@implementation SampleViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(receiveDidEnterBackgroundNotification:) name:UIApplicationDidEnterBackgroundNotification object:nil];
}

- (void)receiveDidEnterBackgroundNotification:(NSNotification *)notification
{
    // do something
}

- (void)dealloc
{
    // You have to remove observer, but sometimes forget about it.
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

@end

@implementation SampleViewController {
    CSNNotificationObserver *_observer;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    _observer = [[CSNNotificationObserver alloc] initWithObserver:self selector:@selector(receiveDidEnterBackgroundNotification:) name:UIApplicationDidEnterBackgroundNotification object:nil];
}

- (void)receiveDidEnterBackgroundNotification:(NSNotification *)notification
{
    // do something
}

- (void)dealloc
{
    // You don't have to worry about to remove observer
}

@end

@implementation SampleViewController {
    id _didEnterBackgroundNotificationObserver;
    id _willEnterForegroundNotificationObserver;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    _didEnterBackgroundNotificationObserver = [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidEnterBackgroundNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notification) {
        // do something
    }];

    _willEnterForegroundNotificationObserver = [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationWillEnterForegroundNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notification) {
        // do something
    }];
}

- (void)dealloc
{
    // You have to remove observer, but sometimes forget about it.
    [[NSNotificationCenter defaultCenter] removeObserver:_didEnterBackgroundNotificationObserver];
    [[NSNotificationCenter defaultCenter] removeObserver:_willEnterForegroundNotificationObserver];
}

@end

@implementation SampleViewController {
    CSNNotificationObserver *_observer;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    _observer = [[CSNNotificationObserver alloc] initWithName:UIApplicationDidEnterBackgroundNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notification) {
        // do something
    }];

    [_observer addObserverForName:UIApplicationWillEnterForegroundNotification object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *notification) {
        // do something
    }];
}

- (void)dealloc
{
    // You don't have to worry about to remove observer
}

@end