如何验证Codeigniter中结合的多个字段？

$this->load->library('form_validation');

$this->form_validation->set_rules('username', 'Username', 'required');
$this->form_validation->set_rules('password', 'Password', 'required');

if ($this->form_validation->run() == FALSE)
{
    // 验证失败，显示表单页面
    $this->load->view('my_form');
}
else
{
    // 验证成功，处理表单数据
    $this->load->view('success_page');
}

<form action="<?php echo site_url('my_controller/my_method'); ?>" method="post">
   <label for="username">Username:</label>
   <input type="text" name="username" id="username" value="<?php echo set_value('username'); ?>" />
    <span class="error"><?php echo form_error('username'); ?></span>

   <label for="password">Password:</label>
   <input type="password" name="password" id="password" value="<?php echo set_value('password'); ?>" />
    <span class="error"><?php echo form_error('password'); ?></span>

   <input type="submit" value="Submit" />
</form>