Php 为什么 AJAX 请求没有通过 is_ajax_request() codeigniter 的条件

在表单提交时，我使用jQuery$.ajax发出一个AJAX请求:

$('form.sendMessage').off('submit').on('submit', function (e){
            e.preventDefault();
            var form = $('form.sendMessage');
        
            if (form.valid(){
                $.ajax({
                    url: base_url + <?=$moduleUrl?> + '/sendMessage',
                    data: form.serialize();
                    type: 'POST',
                    beforeSubmit: function () {
                        if (!confirm('Are you sure you want to send this message?')) {
                            return false;
                        }
                    },
                    success: function () {
                        toastr.success("Message sent", "Success");
                    },
                    error: function (response) {
                        toastr.error(response, "Error");
                    }
                });
            }
        )
        });

在控制器中，对于发出请求的sendMessage方法，我有一个条件，如果传递该条件，将继续到sendingMessage方法:

public function sendMessage()
    {
        $post = $this->input->post(null, true);

        if ($this->input->is_ajax_request() && !empty($post)) {
            try {
                $this->sendingMessage($post);
            } catch (Exception $e) {
                $this->output->set_status_header('401')
                    ->set_output($e->getMessage());
            }
        }
    }

private function sendingMessage($post)
    {
        if (!empty($post) && (int)$post['device'] > 0 && isset($post['message'])) {
            $this->load->model('protectee/device_messages');
            // save in db
            $data = [
                'device_id' => $post['device'],
                'user_id'   => $_SESSION['UserID'],
                'message'   => $post['message'],
                'date'      => date('Y-m-d H:i:s'),
            ];
            $messageId = $this->device_messages->insertMessage($data);

            if ($messageId > 0) {
                $this->insertCommand($post['device'], 'MSG', '<br>' . $post['message']);
            } else {
                throw new Exception('Message could not be send! Please try again..');
            }
        } else {
            throw new Exception('An error occured! Please try again..');
        }
    }

当我提交表单时，它转到一个空白页面，URL更改为.../sendMessage. 因此，我认为，它不符合成为AJAX_Requestif ($this-\>input-\>is_ajax_request() && !empty($post)的条件.