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) 2013-2014 Vladimir Grichina. All rights reserved.
//

#import "NSObject+KVOWeakPropertyDebug.h"

#import <objc/runtime.h>

static 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);
    }
}

static BOOL DoesPropertyTypeStringIncludeWeak(const char* string)
{
    // Assume the string is ASCII.
    // Traverse the string char by char, ignore everything before the first comma, return YES if a 'W' is found after it, NO otherwise.
    size_t strLen = string ? strlen(string) : 0;
    BOOL foundFirstComma = NO;
    for (size_t i = 0; i < strLen; i++) {
        if (string[i] == ',') {
            foundFirstComma = YES;
        }
        else if (foundFirstComma && (string[i] == 'W')) {
            return YES;
        }
    }
    return NO;
}

@implementation NSObject (KVOWeakPropertyDebug)

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

- (BOOL)private_isWeak:(NSString *)keyPath
{
    BOOL isWeak = NO;
    
    // TODO: Support complex keyPath variants. Until then, keep just what's before the first '.'
    char* copy = strdup(keyPath.UTF8String);
    assert(copy);
    copy = strsep(&copy, ".");
    assert(copy);
    
    objc_property_t property = class_getProperty(self.class, copy);
    if (property) {
        const char* tmp = property_getAttributes(property);
        isWeak = DoesPropertyTypeStringIncludeWeak(tmp);
    }

    free(copy);

    return isWeak;
}

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

- (void)private_addObserver:(NSObject *)observer
         toObjectsAtIndexes:(NSIndexSet *)indexes
                 forKeyPath:(NSString *)keyPath
                    options:(NSKeyValueObservingOptions)options
                    context:(void *)context
{
    if ([self private_isWeak:keyPath]) {
        NSLog(@"WARNING: object %p is observing '%@' property, which is weak and so isn't KVO-compliant", observer, keyPath);
    }
    
    [self private_addObserver:observer
           toObjectsAtIndexes:indexes
                   forKeyPath:keyPath
                      options:options
                      context:context];
}

@end

On 10.11 we need something like

static BOOL DoesPropertyTypeStringIncludeWeak(const char* string)
{
return strstr(string, ",W,") != NULL ? YES : NO; // 10.11 variant
}