fetching cached configuration using dispatch groups and early return

snippet.m

@interface ViewController ()
@end
@implementation ViewController

static dispatch_queue_t serialq;
static dispatch_queue_t concurrentq;

- (void)getConfigurationWithCompletion:(void (^)(NSString *))completionBlock {
    static NSString *configuration;

    dispatch_async(serialq, ^{
        if (configuration) {
            NSLog(@"returning cached config.");
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(configuration);
            });
            return;
        }

        NSLog(@"grouping...");
        dispatch_group_t group = dispatch_group_create();
        dispatch_group_enter(group);
        [self GET:^(NSString *result) {
            configuration = result;
            dispatch_async(dispatch_get_main_queue(), ^{
                completionBlock(configuration);
            });
            dispatch_group_leave(group);
        }];
        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    });
}

- (void)GET:(void (^)(NSString *))completionBlock {
    dispatch_async(concurrentq, ^{
        NSLog(@"requesting network...");
        sleep(1);
        NSLog(@"network request done...");
        completionBlock(@"WAT");
    });
}

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    concurrentq = dispatch_queue_create("work q", DISPATCH_QUEUE_CONCURRENT);
    serialq = dispatch_queue_create("exec q", DISPATCH_QUEUE_SERIAL);
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    for (int i = 1; i <= 5; i ++) {
        NSLog(@"Getting configuration for the %d time.", i);
        [self getConfigurationWithCompletion:^(NSString *conf) {
            NSLog(@"Got configuration %d: %@", i, conf);
        }];
    }
}

@end