How to debug KVO
- by user8472
In my program I use KVO manually to observe changes to values of object properties. I receive an EXC_BAD_ACCESS signal at the following line of code inside a custom setter:
[self willChangeValueForKey:@"mykey"];
The weird thing is that this happens when a factory method calls the custom setter and there should not be any observers around. I do not know how to debug this situation.
Update: The way to list all registered observers is observationInfo. It turned out that there was indeed an object listed that points to an invalid address. However, I have no idea at all how it got there.
Update 2: Apparently, the same object and method callback can be registered several times for a given object - resulting in identical entries in the observed object's observationInfo. When removing the registration only one of these entries is removed. This behavior is a little counter-intuitive (and it certainly is a bug in my program to add multiple entries at all), but this does not explain how spurious observers can mysteriously show up in freshly allocated objects (unless there is some caching/reuse going on that I am unaware of).
Modified question: How can I figure out WHERE and WHEN an object got registered as an observer?
Update 3: Specific sample code.
ContentObj is a class that has a dictionary as a property named mykey. It overrides:
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)theKey {
BOOL automatic = NO;
if ([theKey isEqualToString:@"mykey"]) {
automatic = NO;
} else {
automatic=[super automaticallyNotifiesObserversForKey:theKey];
}
return automatic;
}
A couple of properties have getters and setters as follows:
- (CGFloat)value {
return [[[self mykey] objectForKey:@"value"] floatValue];
}
- (void)setValue:(CGFloat)aValue {
[self willChangeValueForKey:@"mykey"];
[[self mykey] setObject:[NSNumber numberWithFloat:aValue]
forKey:@"value"];
[self didChangeValueForKey:@"mykey"];
}
The container class has a property contents of class NSMutableArray which holds instances of class ContentObj. It has a couple of methods that manually handle registrations:
+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)theKey {
BOOL automatic = NO;
if ([theKey isEqualToString:@"contents"]) {
automatic = NO;
} else {
automatic=[super automaticallyNotifiesObserversForKey:theKey];
}
return automatic;
}
- (void)observeContent:(ContentObj *)cObj {
[cObj addObserver:self
forKeyPath:@"mykey"
options:0
context:NULL];
}
- (void)removeObserveContent:(ContentObj *)cObj {
[cObj removeObserver:self
forKeyPath:@"mykey"];
}
- (void)observeValueForKeyPath:(NSString *)keyPath
ofObject:(id)object
change:(NSDictionary *)change
context:(void *)context {
if (([keyPath isEqualToString:@"mykey"]) &&
([object isKindOfClass:[ContentObj class]])) {
[self willChangeValueForKey:@"contents"];
[self didChangeValueForKey:@"contents"];
}
}
There are several methods in the container class that modify contents. They look as follows:
- (void)addContent:(ContentObj *)cObj {
[self willChangeValueForKey:@"contents"];
[self observeDatum:cObj];
[[self contents] addObject:cObj];
[self didChangeValueForKey:@"contents"];
}
And a couple of others that provide similar functionality to the array. They all work by adding/removing themselves as observers. Obviously, anything that results in multiple registrations is a bug and could sit somewhere hidden in these methods.
My question targets strategies on how to debug this kind of situation. Alternatively, please feel free to provide an alternative strategy for implementing this kind of notification/observer pattern.
