MBProgressHUD 画不出来解决办法

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- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {    MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.navigationController.view animated:YES];    hud.dimBackground = YES;//block main线程 代码原本在此， 但因为这个方法(块)执行完之前， 画图的代码没有机会执行，所以MBProgressHUD没法被画出来， //所以要想办法 让这段代码执行完，有画图的机会，然后在回到主线程去执行 那种必须在主线程执行的代码（pushViewController）    dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{          dispatch_async(dispatch_get_main_queue(), ^{            NSString *filename = [[self fileList]objectAtIndex:indexPath.row];            ViewController *browserController = [[ViewController alloc] initWithFileName:filename];            browserController.title =  filename ;            [[self navigationController] pushViewController:browserController animated:YES];            [browserController release];            [MBProgressHUD hideHUDForView:self.navigationController.view animated:YES];        });     });}