Example implementation of listening for iTunes/Spotify track changes and displaying information in the NotificationCenter. Article reference: http://blog.corywiles.com/now-playing-with-spotify-and-itunes

gistfile1.m

//
//  AppDelegate.m
//  
//
//  Created by Cory D. Wiles on 10/8/12.
//  Copyright (c) 2013 Cory D. Wiles. All rights reserved.
//

#import "AppDelegate.h"
#import "iTunes.h"
#import "SpotifyClient.h"

@interface AppDelegate()
- (void)updateTrackInfoFromITunes:(NSNotification *)notification;
- (void)updateTrackInfoFromSpotify:(NSNotification *)notification;
@end

@implementation AppDelegate

- (void)dealloc {

  NSDistributedNotificationCenter *dnc = [NSDistributedNotificationCenter defaultCenter];

  [dnc removeObserver:self name:@"com.apple.iTunes.playerInfo" object:nil];
  [dnc removeObserver:self name:@"com.spotify.client.PlaybackStateChanged" object:nil];
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {

  self.masterViewController = [[WNMasterViewController alloc] initWithNibName:@"WNMasterViewController" bundle:nil];

  [self.window.contentView addSubview:self.masterViewController.view];

  self.masterViewController.view.frame = ((NSView*)self.window.contentView).bounds;

  iTunesApplication *iTunes         = [SBApplication applicationWithBundleIdentifier:@"com.apple.iTunes"];
  SpotifyClientApplication *spotify = [SBApplication applicationWithBundleIdentifier:@"com.spotify.client"];

  if ([iTunes isRunning]) {

    NSDistributedNotificationCenter *dnc = [NSDistributedNotificationCenter defaultCenter];

    [dnc addObserver:self
            selector:@selector(updateTrackInfoFromITunes:)
                name:@"com.apple.iTunes.playerInfo"
              object:nil];

  } else if (spotify.isRunning) {

    NSDistributedNotificationCenter *dnc = [NSDistributedNotificationCenter defaultCenter];

    [dnc addObserver:self
            selector:@selector(updateTrackInfoFromSpotify:)
                name:@"com.spotify.client.PlaybackStateChanged"
              object:nil];
  } else {

    [[NSAlert alertWithMessageText:NSLocalizedString(@"iTunes nor Spotify are Running", nil)
                     defaultButton:NSLocalizedString(@"OK", nil)
                   alternateButton:nil
                       otherButton:nil
         informativeTextWithFormat:@"iTunes nor Spotify are currently running, thus songs won't be sync'd"] runModal];
  }

  [self.window makeKeyAndOrderFront:self];
}

- (BOOL)applicationShouldHandleReopen:(NSApplication *)application
                    hasVisibleWindows:(BOOL)flag {

  [self.window makeKeyAndOrderFront:self];

  return YES;
}

#pragma mark - Methods

- (void)updateTrackInfoFromITunes:(NSNotification *)notification {

  NSLog(@"notification payload: %@", notification.userInfo);

  iTunesApplication *iTunes = [SBApplication applicationWithBundleIdentifier:@"com.apple.iTunes"];

  if ([iTunes isRunning]) {

    NSUserNotification *notification = [[NSUserNotification alloc] init];

    [notification setTitle:[[[iTunes currentTrack] get] name]];
    [notification setInformativeText:[[[iTunes currentTrack] get] artist]];
    [notification setDeliveryDate:[NSDate dateWithTimeInterval:2 sinceDate:[NSDate date]]];
    [notification setSoundName:NSUserNotificationDefaultSoundName];

    NSUserNotificationCenter *center = [NSUserNotificationCenter defaultUserNotificationCenter];

    [center scheduleNotification:notification];
  }
}

- (void)updateTrackInfoFromSpotify:(NSNotification *)notification {

  SpotifyClientApplication *spotify = [SBApplication applicationWithBundleIdentifier:@"com.spotify.client"];
  SpotifyClientTrack *spotifyTrack  = [spotify currentTrack];
  
  NSLog(@"notification payload: %@", notification.userInfo);

  if ([[notification.userInfo valueForKey:@"Player State"] isEqualToString:@"Stopped"]) {
    return;
  }

  /**
   * If you don't have a paid account then you'll get adverts that register as 
   * 'songs'. So you'll probably want to skip the notification in a real app.
   */

  if ([[spotifyTrack album] hasPrefix:@"http"]) {
    NSLog(@"Album prefix = http (%@)", [spotifyTrack album]);
  }

  NSUserNotification *notif = [[NSUserNotification alloc] init];

  [notif setTitle:[spotifyTrack name]];
  [notif setInformativeText:[spotifyTrack albumArtist]];
  [notif setDeliveryDate:[NSDate dateWithTimeInterval:2 sinceDate:[NSDate date]]];
  [notif setSoundName:NSUserNotificationDefaultSoundName];

  NSUserNotificationCenter *center = [NSUserNotificationCenter defaultUserNotificationCenter];

  [center scheduleNotification:notif];
}

@end

Hi there, I made some iOS app and I would like to sent notifications to those app when a new app is available.
What do I have to do to be able to send such notifications from my Mac ?
Thanks in advance for your help.