KVO with multi-key path where observer is added/removed while non-last-key property is mutated, crashes

KVOCrash.m

/* 2016-12-05 19:29:16.988026 Compass[71668:644974] [General] An uncaught exception was raised

2016-12-05 19:29:16.988060 Compass[71668:644974] [General] Cannot update for observer <TestObserver 0x600000001060> for the key path "nestedObject.nestedField" from <TestObject 0x600000026ee0>, most likely because the value for the key "nestedObject" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the TestObject class.
2016-12-05 19:29:16.988395 Compass[71668:644974] [General] (
	0   CoreFoundation                      0x00007fffaa71ee7b __exceptionPreprocess + 171
	1   libobjc.A.dylib                     0x00007fffbf303cad objc_exception_throw + 48
	2   CoreFoundation                      0x00007fffaa79d99d +[NSException raise:format:] + 205
	3   Foundation                          0x00007fffac0f5a62 -[NSKeyValueNestedProperty object:withObservance:didChangeValueForKeyOrKeys:recurse:forwardingValues:] + 830
	4   Foundation                          0x00007fffac0c8e88 NSKeyValueDidChange + 186
	5   Foundation                          0x00007fffac207c37 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 944
	6   Foundation                          0x00007fffac08cd1d -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 60
	7   Foundation                          0x00007fffac0f559b _NSSetObjectValueAndNotify + 261
	8   Compass                             0x000000010000277f -[TestObserver set] + 255
	9   Compass                             0x0000000100002866 __19-[TestObserver set]_block_invoke + 38
	10  libdispatch.dylib                   0x00000001000be6e5 _dispatch_call_block_and_release + 12
	11  libdispatch.dylib                   0x00000001000b4f5c _dispatch_client_callout + 8
	12  libdispatch.dylib                   0x00000001000c59c7 _dispatch_queue_override_invoke + 1360
	13  libdispatch.dylib                   0x00000001000b71d7 _dispatch_root_queue_drain + 671
	14  libdispatch.dylib                   0x00000001000b6ee8 _dispatch_worker_thread3 + 114
	15  libsystem_pthread.dylib             0x000000010012c89a _pthread_wqthread + 1299
	16  libsystem_pthread.dylib             0x000000010012c375 start_wqthread + 13
)
2016-12-05 19:29:16.997343 Compass[71668:644974] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot update for observer <TestObserver 0x600000001060> for the key path "nestedObject.nestedField" from <TestObject 0x600000026ee0>, most likely because the value for the key "nestedObject" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the TestObject class.'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fffaa71ee7b __exceptionPreprocess + 171
	1   libobjc.A.dylib                     0x00007fffbf303cad objc_exception_throw + 48
	2   CoreFoundation                      0x00007fffaa79d99d +[NSException raise:format:] + 205
	3   Foundation                          0x00007fffac0f5a62 -[NSKeyValueNestedProperty object:withObservance:didChangeValueForKeyOrKeys:recurse:forwardingValues:] + 830
	4   Foundation                          0x00007fffac0c8e88 NSKeyValueDidChange + 186
	5   Foundation                          0x00007fffac207c37 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:usingBlock:] + 944
	6   Foundation                          0x00007fffac08cd1d -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:] + 60
	7   Foundation                          0x00007fffac0f559b _NSSetObjectValueAndNotify + 261
	8   Compass                             0x000000010000277f -[TestObserver set] + 255
	9   Compass                             0x0000000100002866 __19-[TestObserver set]_block_invoke + 38
	10  libdispatch.dylib                   0x00000001000be6e5 _dispatch_call_block_and_release + 12
	11  libdispatch.dylib                   0x00000001000b4f5c _dispatch_client_callout + 8
	12  libdispatch.dylib                   0x00000001000c59c7 _dispatch_queue_override_invoke + 1360
	13  libdispatch.dylib                   0x00000001000b71d7 _dispatch_root_queue_drain + 671
	14  libdispatch.dylib                   0x00000001000b6ee8 _dispatch_worker_thread3 + 114
	15  libsystem_pthread.dylib             0x000000010012c89a _pthread_wqthread + 1299
	16  libsystem_pthread.dylib             0x000000010012c375 start_wqthread + 13
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) */

@interface NestedTestObject : NSObject
@property NSString *nestedField;
@end

@implementation NestedTestObject @end

@interface TestObject : NSObject
@property NSString *field;
@property NestedTestObject *nestedObject;
@end

@implementation TestObject @end

@interface TestObserver : NSObject
@property TestObject *object;
@end

@implementation TestObserver

- (instancetype)init {
    self = [super init];

    _object = [TestObject new];

    // Dummy observer for safety
    [_object addObserver:self
              forKeyPath:@"nestedObject.nestedField"
                 options:(NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)
                 context:observerContext];

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self observe];
    });

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self set];
    });

    return self;
}

- (void)set {
    if (arc4random_uniform(10) > 5) {
        self.object.nestedObject.nestedField = @"no prob";
    } else {
        self.object.nestedObject = [NestedTestObject new];
    }

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [self set];
    });
}

-(void)observe {
    [self.object addObserver:self
                  forKeyPath:@"nestedObject.nestedField"
                     options:(NSKeyValueObservingOptionInitial | NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)
                     context:observerContext];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        [self.object removeObserver:self forKeyPath:@"nestedObject.nestedField" context:observerContext];

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
            [self observe];
        });
    });
}

static void *observerContext = &observerContext;

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary<NSKeyValueChangeKey,id> *)change
                       context:(void *)context {

    if (context != observerContext) {
        return [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }

//    NSLog(@"Changed");
}

@end

Hello Mathew; I’ve run into this problem, too. I wonder if you have ever figured out a workaround?

Hahaha this was a while ago. But yes I came up with a horrendous workaround.

First of all...

static void synchronizeKVOSetter(Class clazz) {
    SEL selector = NSSelectorFromString(@"_changeValueForKeys:count:maybeOldValuesDict:usingBlock:");
    Method method = class_getInstanceMethod(clazz, selector);
    IMP superIMP = method_getImplementation(method);

    IMP newIMP = imp_implementationWithBlock(^(id self, id *keys, unsigned int count, id oldValues, id block) {
        @synchronized(self) {
            ((void (*)(id, SEL, id*, unsigned int, id, id))superIMP)(self, selector, keys, count, oldValues, block);
        }
    });

    class_addMethod(clazz, selector, newIMP, method_getTypeEncoding(method));
}

// e.g. synchronizeKVOSetter([NSObject class]);

If that's not enough, then you also need to make your class's add/remove observer look like this (if you're working with a shared base class, just paste this in, otherwise if you're working with classes outside of your control, you'll need to do some more swizzling)...

- (void)removeObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath {
    @synchronized(self) {
        @synchronized ([self valueForKeyPath:[keyPath componentsSeparatedByString:@"."][0]]) {
            [super removeObserver:observer forKeyPath:keyPath];
        }
    }
}

- (void)addObserver:(NSObject *)observer
         forKeyPath:(NSString *)keyPath
            options:(NSKeyValueObservingOptions)options
            context:(nullable void *)context {

    @synchronized (self) {
        @synchronized ([self valueForKeyPath:[keyPath componentsSeparatedByString:@"."][0]]) {
            [super addObserver:observer forKeyPath:keyPath options:options context:context];
        }
    }
}