Blocking asynchronous operation.

block asynchronous

NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
    dispatch_group_t dispatchGroup = dispatch_group_create();
    dispatch_group_enter(dispatchGroup);
    [task runWithCompletion:^{
        dispatch_group_leave(dispatchGroup);
    }];
    dispatch_group_wait(dispatchGroup, DISPATCH_TIME_FOREVER);
}];