Question

[To Make things short and clear]

I’ve written a custom segue.

-(void)perform {
UIView *preV = ((UIViewController *)self.sourceViewController).view;
UIView *newV = ((UIViewController *)self.destinationViewController).view;

[preV.window insertSubview:newV aboveSubview:preV];
newV.center = CGPointMake(preV.center.x + preV.frame.size.width, newV.center.y);
[UIView animateWithDuration:0.4
 animations:^{
     newV.center = CGPointMake(preV.center.x, newV.center.y);
     preV.center = CGPointMake(0- preV.center.x, newV.center.y);}
    completion:^(BOOL finished){ [preV removeFromSuperview]; }];
}

When the segue is triggered there is no exception. However it would deallocate destinationViewController.

The app would crash when a button, that triggers another segue, in destinationViewController is clicked.

I tried removing [preV removeFromSuperview] but to no avail.

[Details]

I’ve recently started working with Object-C and I wrote a custom segue that simulates a push segue.

The first time it is been triggered, everything works fine.

But after that, no matter what segue is been triggered,
the app crashes and I would receive a EXC_BAD_ACCESS error.

---

My first guess is that this has to do with memory management. Something out there has to be deallocated but I have no idea what it is.

My second guess is that this has to do with the infrastructure provided by UIView and UIWindow . But once again, due to my lack of knowledge and experience, I can’t figure out what the real problem is.

I know I can actually take an easy approach and create a push segue by using a rootviewcontroller and simply hiding the navigation bar, but I really want to know what exactly is wrong with my seemingly well-constructed custom segue and learn what is going on underneath the fabric of codes.

---

[Updates]

Thank Phillip Mills & Joachim Isaksson for the suggestions, after conducting some experiments and making use of breakpoints and the Zombie tool,

this is what I realise:

1. After the custom segue has been triggered by a button,the app would only crash when the next segue is also triggered by a button. Triggering the next segue using viewDidAppear would not lead to any crash.

2. The main reason behind the crash:

An Objective-C message was sent to a deallocated object (zombie)

[#, event type, refCt, Library, Caller]

0   Malloc  1   UIKit   -[UIClassSwapper initWithCoder:]
1   Retain  2   UIKit   -[UIRuntimeConnection initWithCoder:]
2   Retain  3   UIKit   -[UIRuntimeConnection initWithCoder:]
3   Retain  4   UIKit   -[UIRuntimeConnection initWithCoder:]
4   Retain  5   UIKit   -[UIRuntimeConnection initWithCoder:]
5   Retain  6   UIKit   -[UIRuntimeConnection initWithCoder:]
6   Retain  7   UIKit   -[UIRuntimeConnection initWithCoder:]
7   Retain  8   UIKit   -[UIRuntimeConnection initWithCoder:]
8   Retain  9   UIKit   UINibDecoderDecodeObjectForValue
9   Retain  10  UIKit   UINibDecoderDecodeObjectForValue
10  Retain  11  UIKit   -[UIStoryboardScene setSceneViewController:]
11  Retain  12  UIKit   -[UINib instantiateWithOwner:options:]
12  Release 11  UIKit   -[UINibDecoder finishDecoding]
13  Release 10  UIKit   -[UINibDecoder finishDecoding]
14  Release 9   UIKit   -[UIRuntimeConnection dealloc]
15  Release 8   UIKit   -[UIRuntimeConnection dealloc]
16  Release 7   UIKit   -[UIRuntimeConnection dealloc]
17  Release 6   UIKit   -[UIRuntimeConnection dealloc]
18  Release 5   UIKit   -[UINibDecoder finishDecoding]
19  Release 4   UIKit   -[UIRuntimeConnection dealloc]
20  Release 3   UIKit   -[UIRuntimeConnection dealloc]
21  Release 2   UIKit   -[UIRuntimeConnection dealloc]
22  Retain  3   UIKit   -[UIStoryboardSegue initWithIdentifier:source:destination:]
23  Retain  4   ProjectX    -[pushlike perform]
24  Retain  5   UIKit   -[UINib instantiateWithOwner:options:]
25  Retain  6   UIKit   +[UIProxyObject addMappingFromIdentifier:toObject:forCoder:]
26  Retain  7   UIKit   -[UIProxyObject initWithCoder:]
27  Retain  8   UIKit   -[UIRuntimeConnection initWithCoder:]
28  Retain  9   UIKit   UINibDecoderDecodeObjectForValue
29  Retain  10  UIKit   UINibDecoderDecodeObjectForValue
30  Release 9   UIKit   -[UINib instantiateWithOwner:options:]
31  Release 8   UIKit   +[UIProxyObject removeMappingsForCoder:]
32  Release 7   UIKit   -[UINibDecoder finishDecoding]
33  Release 6   UIKit   -[UIRuntimeConnection dealloc]
34  Release 5   UIKit   -[UINibDecoder finishDecoding]
35  Release 4   UIKit   -[UINibDecoder finishDecoding]
36  Release 3   ProjectX    -[pushlike perform]
37  Retain  4   libsystem_sim_blocks.dylib  _Block_object_assign
38  Retain  5   UIKit   -[UIApplication _addAfterCACommitBlockForViewController:]
39  Release 4   UIKit   -[UIStoryboardSegue dealloc]
40  Release 3   UIKit   _UIApplicationHandleEvent
41  Release 2   UIKit   -[UIStoryboardScene dealloc]
42  Retain  3   UIKit   _applyBlockToCFArrayCopiedToStack
43  Release 2   UIKit   _applyBlockToCFArrayCopiedToStack
44  Release 1   UIKit   __destroy_helper_block_739
45  Release 0   UIKit   _applyBlockToCFArrayCopiedToStack
46  Zombie  -1  UIKit   -[UIApplication sendAction:to:from:forEvent:]

which implies that (if I’m not wrong)

the custom segue has somehow triggered something that deallocates objects, to which an objective-C message would be sent after button (in destinationViewController) that triggers another segue is clicked.

---

More Details

Not a single prepareForSegue is been called since I don’t need to pass data between views.

My segues are all triggered the same way:

- (void)viewDidLoad
{
    [super viewDidLoad];
    CGRect buttonFrame = CGRectMake( 10, 40, 200, 50 );
    UIButton *button = [[UIButton alloc] initWithFrame: buttonFrame];
    [button setTitle: @"Go" forState: UIControlStateNormal];
    [button addTarget:self action:@selector(nextView) forControlEvents:UIControlEventTouchUpInside];
    [button setTitleColor: [UIColor blackColor] forState: UIControlStateNormal];
    [self.view addSubview:button]; 
}

- (void)nextView{
    [self performSegueWithIdentifier:@"push" sender:self];
}

I have my ARC enabled so I didn’t really do any of the deallocation myself..

---

[UPDATES 2]

The object that has been turned into a zombie is the destinationViewController of the custom segue.

Not calling removeFromSuperview in the custom segue does not stop the object from turning into zombie.

As long as I use a regular model segue or a push segue (with rootViewController) instead the custom one I made, there won’t be any zombie and everything would work fine.

Answer 1

You’re getting a crash just because your new controller not retained after segue execution.

What you do is this:

• src and dest controllers are instantiated
• you perform your animations
• in completion you remove src view
• your src controller gets released, but window’s rootViewController still pointing to it and your destination view controller not added to window’s hierarchy.

This will work just as intended:

-(void)perform {
  UIView *preV = ((UIViewController *)self.sourceViewController).view;
  UIView *newV = ((UIViewController *)self.destinationViewController).view;

  UIWindow *window = [[[UIApplication sharedApplication] delegate] window];
  newV.center = CGPointMake(preV.center.x + preV.frame.size.width, newV.center.y);
  [window insertSubview:newV aboveSubview:preV];

  [UIView animateWithDuration:0.4
                   animations:^{
                       newV.center = CGPointMake(preV.center.x, newV.center.y);
                       preV.center = CGPointMake(0- preV.center.x, newV.center.y);}
                   completion:^(BOOL finished){
                       [preV removeFromSuperview];
                       window.rootViewController = self.destinationViewController;
                   }];
}