Zend 表单验证不起作用

$inputFilter = new InputFilter();
$inputFilter->add([
    'name' => 'username',
    'required' => true,
    'filters' => [
        ['name' => 'StringTrim'],
    ],
    'validators' => [
        [
            'name' => 'StringLength',
            'options' => [
                'min' => 6,
                'max' => 255,
            ],
        ],
    ],
]);

$form = new Form();
$form->setInputFilter($inputFilter);

public function submitAction()
{
    $form = new Form();
    $form->setInputFilter($inputFilter);

    $request = $this->getRequest();
    if ($request->isPost()) {
        $form->setData($request->getPost());
        if ($form->isValid()) {
            // 处理表单数据
        }
    }

    return new ViewModel([
        'form' => $form,
    ]);
}

<?php
$form = $this->form;
$form->setAttribute('action', $this->url('submit'));
$form->prepare();

echo $this->form()->openTag($form);
echo $this->formHidden($form->get('csrf'));
echo $this->formRow($form->get('username'));
echo $this->formSubmit($form->get('submit'));
echo $this->form()->closeTag();
?>