Nette\Forms\Controls\BaseControl::setRequired PHP Method

    public function setRequired($value = TRUE)
    {
        $this->rules->setRequired($value);
        return $this;
    }

 /**
  * @param \Nette\Forms\Controls\BaseControl $control
  * @param string|null $errorMessage
  * @param bool|callable $requiring
  */
 private function resolveRequiring(\Nette\Forms\Controls\BaseControl $control, $errorMessage = null, $requiring = false)
 {
     if ($requiring === true) {
         $control->setRequired($errorMessage !== null ? $errorMessage : true);
     } elseif (is_callable($requiring)) {
         $condition = $requiring($control);
         if (!$condition instanceof \Nette\Forms\Rules) {
             throw new RequiredFormFieldResolutionException('Requiring call must return instance of \\Nette\\Forms\\Rules.');
         }
         $condition->setRequired($errorMessage !== null ? $errorMessage : true);
     }
 }