iOS: crush when save Core Data

如果一个页面拥有一个 NSFetchedResultsController 的引用，那么在这个界面将要结束时，一定要 remove 它的 observer

- (void)dealloc {
    //...
    [[NSNotificationCenter defaultCenter] removeObserver:self.fetchedResultsController];
    _fetchedResultsController.delegate = nil;
    [_fetchedResultsController release];
    [super dealloc];
}

为什么呢？

看下崩溃日志（友盟统计捕获的崩溃日志）

2014-03-26 17:39:17.128 XiaoJiZhuoMi[13394:90b] Umeng: -[UIButton controllerWillChangeContent:]: unrecognized selector sent to instance 0xce92db0
(null)
(
    0   CoreFoundation                      0x01daa1e4 __exceptionPreprocess + 180
    1   libobjc.A.dylib                     0x041fb8e5 objc_exception_throw + 44
    2   CoreFoundation                      0x01e47243 -[NSObject(NSObject) doesNotRecognizeSelector:] + 275
    3   CoreFoundation                      0x01d9a50b ___forwarding___ + 1019
    4   CoreFoundation                      0x01d9a0ee _CF_forwarding_prep_0 + 14
    5   CoreData                            0x03fda640 -[NSFetchedResultsController(PrivateMethods) _managedObjectContextDidChange:] + 2080
    6   Foundation                          0x0395b049 __57-[NSNotificationCenter addObserver:selector:name:object:]_block_invoke + 40
    7   CoreFoundation                      0x01e05f04 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 20
    8   CoreFoundation                      0x01d5defb _CFXNotificationPost + 2859
    9   Foundation                          0x03894e41 -[NSNotificationCenter postNotificationName:object:userInfo:] + 98
    10  CoreData                            0x03edda13 -[NSManagedObjectContext(_NSInternalNotificationHandling) _postObjectsDidChangeNotificationWithUserInfo:] + 83
    11  CoreData                            0x03f7cfaf -[NSManagedObjectContext(_NSInternalChangeProcessing) _createAndPostChangeNotification:withDeletions:withUpdates:withRefreshes:] + 367
    12  CoreData                            0x03ed8eb8 -[NSManagedObjectContext(_NSInternalChangeProcessing) _processRecentChanges:] + 2152
    13  CoreData                            0x03edc2fc -[NSManagedObjectContext save:] + 140

...

当 [NSManagedObjectContext save:] 后，会发送一个通知，这时与之绑定的 NSFetchedResultsController，拿到通知后，负责给它的代理更新页面（或其他）。而如果持有 NSFetchedResultsController 的页面已经被销毁，及 NSFetchedResultsController 已经被 release，那么它收到通知会怎么办？只有崩溃（如崩溃日志第一行所示）。