An -observeValueForKeyPath:ofObject:change:context: message was received but not handled

Question

I am relatively new to KVO, so there is a good chance that I am violating some fundamental rule. I am using Core Data.

My app crashes with the following message: And what I can't understand is why a CGImage is getting involved in observing a value that is set on a MeasurementPointer object.

        *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '<CGImage 0x276fc0>: An -observeValueForKeyPath:ofObject:change:context: message was received but not handled. Key path: measurementDescriptor Observed object: <MeasurementPointer: 0x8201640> (entity: MeasurementPointer; id: 0x8200410 <x-coredata://EBEE0687-D67D-4B03-8C95-F4C60CFDC20F/MeasurementPointer/p75> ; data: {     measurementDescriptor = "0x260fd0 <x-coredata://EBEE0687-D67D-4B03-8C95-F4C60CFDC20F/MeasurementDescriptor/p22>"; }) Change: {     kind = 1;     new = "<MeasurementDescriptor: 0x262530> (entity: MeasurementDescriptor; id: 0x260fd0 <x-coredata://EBEE0687-D67D-4B03-8C95-F4C60CFDC20F/MeasurementDescriptor/p22> ; data: {\n    measurementName = Temperature;\n    measurementUnits = \"\\U00b0C\";\n    sortString = nil;\n})"; } Context: 0x0' *** Call stack at first throw: (     0   CoreFoundation                      0x30897ed3 __exceptionPreprocess + 114     1   libobjc.A.dylib                     0x3002f811 objc_exception_throw + 24     2   CoreFoundation                      0x30897d15 +[NSException raise:format:arguments:] + 68     3   CoreFoundation                      0x30897d4f +[NSException raise:format:] + 34     4   Foundation                          0x34a13779 -[NSObject(NSKeyValueObserving) observeValueForKeyPath:ofObject:change:context:] + 60     5   Foundation                          0x349b6acd NSKeyValueNotifyObserver + 216     6   Foundation                          0x349b6775 NSKeyValueDidChange + 236     7   Foundation                          0x349ae489 -[NSObject(NSKeyValueObserverNotification) didChangeValueForKey:] + 76     8   CoreData                            0x3165b577 _PF_ManagedObject_DidChangeValueForKeyIndex + 102     9   CoreData                            0x3165ac51 _sharedIMPL_setvfk_core + 184     10  CoreData                            0x3165dc83 _svfk_0 + 10     11  SPARKvue                            0x000479f1 -[MeasurementViewController doneAction:] + 152     12  CoreFoundation                      0x3083f719 -[NSObject(NSObject) performSelector:withObject:withObject:] + 24     13  UIKit                               0x31eb1141 -[UIApplication sendAction:to:from:forEvent:] + 84     14  UIKit                               0x31f08315 -[UIBarButtonItem(UIInternal) _sendAction:withEvent:] + 92     15  CoreFoundation                      0x3083f719 -[NSObject(NSObject) performSelector:withObject:withObject:] + 24     16  UIKit                               0x31eb1141 -[UIApplication sendAction:to:from:forEvent:] + 84     17  UIKit                               0x31eb10e1 -[UIApplication sendAction:toTarget:fromSender:forEvent:] + 32     18  UIKit                               0x31eb10b3 -[UIControl sendAction:to:forEvent:] + 38     19  UIKit                               0x31eb0e05 -[UIControl(Internal) _sendActionsForEvents:withEvent:] + 356     20  UIKit                               0x31eb1453 -[UIControl touchesEnded:withEvent:] + 342     21  UIKit                               0x31eafddd -[UIWindow _sendTouchesForEvent:] + 368     22  UIKit                               0x31eaf757 -[UIWindow sendEvent:] + 262     23  UIKit                               0x31eaa9ff -[UIApplication sendEvent:] + 298     24  UIKit                               0x31eaa337 _UIApplicationHandleEvent + 5110     25  GraphicsServices                    0x31e4504b PurpleEventCallback + 666     26  CoreFoundation                      0x3082cce3 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 26     27  CoreFoundation                      0x3082cca7 __CFRunLoopDoSource1 + 166     28  CoreFoundation                      0x3081f56d __CFRunLoopRun + 520     29  CoreFoundation                      0x3081f277 CFRunLoopRunSpecific + 230     30  CoreFoundation                      0x3081f17f CFRunLoopRunInMode + 58     31  GraphicsServices                    0x31e445f3 GSEventRunModal + 114     32  GraphicsServices                    0x31e4469f GSEventRun + 62     33  UIKit                               0x31e51123 -[UIApplication _run] + 402     34  UIKit                               0x31e4f12f UIApplicationMain + 670     35  SPARKvue                            0x000031ff main + 70     36  SPARKvue                            0x000031b4 start + 40 ) terminate called after throwing an instance of 'NSException' Program received signal:  “SIGABRT”. 

All that is happening to trigger this is:

[[self measurementPointer] setMeasurementDescriptor:descriptor]; 

Given this,

[[meterDisplay measurementPointer] addObserver:self              forKeyPath:@"measurementDescriptor"             options:NSKeyValueObservingOptionNew             context:nil]; 

Basically, MeasurementPointer objects point to MeasurementDescriptor objects - and both are NSManagedObject subclasses. MeasurementDescriptor objects describe a specific 'measurement' and 'unit' combination (e.g., "Temperature (°C)" or "Wind Speed (mph)"). MeasurementDescriptors are something like singletons to the extent that there is only one for each unique measurement-unit combo.

MeasurementPointers are referenced by other objects - both Model objects and Controller objects. A MeasurementPointer references a MeasurementDescriptor. Many objects are interested in knowing when a MeasurementPointer starts referencing a new/different MeasurementDescriptor. Such a change might cause a graph display's axis to change, for example. Or, in the code above, might cause a meter display to show a different sample (from a selected set of samples).

I think that fundamental problem is that a CGImage is receiving a message that is not intended for it... unfortunately, this is intermittent, so I have not been able to find a pattern that triggers it.

Answer 1

You have an object that got dealloc'ed and did not stop observing another object. Walk through all of your -addObserver... calls and make sure they are matched with -removeObserver... calls at least in the -dealloc and possibly in the -viewDidUnload depending on your application structure.