NSObject category that logs calls to addObserver for weak properties (as they aren't fully KVO-compliant)

NSObject+KVOWeakPropertyDebug.h

//
//  NSObject+KVOWeakPropertyDebug.h
//  KVOWeakPropertyDebug
//
//  Created by Vladimir Grichina on 12.01.13.
//  Copyright (c) 2013 Vladimir Grichina. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface NSObject (KVOWeakPropertyDebug)

@end

NSObject+KVOWeakPropertyDebug.m

//
//  NSObject+KVOWeakPropertyDebug.m
//  KVOWeakPropertyDebug
//
//  Created by Vladimir Grichina on 12.01.13.
//  Copyright (c) 2013 Vladimir Grichina. All rights reserved.
//

#import "NSObject+KVOWeakPropertyDebug.h"

#import <objc/runtime.h>

void MethodSwizzle(Class c, SEL origSEL, SEL overrideSEL)
{
    Method origMethod = class_getInstanceMethod(c, origSEL);
    Method overrideMethod = class_getInstanceMethod(c, overrideSEL);
    if (class_addMethod(c, origSEL, method_getImplementation(overrideMethod), method_getTypeEncoding(overrideMethod))) {
        class_replaceMethod(c, overrideSEL, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    } else {
        method_exchangeImplementations(origMethod, overrideMethod);
    }
}

@implementation NSObject (KVOWeakPropertyDebug)

+ (void)load
{
    MethodSwizzle(self, @selector(addObserver:forKeyPath:options:context:), @selector(_addObserver:forKeyPath:options:context:));
}

- (BOOL)_isWeak:(NSString *)keyPath
{
    // TODO: Support complex keyPath variants
    objc_property_t property = class_getProperty(self.class, keyPath.UTF8String);
    if (property) {
        return property_getAttributes(property)[3] == 'W';
    }

    return NO;
}

- (void)_addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context
{
    if ([self _isWeak:keyPath]) {
        NSLog(@"WARNING: observing '%@' property, which is weak and so isn't fully KVO-compliant", keyPath);
    }

    [self _addObserver:observer forKeyPath:keyPath options:options context:context];
}

@end

Thanks for creating this!

I've made a few improvements: 1) fixed what steipete pointed out (better check for 'W') 2) also swizzle and check addObserver:toObjectsAtIndexes:forKeyPath:options:context: 3) for keypaths, strip everything after the first dot. 4) renamed methods that start with underscore, which is reserved by Apple.

https://gist.github.com/seanm/00938c5f6e456db8fcb1