * Add subsites-specific fields to the folder editor.
+     * @param FieldList $fields
+     */
+    public function updateFormFields(FieldList $fields)
+    {
+        $sites = Subsite::accessible_sites('CMS_ACCESS_AssetAdmin');
+        $values = [];
+        $values[0] = _t(__CLASS__ . '.AllSitesDropdownOpt', 'All sites');
+        foreach ($sites as $site) {
+            $values[$site->ID] = $site->Title;
+        }
+        ksort($values);
+        if ($sites) {
+            // Dropdown needed to move folders between subsites
+            /** @var @skipUpgrade */
+            $dropdown = DropdownField::create(
+                'SubsiteID',
+                _t(__CLASS__ . '.SubsiteFieldLabel', 'Subsite'),
+                $values
+            );
+            $dropdown->addExtraClass('subsites-move-dropdown');
+            $fields->push($dropdown);
+            $fields->push(LiteralField::create(
+                'Message',
+                '<p class="message notice">' .
+                _t(
+                    __CLASS__ . '.SUBSITENOTICE',
+                    'Folders and files created in the main site are accessible by all subsites.'
+                )
+                . '</p>'
+            ));
+        }
+    }
+}