UIProgressHUD is a view which occupies the whole window for showing a short progress message. This view is private, that means you shouldn't use them for AppStore apps, but alternatives[1] exist.
Also note that UIProgressHUD is just a normal view. It will not block interactions to other background views.
Using UIProgressHUD
UIProgressHUD* hud = [[UIProgressHUD alloc] initWithFrame:CGRectZero];
[hud setText:@"Loading, please wait."];
[hud showInView:someView];
...
[hud done];
...
[hud hide];
...
[hud release];
References
- ↑ MBProgressHUD: http://www.bukovinski.com/2009/04/08/mbprogresshud-for-iphone/