iOS - 通过委托工作一次解除视图控制器本身

Question

我提出customViewController并试图通过委托解雇自己。它只适用于第一次。当我再次presentCustomViewController时，会出现view controller，但它无法解散。但是，当我尝试通过按钮解除它时，它每次都有效。你能告诉我原因是什么吗？

- (void)presentCustomViewController {
    CustomViewController *customViewController = [self.window.rootViewController.storyboard instantiateViewControllerWithIdentifier:@"CustomViewController"];

    customViewController.modalPresentationStyle = UIModalPresentationOverCurrentContext;
    customViewController.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
    customViewController.delegate = self;

    [self.window.rootViewController presentViewController:customViewController animated:YES completion:nil];
}

#pragma mark - CustomViewController

@implementation CustomViewController {
    NSURLSession *backgroundSession;
    NSURLSessionDownloadTask *downloadTask;
}

- (void)viewDidLoad {
    [super viewDidLoad];
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];

    backgroundSession = nil;
    self.mailComposeViewController = nil;
    self.documentInteractionController = nil;

    [self startDownload];
}

- (void)startDownload {
    NSURLSessionConfiguration *backgroundConfigurationObject = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"myBackgroundSessionIdentifier"];
    backgroundSession = [NSURLSession sessionWithConfiguration:backgroundConfigurationObject delegate:self delegateQueue:[NSOperationQueue mainQueue]];

    downloadTask = [backgroundSession downloadTaskWithURL:[NSURL URLWithString:_attachment.url]];
    [downloadTask resume];
}

- (IBAction)cancelButtonClicked:(id)sender {
    [downloadTask cancel];

    self.mailComposeViewController = nil;
    self.documentInteractionController = nil;

    [self dismissViewControllerAnimated:YES completion:nil];
}

#pragma mark - CustomViewControllerDelegate

- (void)viewController:(CustomViewController *)viewController {    
    [viewController dismissViewControllerAnimated:YES completion:nil];

    NSLog(@"%@, %d", viewController, [viewController isBeingDismissed]);
 }

日志：

<CustomViewController: 0x100691390>, 1
<CustomViewController: 0x100691390>, 0