app\widgets\Alert::init PHP Method

    public function init()
    {
        parent::init();
        $session = \Yii::$app->getSession();
        $flashes = $session->getAllFlashes();
        $appendCss = isset($this->options['class']) ? ' ' . $this->options['class'] : '';
        foreach ($flashes as $type => $data) {
            if (isset($this->alertTypes[$type])) {
                $data = (array) $data;
                foreach ($data as $message) {
                    /* initialize css class for each alert box */
                    $this->options['class'] = $this->alertTypes[$type] . $appendCss;
                    /* assign unique id to each alert box */
                    $this->options['id'] = $this->getId() . '-' . $type;
                    echo \yii\bootstrap\Alert::widget(['body' => $message, 'closeButton' => $this->closeButton, 'options' => $this->options]);
                }
                $session->removeFlash($type);
            }
        }
    }