Understanding the validation in the widget?

Question

Yii

I use this widget https://github.com/unclead/yii2-multiple-input/wik... I have a limit in the line, there must be 50 characters. And if the user writes more than 50 characters in the input, for example 70 characters, then my page reloads and displays this entry on a white background )
But how to make it so that when he drove in the same page and went to this page or ajax checked the data for validity and immediately indicated this error
Here is the code in the controller

public function actionCreate()
    {
        $notification = $this->findNotification();
        $models = [new Custom()];
        $request = Yii::$app->getRequest();
        if ($request->isPost && $request->post('ajax') !== null){
            $data = Yii::$app->request->post('Custom', []);
            foreach (array_keys($data) as $index) {
                $model[$index] = new Custom();
            }
            Model::loadMultiple($models, Yii::$app->request->post());
            Yii::$app->response->format = Response::FORMAT_JSON;
            $result = ActiveForm::validateMultiple($models);
            return $result;
        }

        if(Model::loadMultiple($models, Yii::$app->request->post())) {
            foreach ($models as $custom) {
                if ($custom->save()){
                    return $this->redirect(Yii::$app->request->referrer);
                } else{
                   print_r($custom->getErrors());
                }
            }
        } else {
            return $this->render('create', [
               'models' => $models,
            ]);
        }
    }

Answer 1

use ajax validation:
reading on tostra
reading on yandex