如何在执行计算密集型任务之前向视图中添加微调器

我有一个uitableviewcontroller,当我将一个特定的视图推到堆栈上时,需要花费很长时间,所以我想在移动之前向单元格中添加一个微调器。我遇到的问题是,在将新视图推送到控制器堆栈后,会添加微调器。但我认为信息是同步的?

因此,在转到下一个视图之前,如何使此微调器显示?提前谢谢!

- (void)
    tableView: (UITableView *) tableView
    didSelectRowAtIndexPath: (NSIndexPath *) indexPath {

    UITableViewCell *cell = [ self.tableView cellForRowAtIndexPath: indexPath ];

    if (cell.accessoryType == UITableViewCellAccessoryDisclosureIndicator) {
        UIActivityIndicatorView *activityIndicator = [ [ UIActivityIndicatorView alloc ] initWithFrame: CGRectMake(260.0, 10.0, 25.0, 25.0) ];
        activityIndicator.hidesWhenStopped = YES;
        activityIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleGray;

        [ cell addSubview: activityIndicator ];
        [ activityIndicator startAnimating ];

        TableDemoViewController *newViewController = [ [ TableDemoViewController alloc ] initWithPath: cell.textLabel.text ];
        UINavigationController *navigationController = [ appDelegate navigationController ];
        [ navigationController pushViewController: newViewController animated: YES ];
        [ activityIndicator stopAnimating ];
    }
}