typedef void(^LoopBlock)(dispatch_block_t, int);

LoopBlock *loop_blk = malloc(sizeof(LoopBlock));
*loop_blk = ^(dispatch_block_t done_blk, int i) {
    ((i > 0)
        ? (printf("executing loop %d more times\n", i),
          (*loop_blk)(done_blk, i - 1))
        : (done_blk(), free(loop_blk)));
};

(*loop_blk)(^{ printf("ok done\n"); }, 20);