Weak KVO example

main.m

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface Person : NSObject

@property (strong) NSString *name;

@end

@interface Observer : NSObject

- (id)initObserving:(id)object keyPath:(NSString *)keyPath;
@property (assign) id observedObject;
@property (copy) NSString *keyPath;

@end

@implementation Person

- (void)dealloc
{
    NSLog(@"Person dealloc");
}

@end

@implementation Observer

- (id)initObserving:(id)object keyPath:(NSString *)keyPath
{
    self = [super init];
    
    if (self) {
        _observedObject = object;
        _keyPath = keyPath;
        [object addObserver:self forKeyPath:keyPath options:0 context:NULL];
        objc_setAssociatedObject(object, "observer", self, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
    }
    
    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object 
                        change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"Change observed!");
}

- (void)dealloc
{
    NSLog(@"Observer dealloc");
    [self.observedObject removeObserver:self forKeyPath:_keyPath];
}

@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        {
            Person *p1 = [Person new];
            
            {
                Observer *o1 = [[Observer alloc] initObserving:p1 keyPath:@"name"];
            }
        }
        
        return 0;
    }
}

output

2016-03-16 13:52:15.933 KVO dealloc ordering[81591:15373864] Person dealloc
2016-03-16 13:52:15.934 KVO dealloc ordering[81591:15373864] Observer dealloc

Works great! Here is a Swift 4.0 version (but only of the observer part): https://gist.github.com/danurna/3b0c2ac973d07877a882644beec7e250