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Asked: 2026-06-15T02:17:24+00:00

I am writing a multithreaded application which needs to upload photos from the ALAssetsLibrary

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I am writing a multithreaded application which needs to upload photos from the ALAssetsLibrary en masse in the background. So I have an NSOperation subclass which finds the appropriate ALAsset via the asset’s URL and adds the image to an upload queue.

In the upload queue for the current ALAsset, I need to get the metadata from the image, but I’ve encountered a problem: both the -metadata and the -fullResolutionImage methods never return when they are called on the ALAssetRepresentation of the ALAsset. They simply hang there indefinitely. I tried printing the value of each of these methods in LLDB, but it hung the debugger up, and I ended up killing Xcode, signal 9 style. These methods are being called on a background queue.

I am testing these on an iPad 2. This is the method in which the ALAsset is added to the upload queue when it is found in the success block of -assetForURL:resultBlock:failureBlock:

- (void)addMediaToUploadQueue:(ALAsset *)media {
    @autoreleasepool {
        ALAssetRepresentation *defaultRepresentation = [media defaultRepresentation];
        CGImageRef fullResolutionImage = [defaultRepresentation fullResolutionImage];

        // Return if the user is trying to upload an image which has already been uploaded
        CGFloat scale = [defaultRepresentation scale];
        UIImageOrientation orientation = [defaultRepresentation orientation];

        UIImage *i = [UIImage imageWithCGImage:fullResolutionImage scale:scale orientation:orientation];
        if (![self isImageUnique:i]) return;

        NSDictionary *imageDictionary = [self dictionaryForAsset:media withImage:i];

        dispatch_async(self.background_queue, ^{
            NSManagedObjectContext *ctx = [APPDELEGATE createManagedObjectContextForThread];
            [ctx setUndoManager:nil];

            [ctx performBlock:^{
                ImageEntity *newImage = [NSEntityDescription insertNewObjectForEntityForName:@"ImageEntity"
                                                                    inManagedObjectContext:ctx];

                [newImage updateWithDictionary:imageDictionary
                         inManagedObjectContext:ctx];

                [ctx save:nil];

                [APPDELEGATE saveContext];

                dispatch_async(dispatch_get_main_queue(), ^{
                    [self.fetchedResultsController performFetch:nil];
                });

                    if (!currentlyUploading) {
                        currentlyUploading = YES;
                        [self uploadImage:newImage];
                    }
            }];
        });
    }
}
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1 Answer

  1. Editorial Team

    I had a similar problem and I was tearing my hair out trying to figure it out.

    Turns out while I had thought I setup a singleton for ALAssetsLibrary, my code was not calling it properly and some ALAssets were returning an empty ‘fullResolutionImage’

    In all of my NSLogs I must have missed the most important message from Xcode:

    “invalid attempt to access ALAssetPrivate past the lifetime of its owning ALAssetsLibrary”

    Follow this link

    http://www.daveoncode.com/2011/10/15/solve-xcode-error-invalid-attempt-to-access-alassetprivate-past-the-lifetime-of-its-owning-alassetslibrary/

    I hope that helps

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