在UISearchController处于活动状态时显示UIAlertController

class ViewController: UIViewController, UISearchControllerDelegate {
    var searchController: UISearchController!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 创建UISearchController对象
        searchController = UISearchController(searchResultsController: nil)
        searchController.delegate = self
        
        // 设置UISearchController的属性
        
        // ...
    }
    
    func willPresentSearchController(_ searchController: UISearchController) {
        // 当搜索框处于活动状态时，显示UIAlertController
        
        // 创建UIAlertController对象
        let alertController = UIAlertController(title: "提示", message: "搜索框处于活动状态", preferredStyle: .alert)
        
        // 添加关闭按钮
        let closeAction = UIAlertAction(title: "关闭", style: .default, handler: nil)
        alertController.addAction(closeAction)
        
        // 呈现UIAlertController
        present(alertController, animated: true, completion: nil)
    }
}