How to set the activity indicator in the navigation bar?

I add the below piece of code in the view where i wanted the activity indicator in the navigation bar.

activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
UIBarButtonItem * barButton = [[UIBarButtonItem alloc] initWithCustomView:activityIndicator];
[self navigationItem].rightBarButtonItem = barButton;
[activityIndicator startAnimating];