如何动态设置CodeIgniter的表单验证规则？

$validation_rules = array(
    'field_name' => 'required|trim',
    'another_field_name' => 'required|trim'
);

$this->form_validation->set_rules($validation_rules);

if ($this->form_validation->run() == FALSE) {
    // 显示表单
} else {
    // 处理表单数据
}