Show UIActivityIndicator during segue navigation, after UIBarButtonItem press - objective-c

Question

I have a UIBarButtonItem in the navigation bar which switches to another screen, using a segue. This other screen takes some time to initialize, and I wanted to put a UIActivityIndicator on the UIBarButtonItem to show the tap has been registered, and the iPad is busy executing the action.
My approach was to add a UIActivityIndicator to the UIBarButtonItem after it was pressed, then call performSegueWithIdentifier:, and in the viewDidLoad method of the second view, put the initialization into a dispatch_sync() call. You can guess it does not work... why?
The IBAction on the first screen:
- (void)tappedEdit: (UIBarButtonItem *)editButton {
// put activity indicator somewhere
UIActivityIndicatorView *indicator;
indicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleWhiteLarge];
indicator.frame = CGRectMake (200, 5, 50, 50);
[self addSubview: indicator];
[indicator startAnimating];
// follow segue
[self performSegueWithIdentifier: SegueShowDesignMode sender: editButton];
}
The initialization on the second screen:
- (void)viewDidLoad {
[super viewDidLoad];
// put costly operations into another queue to free main queue for activity indicator
dispatch_sync (dispatch_get_global_queue (0, 0),
^{ // do initialization here
});
}
The effect of this is that the UIBarButtonItem stays tapped while the initialization is performed, then the UIActivityIndicator is visible for a quick moment, and lastly the segue animation is shown, displaying the second screen.
Any idea? Thanks a lot!
EDIT: Probably an addition to the problem is that the initialization does some UIKit stuff: When I tried to use a semaphore, the initialization dumps with a BAD ACCESS in some UITextView method. I guess this is because it runs on some 'get_global_queue' and not on the 'get_main_queue'.
EDIT AGAIN: Well, no. Using 'get_main_queue' results in a dead-lock, as announced in the Apple docs. So the question boils down to
"How can I do background UIView creation (lots of!) while still having a spinner running?"

Answer 1

I think one problem is that you are using dispatch_sync in your viewDidLoad method in your second screen. dispatch_sync will block your thread until the block with // do initialization here has completed. Try changing it to dispatch_async instead.
The other thing I'd consider doing if I were you is getting the UIActivityIndicator to appear as part of the second screen, not the first. That way, the second screen can also dismiss it after your view has finished the costly operations. Of course, this assumes that the view can actually display before those operations have completed.

Answer 2

if you are using a storyboard
ButtonBarItem.h
#import "CheckWifi.h"
interface LoadingViewController : UIViewController
-(IBAction)BarItemPressed:(id)sender;
ButtonBarItem.m:
-(IBAction)BarItemPressed:(id)sender {LoadingViewController *DIS = [self.storyboard instantiateViewControllerWithIdentifier:#"CheckWifi"];
DIS.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
[self presentModalViewController:DIS animated:YES];}
LoadingViewController.h:
#interface LoadingViewController : UIViewController {
IBOutlet UIActivityIndicatorView *spinner
}
-(IBAction)StopAnimating;
#end
LoadingViewController.m:
-(void)ViewDidLoad {
spinner.hidden = NO;
}
-(IBAction)StopAnimating {
spinner.hidden = YES;
}
If you're using an XIB File, then I cannot help you really, Sorry.