NSNotification

main.m

#import <UIKit/UIKit.h>
#import "AppDelegate.h"
#import "TTUserObject.h"

int main(int argc, char * argv[]) {
    @autoreleasepool {
        TTUserObject *userObject = [[TTUserObject alloc] init];
        [userObject postNicknameDidChangedNotification];
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}

TTUserObject.h

#import <Foundation/Foundation.h>

@interface TTUserObject : NSObject

- (void)postNicknameDidChangedNotification;

@end

TTUserObject.m

#import "TTUserObject.h"

static NSString * const kUserDidChangeNicknameNotification = @"kUserDidChangeNicknameNotification";

@implementation TTUserObject

- (instancetype)init {
    if (self = [super init]) {
        [[NSNotificationCenter defaultCenter] addObserver:self
                                                 selector:@selector(onUserDidChangeNickname:)
                                                     name:kUserDidChangeNicknameNotification
                                                   object:nil];
    }
    
    return self;
}

- (void)postNicknameDidChangedNotification {
    NSLog(@"A");
    [[NSNotificationCenter defaultCenter] postNotificationName:kUserDidChangeNicknameNotification
                                                        object:nil];
    NSLog(@"C");
}

- (void)onUserDidChangeNickname:(NSNotification *)notification {
    NSLog(@"B");
}

- (void)dealloc {
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

@end