iOS crash calling rollback on NSManagedObjectContext after resuming from background

I've been bashing my head over this issue for several days now to no avail and have lost all sanity. :|

My app is setup to track the device's location while active and in the background and I'm using CoreData to create and store the track data. The primary CoreData entity is "Route" which has many "RoutePoint" entities. As the device receives a new location from the app's CLLocationManager, a new RoutePoint object is added to its parent Route.

When the user stops tracking, they have the option to save or discard the route. Discarding the route simply calls [_moc rollback], and of course save will call [_moc save]. I'm using the method of referencing my NSManagedObjectContext instance on the AppDelegate rather than a singleton object, which is not being retained nor released in any of my UIViewControllers.

The issue arises if the user sends the app into the background while tracking is active (thus the Route and RoutePoint entities are not actually saved to the _moc). When the app is returned to the active state, the user stops the route chooses to discard it which is when the app crashes (calling "rollback"). However, if the same process is repeated but the u开发者_如何学JAVAser saves the route rather than discarding, there is no crash and it saves successfully. Huh?!

The only thing I get in gdb when this happens is EXC_BAD_ACCESS, so I do a backtrace and here's the result:

#0  0x34a80464 in objc_msgSend ()
#1  0x356ea6d6 in -[_PFManagedObjectReferenceQueue _processReferenceQueue:] ()
#2  0x356ea3d4 in -[NSManagedObjectContext(_NSInternalNotificationHandling) _processReferenceQueue:] ()
#3  0x356ea084 in -[NSManagedObjectContext(_NSInternalChangeProcessing) _processRecentChanges:] ()
#4  0x3571fb14 in -[NSManagedObjectContext processPendingChanges] ()
#5  0x357603ee in -[NSManagedObjectContext rollback] ()
#6  0x00031862 in -[RouteHistoryDetailViewController discardRoute] (self=0x4a0fc30, _cmd=0x100941)
#7  0x0003064c in -[RouteHistoryDetailViewController alertView:clickedButtonAtIndex:] (self=0x4a0fc30, _cmd=0x344b7f5f, alertView=0x390aa0, buttonIndex=0)
#8  0x3445d63e in -[UIAlertView(Private) _buttonClicked:] ()
#9  0x35821fec in -[NSObject(NSObject) performSelector:withObject:withObject:] ()
#10 0x341c84ac in -[UIApplication sendAction:to:from:forEvent:] ()
#11 0x341c844c in -[UIApplication sendAction:toTarget:fromSender:forEvent:] ()
#12 0x341c841e in -[UIControl sendAction:to:forEvent:] ()
#13 0x341c8170 in -[UIControl(Internal) _sendActionsForEvents:withEvent:] ()
#14 0x341c89ce in -[UIControl touchesEnded:withEvent:] ()
#15 0x341be354 in -[UIWindow _sendTouchesForEvent:] ()
#16 0x341bdcce in -[UIWindow sendEvent:] ()
#17 0x341a8fc6 in -[UIApplication sendEvent:] ()
#18 0x341a8906 in _UIApplicationHandleEvent ()
#19 0x320c8f02 in PurpleEventCallback ()
#20 0x3580f6fe in __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ ()
#21 0x3580f6c2 in __CFRunLoopDoSource1 ()
#22 0x35801f7c in __CFRunLoopRun ()
#23 0x35801c86 in CFRunLoopRunSpecific ()
#24 0x35801b8e in CFRunLoopRunInMode ()
#25 0x320c84aa in GSEventRunModal ()
#26 0x320c8556 in GSEventRun ()
#27 0x341dc328 in -[UIApplication _run] ()
#28 0x341d9e92 in UIApplicationMain ()
#29 0x00002982 in main (argc=1, argv=0x2fdff5a8)

So it appears to be an issue with calling [_moc rollback], but I've done extensive searching and tried various things, even retaining and releasing my MOC instance in that UIViewController, but nothing has worked yet. Clearly something is going awry when the app goes into the background, but it were related to the MOC then wouldn't a similar crash happen when saving as well?

Suggestions are greatly appreciated! :)

I've always just had a flag on my items and performed a save in willResignActive with any "non-confirmed" items having flag=true. Then when I resume (or startup) if I want to discard them, I can delete where flag=true and erase them... that way I can still save or undo even if iOS shuts my app down due to memory pressure.

IIRC Your current design means if someone opens up Safari, etc and your app closes, the route is permanently gone even if the user wants to come back and save it because the changes were only in memory.