Why removeObserver causes objc_msgSend SIGSEGV in viewWillDisappear in this code? - ios

Question

First of all, it is a crash that I can not reproduce so I can not debug in Xcode. So I'm trying to reason it.
Stack trace:
Exception Type: SIGSEGV
Exception Codes: SEGV_ACCERR at 0xc
Crashed Thread: 0
Thread 0 Crashed:
0 libobjc.A.dylib 0x3a816b26 objc_msgSend + 5
1 Voodo 2 0x000d8933 -[TagListViewController viewWillDisappear:] (TagListViewController.m:107)
2 UIKit 0x32b68169 -[UIViewController _setViewAppearState:isAnimating:] + 472
3 UIKit 0x32bc602b -[UIViewController viewWillMoveToWindow:] + 290
4 UIKit 0x32b4be89 -[UIView _willMoveToWindow:] + 468
5 UIKit 0x32b6e4df -[UIScrollView _willMoveToWindow:] + 122
6 UIKit 0x32b4c7b5 __85-[UIView(Hierarchy) _makeSubtreePerformSelector:withObject:withObject:copySublayers:]_block_invoke + 77
7 UIKit 0x32b4c6e5 -[UIView _makeSubtreePerformSelector:withObject:withObject:copySublayers:] + 376
8 UIKit 0x32b4c7d5 __85-[UIView(Hierarchy) _makeSubtreePerformSelector:withObject:withObject:copySublayers:]_block_invoke + 109
9 UIKit 0x32b4c6e5 -[UIView _makeSubtreePerformSelector:withObject:withObject:copySublayers:] + 376
10 UIKit 0x32b4c7d5 __85-[UIView(Hierarchy) _makeSubtreePerformSelector:withObject:withObject:copySublayers:]_block_invoke + 109
11 UIKit 0x32b4c6e5 -[UIView _makeSubtreePerformSelector:withObject:withObject:copySublayers:] + 376
12 UIKit 0x32b4c7d5 __85-[UIView(Hierarchy) _makeSubtreePerformSelector:withObject:withObject:copySublayers:]_block_invoke + 109
13 UIKit 0x32b4c6e5 -[UIView _makeSubtreePerformSelector:withObject:withObject:copySublayers:] + 376
14 UIKit 0x32de333b __UIViewWillBeRemovedFromSuperview + 379
15 UIKit 0x32b4badd -[UIView removeFromSuperview] + 56
16 UIKit 0x32b4e407 -[UIView dealloc] + 366
17 UIKit 0x32d3c0f1 -[UIWindow dealloc] + 824
18 libobjc.A.dylib 0x3a818007 (anonymous namespace)::AutoreleasePoolPage::pop(void*) + 359
19 CoreFoundation 0x3030e981 _CFAutoreleasePoolPop + 16
20 UIKit 0x32b4624d _wrapRunLoopWithAutoreleasePoolHandler + 37
21 CoreFoundation 0x303a61cd __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 21
22 CoreFoundation 0x303a3b71 __CFRunLoopDoObservers + 285
23 CoreFoundation 0x303a3eb3 __CFRunLoopRun + 731
24 CoreFoundation 0x3030ec27 CFRunLoopRunSpecific + 522
25 CoreFoundation 0x3030ea0b CFRunLoopRunInMode + 106
26 GraphicsServices 0x35035283 GSEventRunModal + 138
27 UIKit 0x32bb2049 UIApplicationMain + 1136
28 Voodo 2 0x000c87df main (main.m:14)
29 libdyld.dylib 0x3ad23ab7 start + 3
The code is very simple and using ARC:
- (void)viewDidAppear:(BOOL)animated {
[super viewDidAppear:animated];
[self refresh];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(refresh) name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];
}
- (void)viewWillDisappear:(BOOL)animated {
[super viewWillDisappear:animated];
[NSObject cancelPreviousPerformRequestsWithTarget:self selector:#selector(refresh) object:nil];
[[NSNotificationCenter defaultCenter] removeObserver:self name:NSManagedObjectContextDidSaveNotification object:self.managedObjectContext];
}
- (void)refresh {
[self.tableView reloadData];
// Refresh every 60s.
[NSObject cancelPreviousPerformRequestsWithTarget:self selector:#selector(refresh) object:nil];
[self performSelector:#selector(refresh) withObject:nil afterDelay:60];
}
The crash occurs at the last line.
The only possible cause I can think of is that the cancelPreviousPerformRequestsWithTarget:selector:object: call releases the last reference to self so self is dangling at the last line. However, I can not find any possible path for this situation to happen.