Nette\Forms\Rendering\DefaultFormRenderer::renderBegin PHP Method

    public function renderBegin()
    {
        $this->counter = 0;
        foreach ($this->form->getControls() as $control) {
            $control->setOption('rendered', FALSE);
        }
        if ($this->form->isMethod('get')) {
            $el = clone $this->form->getElementPrototype();
            $query = parse_url($el->action, PHP_URL_QUERY);
            $el->action = str_replace("?{$query}", '', $el->action);
            $s = '';
            foreach (preg_split('#[;&]#', $query, NULL, PREG_SPLIT_NO_EMPTY) as $param) {
                $parts = explode('=', $param, 2);
                $name = urldecode($parts[0]);
                if (!isset($this->form[$name])) {
                    $s .= Html::el('input', ['type' => 'hidden', 'name' => $name, 'value' => urldecode($parts[1])]);
                }
            }
            return $el->startTag() . ($s ? "\n\t" . $this->getWrapper('hidden container')->setHtml($s) : '');
        } else {
            return $this->form->getElementPrototype()->startTag();
        }
    }

 public function renderBegin()
 {
     $this->controlsInit();
     return parent::renderBegin();
 }