KVO: fix possible duplicate observing of key

[triplef]

Could happen when using dependent key paths, as the dependency handling can have added an observer for restOfKeypath already.

Following is a test case to reproduce the issue, which resulted in a KVO notification after removing the observer. This happened because number was observed twice internally, but only one observer was being removed internally on -removeObserver:forKeyPath:.

@interface Child : NSObject
@property (nonatomic, assign) NSUInteger number;
@end

@implementation Child
@end

@interface Parent : NSObject
@property (nonatomic, assign) BOOL useChild2;
@property (nonatomic, readonly) Child *child;
@property (nonatomic, strong) Child *child1;
@property (nonatomic, strong) Child *child2;
@end

@implementation Parent

- (id)init
{
	self = [super init];
	if (self) {
		self.child1 = [[Child alloc] init];
		self.child1.number = 1;
		self.child2 = [[Child alloc] init];
		self.child2.number = 2;;
	}
	return self;
}

+ (NSSet *)keyPathsForValuesAffectingChild
{
	return [NSSet setWithObject:@"useChild2"];
}

- (Child *)child
{
	return self.useChild2 ? self.child2 : self.child1;
}

@end

@interface Test : NSObject
@end

@implementation Test

- (void)test
{
	Parent *parent = [Parent new];

	[parent addObserver:self forKeyPath:@"child.number" options:NSKeyValueObservingOptionInitial context:NULL];
	[parent removeObserver:self forKeyPath:@"child.number"];

	// *** BUG: incorrectly triggers KVO ***
	parent.child.number = 999;
	// ***
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
	NSLog(@"%@.%@ (change %@) = %@", object, keyPath, change, [object valueForKeyPath:keyPath]);
}

@end

While I don’t expect this to be merged given the state of the project, I wanted to post this here for posterity in case anyone else is using this KVO implementation.

[DHowett]

Wow, good fix. It's been a long time since I've thought about the KVO code.

I won't ask you to write a unit test for our abandoned project; I'm just glad somebody is still able to build it.