Trying to show an UIActivityIndicatorView but it doesn't start until the function...

The UI only updates on the main thread and only on each time round the run loop. So if you are doing something long running on the main thread (which you are) then you are blocking any update to the UI including the adding of your progress indicator. You need to do your long running operation in the background.

 

or wherever you call your long running function set the UIActivityIndicatorView before and after the call.

 

Activity indicator needs one pass through the run loop at least once before it starts its twirl. Make an instance variable and split everything in two methods.

Code:

- (void)startTheJob
{
   UIActivityIndicatorView  *tmpActIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];

   self.actIndicatorView = tmpActIndicator;
   
   [tmpActIndicator release];
   
   self.actIndicatorView.hidden = NO;
   self.actIndicatorView.center = self.view.center;
   
   [self.view addSubview:self.actIndicatorView];
   [self.actIndicatorView startAnimating];

   [self performSelector:@selector(doTheJob) withObject:nil afterDelay:.1];
}

- (void)doTheJob
{
   // Perform lenghty operation here

   [self.actIndicatorView stopAnimating];
   [self.actIndicatorView removeFromSuperview];
   self.actIndicatorView = nil;
}

This will do the trick if you have no other option. But otherwise, NSOperation is so cute and simple to master, you should learn how to use it.