Waiting until AWSS3TransferManagerDownloadRequest is finished (iOS, Objective-C)

Question

I am making an iPhone app and using S3 to store images and now I want to download them and display them in my app. The problem is that I do the AWSS3TransferManagerDownloadRequest right before I display the image. Here is the problem: The first time I want to display the image, it doesn't show up, presumably because the download request has not been finished. After that, whenever I rerun my project, the image shows up ok, presumably because it has already been stored locally. So how can I make the image show up right away. Here is the relevant function.

- (void)makeImageWithBucket:(NSString *)bucket withKey:(NSString *)key atLocation:(NSString *)location{
AWSS3TransferManager *transferManager = [AWSS3TransferManager defaultS3TransferManager];

// Construct the NSURL for the download location.
NSString *downloadingFilePath = [NSTemporaryDirectory() stringByAppendingPathComponent:location];
NSURL *downloadingFileURL = [NSURL fileURLWithPath:downloadingFilePath];

// Construct the download request.
AWSS3TransferManagerDownloadRequest *downloadRequest = [AWSS3TransferManagerDownloadRequest new];

downloadRequest.bucket = bucket;
downloadRequest.key = key;
downloadRequest.downloadingFileURL = downloadingFileURL;

// Download the file.
[[transferManager download:downloadRequest] continueWithExecutor:[AWSExecutor mainThreadExecutor]
                                                       withBlock:^id(AWSTask *task) {
                                                           if (task.error){
                                                               if ([task.error.domain isEqualToString:AWSS3TransferManagerErrorDomain]) {
                                                                   switch (task.error.code) {
                                                                       case AWSS3TransferManagerErrorCancelled:
                                                                       case AWSS3TransferManagerErrorPaused:
                                                                           break;

                                                                       default:
                                                                           NSLog(@"Error: %@", task.error);
                                                                           break;
                                                                   }
                                                               } else {
                                                                   // Unknown error.
                                                                   NSLog(@"Error: %@", task.error);
                                                               }
                                                           }

                                                           if (task.result) {

                                                               AWSS3TransferManagerDownloadOutput *downloadOutput = task.result;

                                                               //File downloaded successfully.
                                                           }
                                                           return nil;
                                                       }];

UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, 375, 667)];
imageView.image = [UIImage imageWithContentsOfFile:downloadingFilePath];
UIScrollView *scrollView = [[UIScrollView alloc] initWithFrame: self.view.frame];
scrollView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
scrollView.contentSize = CGSizeMake(375,imageView.frame.size.height);
[scrollView addSubview:imageView];
[self.view addSubview:scrollView];}

Answer 1

You can update the UI from the 'continue with' block. So, if the image you are displaying is nil, you can update it in this section of your code snippet:

if (task.result) {
   AWSS3TransferManagerDownloadOutput *downloadOutput = task.result;

   //File downloaded successfully.
   //Display the downloaded image here.
}