Dismiss actionSheet before action under it's button is completed - objective-c

Question

My problem is that i have an ActionSheet, which is disappearing from screen only when action under this button is completed. My problem is that i want to click 'save' inside my action sheet, then dismiss action sheet and then to show some alert, informing user to wait until saving is done. Currently it works different: firstly action sheet is shown, then there is saving message UNDER action sheet, finally action sheet is removed from view.. so user doesnt see any alert message.
How to dismiss actionSheet earlier than xcode does it?
Method under sheetActionButton:
- (IBAction)saveAction:(id)sender
{
UIAlertView *alert;
alert = [[[UIAlertView alloc] initWithTitle:#"Saving photo to library\nPlease Wait..." message:nil delegate:self cancelButtonTitle:nil otherButtonTitles: nil] autorelease];
[alert show];
UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
indicator.center = CGPointMake(alert.bounds.size.width / 2, alert.bounds.size.height - 50);
[indicator startAnimating];
[alert addSubview:indicator];
[indicator release];
[self saveImageToCameraRoll];
[alert dismissWithClickedButtonIndex:0 animated:YES];
}

Answer 1

You should move your saveImageToCameraRoll method onto a separate thread, or at least asynchronously on the main thread. Then you can dismiss the alert and saveAction: can return before it completes.
The simplest way to do this would be using dispatch_async. Use dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) to get a queue for a separate thread, or dispatch_get_main_queue() for the main thread. Make sure not to do any UI work (or use any APIs which aren't thread-safe) on other threads!
Edit: more detail:
- (IBAction)saveAction:(id)sender {
UIAlertView *alert;
alert = [[[UIAlertView alloc] initWithTitle:#"Saving photo to library\nPlease Wait..." message:nil delegate:self cancelButtonTitle:nil otherButtonTitles: nil] autorelease];
[alert show];
UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
indicator.center = CGPointMake(alert.bounds.size.width / 2, alert.bounds.size.height - 50);
[indicator startAnimating];
[alert addSubview:indicator];
[indicator release];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Save in the background
[self saveImageToCameraRoll];
dispatch_async(dispatch_get_main_queue(), ^{
// Perform UI functions on the main thread!
[alert dismissWithClickedButtonIndex:0 animated:YES];
});
});
}