Replace CMSForm use with new setValidationResponseCallback() API

Preparing for form schema API, see silverstripe#4938

admin/code/LeftAndMain.php

@@ -13,6 +13,8 @@
  *
  * This is essentially an abstract class which should be subclassed.
  * See {@link CMSMain} for a good example.
+ *
+ * @property FormSchema $schema
  */
 class LeftAndMain extends Controller implements PermissionProvider {
 
@@ -197,7 +199,7 @@ public function schema() {
 		}
 
 		// Make sure it's an AJAX GET request with a valid "X-Formschema-Request" header value.
-		if (!$req->isAjax() || !$req->isGET() || !count($schemaParts)) {
+		if (!$req->isGET() || !count($schemaParts)) {
 			throw new SS_HTTPResponse_Exception(
 				'Invalid request. Check you\'ve set a "X-Formschema-Request" header with "schema" or "state" values.',
 				400
@@ -1312,14 +1314,27 @@ public function getEditForm($id = null, $fields = null) {
 			$actionsFlattened = $actions->dataFields();
 			if($actionsFlattened) foreach($actionsFlattened as $action) $action->setUseButtonTag(true);
 
-			$form = CMSForm::create(
+			$negotiator = $this->getResponseNegotiator();
+			$form = Form::create(
 				$this, "EditForm", $fields, $actions
 			)->setHTMLID('Form_EditForm');
-			$form->setResponseNegotiator($this->getResponseNegotiator());
 			$form->addExtraClass('cms-edit-form');
 			$form->loadDataFrom($record);
 			$form->setTemplate($this->getTemplatesWithSuffix('_EditForm'));
 			$form->setAttribute('data-pjax-fragment', 'CurrentForm');
+			$form->setValidationResponseCallback(function() use ($negotiator, $form) {
+				$request = $this->getRequest();
+				if($request->isAjax() && $negotiator) {
+					$form->setupFormErrors();
+					$result = $form->forTemplate();
+
+					return $negotiator->respond($request, array(
+						'CurrentForm' => function() use($result) {
+							return $result;
+						}
+					));
+				}
+			});
 
 			// Announce the capability so the frontend can decide whether to allow preview or not.
 			if(in_array('CMSPreviewable', class_implements($record))) {
@@ -1368,7 +1383,7 @@ public function getEditForm($id = null, $fields = null) {
 	 * @return Form
 	 */
 	public function EmptyForm() {
-		$form = CMSForm::create(
+		$form = Form::create(
 			$this,
 			"EditForm",
 			new FieldList(
@@ -1387,7 +1402,6 @@ public function EmptyForm() {
 			),
 			new FieldList()
 		)->setHTMLID('Form_EditForm');
-		$form->setResponseNegotiator($this->getResponseNegotiator());
 		$form->unsetValidator();
 		$form->addExtraClass('cms-edit-form');
 		$form->addExtraClass('root-form');

admin/code/ModelAdmin.php

@@ -138,13 +138,12 @@ public function getEditForm($id = null, $fields = null) {
 			$listField->getConfig()->getComponentByType('GridFieldDetailForm')->setValidator($detailValidator);
 		}
 
-		$form = CMSForm::create(
+		$form = Form::create(
 			$this,
 			'EditForm',
 			new FieldList($listField),
 			new FieldList()
 		)->setHTMLID('Form_EditForm');
-		$form->setResponseNegotiator($this->getResponseNegotiator());
 		$form->addExtraClass('cms-edit-form cms-panel-padded center');
 		$form->setTemplate($this->getTemplatesWithSuffix('_EditForm'));
 		$editFormAction = Controller::join_links($this->Link($this->sanitiseClassName($this->modelClass)), 'EditForm');

admin/code/SecurityAdmin.php

@@ -177,13 +177,12 @@ public function getEditForm($id = null, $fields = null) {
 
 		$actions = new FieldList();
 
-		$form = CMSForm::create(
+		$form = Form::create(
 			$this,
 			'EditForm',
 			$fields,
 			$actions
 		)->setHTMLID('Form_EditForm');
-		$form->setResponseNegotiator($this->getResponseNegotiator());
 		$form->addExtraClass('cms-edit-form');
 		$form->setTemplate($this->getTemplatesWithSuffix('_EditForm'));
 		// Tab nav in CMS is rendered through separate template

forms/Form.php

@@ -79,6 +79,11 @@ class Form extends RequestHandler {
 	 */
 	protected $validator;
 
+	/**
+	 * @var callable {@see setValidationResponseCallback()}
+	 */
+	protected $validationResponseCallback;
+
 	/**
 	 * @var string
 	 */
@@ -479,16 +484,45 @@ public function checkAccessAction($action) {
 		);
 	}
 
+	/**
+	 * @return callable
+	 */
+	public function getValidationResponseCallback() {
+		return $this->validationResponseCallback;
+	}
+
+	/**
+	 * Overrules validation error behaviour in {@link httpSubmission()}
+	 * when validation has failed. Useful for optional handling of a certain accepted content type.
+	 *
+	 * The callback can opt out of handling specific responses by returning NULL,
+	 * in which case the default form behaviour will kick in.
+	 *
+	 * @param $callback
+	 * @return self
+	 */
+	public function setValidationResponseCallback($callback) {
+		$this->validationResponseCallback = $callback;
+
+		return $this;
+	}
+
 	/**
 	 * Returns the appropriate response up the controller chain
 	 * if {@link validate()} fails (which is checked prior to executing any form actions).
 	 * By default, returns different views for ajax/non-ajax request, and
 	 * handles 'application/json' requests with a JSON object containing the error messages.
-	 * Behaviour can be influenced by setting {@link $redirectToFormOnValidationError}.
+	 * Behaviour can be influenced by setting {@link $redirectToFormOnValidationError},
+	 * and can be overruled by setting {@link $validationResponseCallback}.
 	 *
 	 * @return SS_HTTPResponse|string
 	 */
 	protected function getValidationErrorResponse() {
+		$callback = $this->getValidationResponseCallback();
+		if($callback && $callbackResponse = $callback()) {
+			return $callbackResponse;
+		}
+
 		$request = $this->getRequest();
 		if($request->isAjax()) {
 				// Special case for legacy Validator.js implementation