From https://stackoverflow.com/questions/20952389/wait-for-completion-block-before-executing-something

semaphore.m

// create results array
__block NSMutableArray* results = [NSMutableArray new];

// create serial queue
dispatch_queue_t queue = dispatch_queue_create("myQueue", 0);

for(NSInteger i = 0; i < 10; i++) {
    // enqueue operation in queue
    dispatch_async(queue, ^{
        // create semaphore
        dispatch_semaphore_t sema = dispatch_semaphore_create(0);

        // do something async, I do use another dispatch_queue for example
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
            // wrap in autoreleasepool to release memory upon completion
            // in your case wrap the resultBlock in autoreleasepool
            @autoreleasepool {
                // here for example the nested operation sleeps for two seconds
                sleep(2);

                // add the operation result to array
                // I construct an array of strings for example
                [results addObject:[NSString stringWithFormat:@"Operation %d has finished.", i]];

                // signal that nested async operation completed
                // to wake up dispatch_semaphore_wait below
                dispatch_semaphore_signal(sema);
            }
        });

        // wait until the nested async operation signals that its finished
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);

        NSLog(@"Finished single operation.");
    });
}

// will be called once all operations complete
dispatch_async(queue, ^{
    NSLog(@"Finished all jobs.");
    NSLog(@"Results: %@", results);
});