Swift:当SearchBar处于活动状态时，应按两次后退按钮

class ViewController: UIViewController, UISearchBarDelegate {
    // ...
    searchBar.delegate = self
    // ...
}

func searchBarCancelButtonClicked(_ searchBar: UISearchBar) {
    searchBar.text = "" // 清空搜索栏的文本
    searchBar.resignFirstResponder() // 放弃第一响应者状态，隐藏键盘
}

override func pressesBegan(_ presses: Set<UIPress>, with event: UIPressesEvent?) {
    super.pressesBegan(presses, with: event)
    
    guard let key = presses.first?.key else { return }
    
    if key.keyCode == .keyboardDelete {
        if searchBar.isFirstResponder {
            searchBarCancelButtonClicked(searchBar)
        }
    }
}