Unable to create unwind segues when using custom view controller containment

Question

I'm attempting to convert our application to storyboards and have hit what I believe is a bug in the handling of unwind segues when dealing with custom container controllers. We have a view controller which displays another and uses the view controller containment api to do this, I wire up the segue in IB then select a custom class for the implementation. The perform method looks something like this:

-(void) perform {
    UIViewController *container = [self sourceViewController];
    UIViewController *child = [self destinationViewController];
    [container addChildViewController:child];
    [container.view addSubview:child.view];
    child.view.center = container.view.center;
    [UIView transitionWithView:container.view
                      duration:0.35
                       options:UIViewAnimationOptionCurveEaseInOut
                    animations:^{
                        child.view.alpha = 1;
                    } completion:^(BOOL finished) {
                        [child didMoveToParentViewController:container];
                    }];
}

That works perfectly, however I can't make it perform the unwind segue back to the container controller. I override viewControllerForUnwindSegueAction: fromViewController: withSender: and ensure that it's returning the correct value:

-(UIViewController *) viewControllerForUnwindSegueAction:(SEL)action fromViewController:(UIViewController *)fromViewController withSender:(id)sender {
    id default = [super viewControllerForUnwindSegueAction:action fromViewController:fromViewController withSender:sender];
    NSAssert1(default == self, @"Expected the default view controller to be self but was %@", default);
    return default;
}

I can also confirm that canPerformUnwindSegueAction:fromViewController:withSender is being called and doing the right thing, but to be sure I overrode it to return YES

-(BOOL) canPerformUnwindSegueAction:(SEL)action fromViewController:(UIViewController *)fromViewController withSender:(id)sender {
    return YES;
}

The next step I would expect to happen is for segueForUnwindingToViewController:fromViewController:identifier: to be called, however it never is. Instead the application crashes with an NSInternalInconsistencyException.

2012-10-01 10:56:33.627 UnwindSegues[12770:c07] *** Assertion failure in -[UIStoryboardUnwindSegueTemplate _perform:], /SourceCache/UIKit_Sim/UIKit-2372/UIStoryboardUnwindSegueTemplate.m:78
2012-10-01 10:56:33.628 UnwindSegues[12770:c07] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Could not find a view controller to execute unwinding for <USCustomContainerViewController: 0x75949a0>'
*** First throw call stack:
(0x1c8e012 0x10cbe7e 0x1c8de78 0xb61f35 0x581711 0x45ab54 0x10df705 0x16920 0x168b8 0xd7671 0xd7bcf 0xd6d38 0x4633f 0x46552 0x243aa 0x15cf8 0x1be9df9 0x1be9ad0 0x1c03bf5 0x1c03962 0x1c34bb6 0x1c33f44 0x1c33e1b 0x1be87e3 0x1be8668 0x1365c 0x1e7d 0x1da5)
libc++abi.dylib: terminate called throwing an exception

Has anyone successfully used unwind segues combined with the view controller containment APIs? Any idea what step I'm missing? I've uploaded a demo project to github which shows the issue in the simplest demonstration project I could come up with.

Answer 1

The problem in your example is that there's no there there. It's too simple. First, you create your container view controller in a rather odd way (you don't use the new IB "container view" which is there to help you do this). Second, you've got nothing to unwind: nothing was pushed or presented on top of anything.

I have a working example showing that canPerformUnwindSegueAction really is consulted up the parent chain, and that viewControllerForUnwindSegueAction and segueForUnwindingToViewController are called and effective, if present in the right place. See:

https://github.com/mattneub/Programming-iOS-Book-Examples/tree/master/ch19p640presentedViewControllerStoryboard2

I have now also created a fork of your original example on github, correcting it so that it illustrates these features:

https://github.com/mattneub/UnwindSegues

It isn't really a situation where "unwind" is needed, but it does show how "unwind" can be used when a custom container view controller is involved.