KVO in a nut. Use it, or don’t?

RGMKeyValueObservance.h

//
//  RGMKeyValueObservance.h
//  RGMKeyValueObservance
//
//  Created by Ryder Mackay on 12/9/2013.
//  Copyright (c) 2013 Ryder Mackay. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface RGMKeyValueObservance : NSObject

+ (instancetype)observeObject:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options target:(__weak id)target action:(SEL)action;
+ (instancetype)observeObject:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(void (^)(NSDictionary *change))block;

@property (nonatomic, strong, readonly) id object;
@property (nonatomic, copy, readonly) NSString *keyPath;

@end

RGMKeyValueObservance.m

//
//  RGMKeyValueObservance.m
//  RGMKeyValueObservance
//
//  Created by Ryder Mackay on 12/9/2013.
//  Copyright (c) 2013 Ryder Mackay. All rights reserved.
//

#import "RGMKeyValueObservance.h"

@interface RGMKeyValueObservance ()
@property (nonatomic, copy) void (^block)(NSDictionary *change);
@property (nonatomic, assign) NSKeyValueObservingOptions options;
@end

@implementation RGMKeyValueObservance

static NSString * RGMKeyValueObservanceContext = @"RGMObservanceContext";

+ (instancetype)observeObject:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options target:(__weak id)target action:(SEL)action
{
    return [self observeObject:object keyPath:keyPath options:options block:^(NSDictionary *change) {
        if (target) {
            __strong id strongTarget = target;
            NSMethodSignature *methodSignature = [strongTarget methodSignatureForSelector:action];
            NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:methodSignature];
            invocation.selector = action;
            [invocation setArgument:&change atIndex:2];
            [invocation invokeWithTarget:strongTarget];
        } else {
            RGMObservanceDeallocatedTargetBreak();
        }
    }];
}

+ (instancetype)observeObject:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(void (^)(NSDictionary *))block
{
    return [[self alloc] initWithObject:object keyPath:keyPath options:options block:block];
}

static inline void RGMObservanceDeallocatedTargetBreak() {
#ifdef DEBUG
    NSLog(@"RGMObservance: target of was deallocated while observing--doing nothing. Set a breakpoint in RGMObservanceDeallocatedTargetBreak for more info.");
#endif
}

- (instancetype)initWithObject:(id)object keyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options block:(void (^)(NSDictionary *))block
{
    NSParameterAssert(object);
    NSParameterAssert(keyPath);
    NSParameterAssert(block);
    if (self = [super init]) {
        _object = object;
        _keyPath = [keyPath copy];
        _options = options;
        _block = [block copy];
        [_object addObserver:self forKeyPath:_keyPath options:_options context:&RGMKeyValueObservanceContext];
    }
    
    return self;
}

- (void)dealloc
{
    [_object removeObserver:self forKeyPath:_keyPath context:&RGMKeyValueObservanceContext];
    NSLog(@"-[%@ %@]", NSStringFromClass([self class]), NSStringFromSelector(_cmd));
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"<%@: %p; object = %@; keyPath = %@; options = %zu;>", NSStringFromClass([self class]), self, self.object, self.keyPath, self.options];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (context == &RGMKeyValueObservanceContext) {
        self.block(change);
    }
}

@end

Considered harmful.

You cheeky bastard!