Question

I have two view controllers: MyParentViewController and MyChildViewController.

The application starts with MyParent. I push the MyChild controller to the top of the navigation stack, so that the chain is like so:

MyParent > MyChild

I log the object ID of MyChild with NSLog(@"%p", self):

2009-11-20 05:08:29.305 MyApp[2213:207] MyChildViewController instance: 0x36afc20

When I press the back button from MyChild this pops MyChild off the stack and returns me to MyParent.

When I rotate the iPhone while viewing MyParent, my application crashes with the following error message:

2009-11-20 05:08:37.671 MyApp[2213:207] *** -[MyChildViewController _existingView]: message sent to deallocated instance 0x36afc20

I have no _existingView method or instance variable in MyChildViewController.

If I pop MyChild off the stack, I think the navigation controller will release it, and I presume that it would be set to nil, and that any messages sent to it would be ignored. Though that s not happening here, obviously.

Does anyone have any ideas why my application crashes on rotation?

Is there a way to find out what is sending the _existingView message to MyChild?

EDIT

Here s the code for pushing MyChild on the stack:

MyChildViewController *_myChildViewController = [[MyChildViewController alloc] initWithNibName:@"MyChildViewController" bundle:nil];
_myChildViewController.managedObjectContext = self.managedObjectContext;
_myChildViewController.title = [_xyz name];
[self.navigationController pushViewController:_myChildViewController animated:YES];
UIBarButtonItem *_backButton = [[UIBarButtonItem alloc] initWithTitle:NSLocalizedString(@"MyChildViewBackBarButtonItemTitle", @"") style:UIBarButtonItemStyleDone target:nil action:nil];
self.navigationItem.backBarButtonItem = _backButton;
[_backButton release];
[_myChildViewController release];

EDIT 2

I think I may have solved this. I have an UISearchDisplayController added to the view controller nib via Interface Builder.

Originally, I set this to nil when the MyChild controller is sent -viewDidUnload, thinking it is usually enough to set IBOutlet instances to nil in this method. But this doesn t appear to be enough for my search display controller. When I release this in -dealloc I don t get the crash. Is this a bug, I wonder, or expected behavior?

Answer 1

There is a way to find out more about who called (found here):

[...] Also, by the time the app is terminated due to the uncaught exception there is no useful backtrace. If you set a breakpoint on objc_exception_throw the debugger will break before the exception is thrown and you ll have a useful backtrace. I do this with a .gdbinit file. Create a file named .gdbinit and place it in your home directory. This is the contents of mine:

fb -[NSException raise]

fb -[_NSZombie release]

fb szone_error

fb objc_exception_throw

It s also possible to set these kinds of breakpoints in the Xcode breakpoints window or in the debugger console. [...]

But I assume that won t fix your problem... _existingView seems to be called by the framework when rotating the phone. The only line I can think of telling the framework to perform this on _myChildController is this one:

myChildViewController.managedObjectContext = self.managedObjectContext;

Doesn t it work without this?

Answer 2

_existingView is an internal variable of an UIViewController instance, defined in UIViewController.h
Maybe you are retaining your MyChildViewController someplace in your code.

Answer 3

I was struggling with this too, and the solution was similar to Alex Reynolds . I release my UISearchDisplayController on -dealloc method, but it had to be before the [super dealloc]. If I release it after super deallocation, it still crashes.

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