autokvo.m

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

@interface NSObject (KVOAutoRemoval)
- (void)cy_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context;
@end

@interface _CYDeallocHandler : NSObject
@property(nonatomic, copy) void (^block)();
@end

@implementation _CYDeallocHandler
- (void)dealloc { _block(); }
@end

@implementation NSObject (KVOAutoRemoval)
- (void)cy_addObserver:(NSObject *)aObserver forKeyPath:(NSString *)aKeyPath options:(NSKeyValueObservingOptions)aOptions context:(void *)aContext
{
    [self addObserver:aObserver forKeyPath:aKeyPath options:aOptions context:aContext];
    
    __block void *selfPtr = (__bridge void *)self;
    __unsafe_unretained id unsafeObserver = aObserver;
    void (^block)() = ^{
        void * const observed = selfPtr;
        if(observed && OSAtomicCompareAndSwapPtr(observed, nil, &selfPtr))
            [(__bridge id)observed removeObserver:unsafeObserver forKeyPath:aKeyPath context:aContext];
    };
    _CYDeallocHandler *handlerA = [_CYDeallocHandler new];
    _CYDeallocHandler *handlerB = [_CYDeallocHandler new];
    handlerA.block = block;
    handlerB.block = block;
    
    objc_setAssociatedObject(self, 
                             (void *)((uintptr_t)(__bridge void *)aObserver ^ (uintptr_t)(__bridge void *)aKeyPath ^ (uintptr_t)aContext), 
                              handlerA, OBJC_ASSOCIATION_RETAIN);
    objc_setAssociatedObject(aObserver, 
                             (void *)((uintptr_t)(__bridge void *)self ^ (uintptr_t)(__bridge void *)aKeyPath ^ (uintptr_t)aContext), 
                             handlerB, OBJC_ASSOCIATION_RETAIN);
}
@end




// --------------------------





@interface Foo : NSObject
@property NSString *test;
@end
@implementation Foo
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@">>> %@ %@ -> %@", keyPath, object, [object valueForKeyPath:keyPath]);
}
@end

int main(int argc, char *argv[]) {
    @autoreleasepool {
        Foo *a = [Foo new];
        Foo *b = [Foo new];
        [a cy_addObserver:b forKeyPath:@"test" options:0 context:NULL];
        a.test = @"asdf";
        b = nil;
        a.test = @"foo";
        a = nil;
    }
}