无法在UISearchBar

Question

    self.searchbar = [[UISearchBar alloc] initWithFrame:CGRectMake(0, 0, 300, 50)];
    self.searchbar.delegate = self;
    UITextField *field = (UITextField *)[[[self.searchbar.subviews firstObject] subviews] lastObject];
    field.clearButtonMode = UITextFieldViewModeWhileEditing;
    field.returnKeyType = UIReturnKeyDone;
    [self.view addSubview:self.searchbar];

- (void)searchBar:(UISearchBar *)searchBar textDidChange:(NSString *)searchText {
    if (searchText.length == 0) {
        [self.searchbar resignFirstResponder];
    }
}

我确定搜索栏代表已经离开;如果我逐个删除字符，它将resignFirstResponder，但如果我按下符号＆＃39; x＆＃39;，它也会出现在此方法中，但它不会resignFirstResponder。为什么会这样？