In my viewDidAppear, how do I know when it's being unwound by a child?

Question

When my child performs an unwind segue, my controller\'s viewDidAppear gets called.

In this method (and this method alone, I need to know whether it was from an unwi

Answer 1

Here is a simple category on UIViewController that you can use to track whether your presented view controller is in the midst of an unwind segue. I suppose it could be flushed out more but I believe this much works for your case.

To use it you need to register the unwind segue from your unwind action method on the destination view controller:

- (IBAction) prepareForUnwind:(UIStoryboardSegue *)segue
{
    [self ts_registerUnwindSegue: segue];
}

That's it. From your intermediate view controller, you can test if you are in the midst of an unwind segue:

- (void) viewDidAppear:(BOOL)animated
{
    [super viewDidAppear: animated];

    BOOL unwinding = [self ts_isUnwinding];

    NSLog( @"%@:%@, unwinding: %@", self.title, NSStringFromSelector(_cmd), unwinding ? @"YES" : @"NO" );
}

There's no need to clean anything up; the segue will self-deregister when it ends.

Here's the full category:

@interface UIViewController (unwinding)

- (void) ts_registerUnwindSegue: (UIStoryboardSegue*) segue;
- (BOOL) ts_isUnwinding;

@end


static NSMapTable* g_viewControllerSegues;

@implementation UIViewController (unwinding)

- (void) ts_registerUnwindSegue: (UIStoryboardSegue*) segue
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{

        g_viewControllerSegues = [NSMapTable weakToWeakObjectsMapTable];
    });

    for ( UIViewController* vc = segue.sourceViewController ; vc != nil ; vc = vc.presentingViewController )
    {
        [g_viewControllerSegues setObject: segue forKey: vc];
    }
}

- (BOOL) ts_isUnwinding
{
    return [g_viewControllerSegues objectForKey: [self ts_topMostParentViewController]] != nil;
}

- (UIViewController *)ts_topMostParentViewController {
    UIViewController *viewController = self;
    while (viewController.parentViewController) {
        viewController = viewController.parentViewController;
    }
    return viewController;
}

@end