ENH Pass form validation result to client

client/src/legacy/LeftAndMain.js

@@ -492,13 +492,12 @@ $.entwine('ss', function($) {
       // Save tab selections so we can restore them later
       this.saveTabState();
 
-
       // Standard Pjax behaviour is to replace the submitted form with new content.
       // The returned view isn't always decided upon when the request
       // is fired, so the server might decide to change it based on its own logic,
       // sending back different `X-Pjax` headers and content
       jQuery.ajax(jQuery.extend({
-        headers: {"X-Pjax" : "CurrentForm,Breadcrumbs"},
+        headers: {"X-Pjax" : "CurrentForm,Breadcrumbs,ValidationResult"},
         url: form.attr('action'),
         data: formData,
         type: 'POST',

code/LeftAndMain.php

@@ -990,6 +990,12 @@ public function getResponseNegotiator()
                             'SilverStripe\\Admin\\CMSBreadcrumbs'
                         ]);
                     },
+                    'ValidationResult' => function () {
+                        return $this->prepareDataForPjax([
+                            'isValid' => true,
+                            'messages' => []
+                        ]);
+                    },
                     'default' => function () {
                         return $this->renderWith($this->getViewer('show'));
                     }
@@ -1438,6 +1444,12 @@ public function getEditForm($id = null, $fields = null)
                 return $negotiator->respond($request, array(
                     'CurrentForm' => function () use ($result) {
                         return $result;
+                    },
+                    'ValidationResult' => function () use ($errors) {
+                        return $this->prepareDataForPjax([
+                            'isValid' => $errors->isValid(),
+                            'messages' => $errors->getMessages()
+                        ]);
                     }
                 ));
             }
@@ -1476,6 +1488,20 @@ public function getEditForm($id = null, $fields = null)
         return $form;
     }
 
+    /**
+     * Convert and arry of data to JSON and wrap it in an HTML tag as pjax is used and jQuery will parse this
+     * as an element on the client side in LeftAndMain.js handleAjaxResponse()
+     * The attribute type="application/json" denotes this is a data block and won't be processed by a browser
+     * https://html.spec.whatwg.org/#the-script-element
+     *
+     * @param array $data
+     * @return string
+     */
+    private function prepareDataForPjax(array $data): string
+    {
+        return '<script type="application/json">' . json_encode($data) . '</script>';
+    }
+
     /**
      * Returns a placeholder form, used by {@link getEditForm()} if no record is selected.
      * Our javascript logic always requires a form to be present in the CMS interface.
@@ -1658,11 +1684,11 @@ public function currentPageID()
         if (isset($this->urlParams['ID']) && is_numeric($this->urlParams['ID'])) {
             return $this->urlParams['ID'];
         }
-        
+
         if (is_numeric($this->getRequest()->param('ID'))) {
             return $this->getRequest()->param('ID');
         }
-        
+
         /** @deprecated */
         $session = $this->getRequest()->getSession();
         return $session->get($this->sessionNamespace() . ".currentPage") ?: null;