Receptionist Pattern

TZReceptionist.h

#import <Foundation/Foundation.h>

typedef void (^RCTaskBlock)(NSString *keyPath, id object, NSDictionary *change);

@interface TZReceptionist : NSObject
+ (id)receptionistForKeyPath:(NSString *)path
                      object:(id)obj
                       queue:(NSOperationQueue *)queue
                        task:(RCTaskBlock)task;
@end

TZReceptionist.m

#import "TZReceptionist.h"

@interface TZReceptionist ()
@property (strong, nonatomic) id observedObject;
@property (copy, nonatomic) NSString *observedKeyPath;
@property (copy, nonatomic) RCTaskBlock task;
@property (strong, nonatomic) NSOperationQueue *queue;
@end

@implementation TZReceptionist

+ (instancetype)receptionistForKeyPath:(NSString *)path object:(id)obj
                                 queue:(NSOperationQueue *)queue task:(RCTaskBlock)task {
    TZReceptionist *receptionist = [TZReceptionist new];
    receptionist.task = task;
    receptionist.observedKeyPath = path;
    receptionist.observedObject = obj;
    receptionist.queue = queue;
    [obj addObserver:receptionist forKeyPath:path
             options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:0];
    return receptionist;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
                        change:(NSDictionary *)change context:(void *)context {
    [self.queue addOperationWithBlock:^{
        self.task(keyPath, object, change);
    }];
}
@end