hasChanged method not working in Form ?

I have a model call ArticleCategory, I have slug field must be unique. I set keepSnapshot equal true. Here is my EditForm init code (Edit form of 1 category):

public function initialize(ArticleCategory $category) {
    // Slug
    $slug = (new Text('slug', [
        'class' => 'form-control',
        'placeholder' => 'Slug'
    ]))->addValidator(new PresenceOf([
            'message' => 'Slug is required'
    ]));
    if($category->hasChanged('slug')){
        $slug->addValidator(new Uniqueness([
            'message' => 'Slug exists',
            'field' => 'slug',
            'model' => 'App\Models\Entities\ArticleCategory'
        ]));
    }
    $slug->addFilter('trim');
    $this->add($slug);
    // other element
}

Controller code:

public function editAction($id) {
    // Get article category by id
    $articleCategory = $this->_articleCategoryService->getById($id);
    if (!$articleCategory) {
        return $this->show404();
    }
    $editForm = new EditForm($articleCategory);
    // Post
    if ($this->request->isPost()) {
        $postData = $this->request->getPost();
        if (!$editForm->isValid($postData)) {
            $editForm->setDefaultsFromObject($articleCategory);
        } else {
            // Form valid
            $editForm->bind($postData, $articleCategory);
            if ($articleCategory->save()) {
                $this->flash->success("Save success.");
                return $this->redirectToIndex();
            } else {
                $this->flash->error("Save fail");
            }
        }
    }
    $this->view->editForm = $editForm;
}

I try to change slug with an slug that exist in db, the category alway save success, not validate unique because the hasChanged return false. My question: How can I add uniqueness validator in EditForm class when slug field change ??? Thank alot !