How to use `-[NSNotificationCenter addObserverForName:object:queue:usingBlock:]`. Drop this in any Objective-C file of your choosing.

notification-with-block.m

#import <libkern/OSAtomic.h>

static int32_t globalCounter;

@interface Test : NSObject

@property uint counter;
@property id observer;

@end

static NSLock *lock;
static NSOperationQueue *queue;

@implementation Test

+ (void)initialize {
	if (self == [Test class]) {
		queue = [[NSOperationQueue alloc] init];
		[queue setName:@"Test"];
	}
}

- (void)doTest
{
	OSAtomicIncrement32(&globalCounter);
	self.counter++;
	NSLog(@"(%@) Global counter: %d; counter: %d", self, globalCounter, self.counter);
}

- (instancetype)init
{
	self = [super init];
	if (self) {
		__weak Test *weakSelf = self;
		NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
		self.observer = [nc addObserverForName:@"TEST"
										object:nil
										 queue:queue
									usingBlock:^(NSNotification *note) {
										[weakSelf doTest];
									}];
	}
	return self;
}

- (void)dealloc
{
	[[NSNotificationCenter defaultCenter] removeObserver:self.observer];
	NSLog(@"Goodbye from %@", self);
}

+ (void)runTest
{
	NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
	
	for (NSUInteger i=0; i<5; ++i) {
		Test *test = [[Test alloc] init];
		[nc postNotificationName:@"TEST" object:nil];
		NSLog(@"(LOOP) Global counter: %d; test counter: %d", globalCounter, test.counter);
	}
}

@end