GCD 保证顺序上传下载

利用线程组保证顺序上传下载, 希望能帮助到一些人对GCD的学习, 也请大神指点.

NSMutableArray *arr = [NSMutableArray arrayWithArray:@[@"1", @"2", @"3", @"4", @"5"]];dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);dispatch_async(queue, ^{    dispatch_group_t group = dispatch_group_create();    dispatch_queue_t sequalQueue = dispatch_queue_create("com.JC.queue", NULL);    __block NSString *cc = @"";    for (int i = 0; i < arr.count; i++) {        dispatch_group_async(group, sequalQueue, ^{            cc = [NSString stringWithFormat:@"%@+%@", cc, arr[i]];            NSLog(@"%@, %d", cc, i);        });    }    dispatch_group_notify(group, dispatch_get_main_queue(), ^{        NSLog(@"~%@~", cc);    });});