Usage of dispatch_group_notify in Objective-C

gistfile1.m

#import "AppDelegate.h"

@interface AppDelegate ()
@property (nonatomic, strong) dispatch_group_t dispatchGroup;
@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    self.dispatchGroup = dispatch_group_create();
    __weak typeof(self) weakSelf = self;
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_group_enter(weakSelf.dispatchGroup);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"block 1");
            dispatch_group_leave(weakSelf.dispatchGroup);
            [weakSelf groupNotifier];
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_group_enter(weakSelf.dispatchGroup);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"block 2");
            dispatch_group_leave(weakSelf.dispatchGroup);
            [weakSelf groupNotifier];
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_group_enter(weakSelf.dispatchGroup);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"block 3");
            dispatch_group_leave(weakSelf.dispatchGroup);
            [weakSelf groupNotifier];
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        dispatch_group_enter(weakSelf.dispatchGroup);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"block 4");
            dispatch_group_leave(weakSelf.dispatchGroup);
            [weakSelf groupNotifier];
        });
    });
    
    return YES;
}

- (void)groupNotifier {
    dispatch_group_notify(self.dispatchGroup, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"Does this only print once?");
    });
}

@end

/* This results in the following trace:
2015-04-08 15:49:52.264 DispatchGroupTest[3491:1504519] block 2
2015-04-08 15:49:52.264 DispatchGroupTest[3491:1504519] block 3
2015-04-08 15:49:52.265 DispatchGroupTest[3491:1504519] block 4
2015-04-08 15:49:52.265 DispatchGroupTest[3491:1504519] block 1
2015-04-08 15:49:52.265 DispatchGroupTest[3491:1505522] Does this only print once?
2015-04-08 15:49:52.265 DispatchGroupTest[3491:1505523] Does this only print once?
2015-04-08 15:49:52.265 DispatchGroupTest[3491:1505522] Does this only print once?
2015-04-08 15:49:52.265 DispatchGroupTest[3491:1505523] Does this only print once?
*/