因为MVC架构，我们自己通过View去创建自己的模态(modal)等待对话框，比较难实现。
一个比较可行的方法就是通过UIAlertView来实现。警告框，正好符合模态的要求。
（1）创建一个警告框
_waitingDialog = [[[UIAlertView alloc] initWithTitle:nil
message:NSLocalizedString(@"Please wait", @"TitleCaptionWattingDialog")
delegate:self
cancelButtonTitle:nil
otherButtonTitles:nil] autorelease];
UIActivityIndicatorView *activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhite];
[activityIndicator setCenter:CGPointMake (132.0f, 60.0f)];
[activityIndicator setActivityIndicatorViewStyle:UIActivityIndicatorViewStyleWhiteLarge];
[activityIndicator startAnimating];
（3）把创建的等待UIActivityIndicatorView，设为警告框UIAlertView的subview.
[_waitingDialog addSubview:activityIndicator];
[_waitingDialog show];
[_waitingDialog dismissWithClickedButtonIndex:0 animated:NO];