KVOHelper. Easy Key-Value Observing

NSObject+KVOHelper.h

// AUTHOR: Kazuo Koga
// LICENSE: MIT
#import <Foundation/Foundation.h>

@interface NSObject (KVOHelper)

- (id)addObserveBlockForKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options with:(void (^)(NSDictionary *change))block;
- (void)removeObserveBlockForKeyPath:(NSString *)keyPath withID:(id)contextID;
+ (NSString *)descriptionOfObserveBlock;

@end

NSObject+KVOHelper.m

// AUTHOR: Kazuo Koga
// LICENSE: MIT
#import "NSObject+KVOHelper.h"

@interface KVOHelperKey : NSObject
@property(nonatomic) NSObject *target;
@property(nonatomic, copy) NSString *keyPath;
@end

@implementation KVOHelperKey
@synthesize target = _target;
@synthesize keyPath = _keyPath;

- (NSString *)description
{
    return [[NSString alloc] initWithFormat:@"<%@ (%@:%p, %@)>", self.class, self.target.class, self.target, self.keyPath];
}

- (NSUInteger)hash
{
    return (self.target.hash ^ self.keyPath.hash);
}

- (BOOL)isEqual:(id)object
{
    KVOHelperKey *other = object;
    return [self.target isEqual:other.target] && [self.keyPath isEqualToString:other.keyPath];
}

@end

@class KVOHelperContext;

@interface KVOHelperContextRef : NSObject
@property(nonatomic, unsafe_unretained) KVOHelperContext *context;
@end

@implementation KVOHelperContextRef
@synthesize context = _context;
@end

@interface KVOHelperContext : NSObject
@property(nonatomic) KVOHelperContextRef *ref;
@property(nonatomic) id block;
@end

@implementation KVOHelperContext
@synthesize ref = _ref;
@synthesize block = _block;

- (void)dealloc
{
    _ref.context = nil;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (context == (__bridge void *)self) {
        ((void (^)(NSDictionary *))self.block)(change);
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

@end

@implementation NSObject (KVOHelper)

static NSMutableDictionary *globalObservers(void)
{
    static NSMutableDictionary *observers = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        observers = [NSMutableDictionary new];
    });
    return observers;
}

- (id)addObserveBlockForKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options with:(void (^)(NSDictionary *change))block
{
    NSParameterAssert(NSThread.currentThread.isMainThread);
    NSParameterAssert(keyPath);
    NSParameterAssert(block);
    KVOHelperContext *context = [KVOHelperContext new];
    context.ref = [KVOHelperContextRef new];
    context.ref.context = context;
    context.block = [block copy];
    KVOHelperKey *key = [KVOHelperKey new];
    key.target = self;
    key.keyPath = keyPath;
    NSMutableDictionary *allObservers = globalObservers();
    NSMutableSet *observers = [allObservers objectForKey:key];
    if (!observers) {
        observers = [NSMutableSet new];
        [allObservers setObject:observers forKey:key];
    }
    [observers addObject:context];
    [self addObserver:context forKeyPath:keyPath options:options context:(__bridge void *)context];
    return context.ref;
}

- (void)removeObserveBlockForKeyPath:(NSString *)keyPath withID:(id)contextID
{
    NSParameterAssert(NSThread.currentThread.isMainThread);
    NSParameterAssert(keyPath);
    KVOHelperKey *key = [KVOHelperKey new];
    key.target = self;
    key.keyPath = keyPath;
    NSMutableDictionary *allObservers = globalObservers();
    NSMutableSet *observers = [allObservers objectForKey:key];
    if (observers) {
        if (contextID) {
            id context = ((KVOHelperContextRef *)contextID).context;
            if (context) {
                [self removeObserver:context forKeyPath:keyPath context:(__bridge void *)context];
                [observers removeObject:context];
                if (observers.count <= 0) {
                    [allObservers removeObjectForKey:key];
                }
            }
        }
        else {
            [observers enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
                [self removeObserver:obj forKeyPath:keyPath];
            }];
            [allObservers removeObjectForKey:key];
        }
    }
}

+ (NSString *)descriptionOfObserveBlock
{
    return [globalObservers() description];
}

@end