KVO, Notification, delegate

delegate.m

// mainViewController

@interface MainViewController () <AwesomeViewController>
_awesomeViewController.delegate = self;

- (void)didSelectLabel;

// awesomeViewController
@protocol AwsomeViewControllerDelegate <NSObject>

@optional
- (void)didSelectLabel;

@end

- (void)action
{
      if ([self.delegate respondsToSelector:@selector(didSelectLabel)]) {
        [self.delegate didSelectLabel];
    } else {
        NSLog(@"no delegate didSelectLabel found");
    }
}

kvo.m

// mainViewController.m
[_tableViewController addObserver:self
                           forKeyPath:@"arr"
                              options:NSKeyValueObservingOptionNew
                              context:NULL];
                              
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"%s", __func__);
    if ([keyPath isEqualToString:@"arr"] && [object isEqual:_tableViewController]) {
        NSLog(@"%@", change);
    }
}

// tableViewController.m
[self willChangeValueForKey:@"arr"];
[self.arr addObject:@"watermelon"];
[self didChangeValueForKey:@"arr"];
// or
self.arr = [@[@"apple", @"pear"] mutableCopy];

notification.m

// use notification center as a event bus

// mainViewController
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(handleNotification:)
                                             name:@"addNewObject"
                                           object:nil];
                                           
- (void)handleNotification: (NSNotification *) notification
{
    NSLog(@"%@", notification);
}

// tableViewController
[[NSNotificationCenter defaultCenter] postNotificationName:@"addNewObject" object:self.arr];