Jens mentioned in a previous thread that he was not quite sure whether to
always remove observations or not.
This was bugging me. I always did it, though more out of habit than absolute
conviction.
The code below confirms that not removing the observation prior to deallocating
the observing object really is toxic.
As shown if linked on OS X 10.9 the code below may run to completion or may
terminate with an access violation (YMMV).
If the code completes a warning is logged stating that the observed object
still has registered observers.
When we mutate the observed key path the deallocated instance gets messaged,
which may or may not be terminal.
If linked on OS X 10.10 then the code never completes.
Either it terminates due to an access violation or it aborts as the previous
console warning is now a NSInternalInconsistencyException and the thread aborts.
This issue can lurk in code without causing problems if the observed and
observing objects tend to get deallocated together.
One can easily see how refactoring could cause the observed object to outlive
its observer and produce instability.
#import <Foundation/Foundation.h>
@interface MGSObservedObject : NSObject
@property (strong) NSString *name;
@end
@interface MGSObservingObject : NSObject
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
MGSObservedObject *observedObject = [MGSObservedObject new];
MGSObservingObject *observingObject = [MGSObservingObject new];
[observedObject addObserver:observingObject forKeyPath:@"name"
options:0 context:NULL];
observedObject.name = @"Laura Palmer";
BOOL removeObservation = NO;
if (removeObservation) {
[observedObject removeObserver:observingObject forKeyPath:@"name"
context:NULL];
}
BOOL deallocObservingObject = YES;
if (deallocObservingObject) {
observingObject = nil;
}
BOOL pokeObservedObject = YES;
if (pokeObservedObject) {
observedObject.name = @"Ronette Pulaski"; // on 10.9 observer may
still register change or we may crash, who knows!
}
}
return 0;
}
@implementation MGSObservingObject
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
change:(NSDictionary *)change context:(void *)context
{
NSLog(@"observed : %@", [object valueForKeyPath:keyPath]);
}
@end
@implementation MGSObservedObject
@end
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