NSURLThumbnailDictionaryKey empty for local file - cocoa

Question

I want to get a thumbnail representation of a file I have to display in my app. I'm using NSURL here:
NSDictionary *thumbnails = nil;
BOOL success = [fileURL getResourceValue:&thumbnails
forKey:NSURLThumbnailDictionaryKey
error: &error];
This works fine if I am connected to iCloud, and the URL is a link to a file stored in iCloud. The fileURL is something like:
file:///Users/me/Library/Mobile%20Documents/BJXXGLR9R3~com~myapp~icloud/FileStorage/contact-page%20copy.png
If I use the same code with a NSURL pointing to a local file, however, the thumbnails dictionary is empty.
Here is an example of the URL in this case:
file:///Users/me/Library/Containers/com.mycompany.mymacapp/Data/Library/Application%20Support/com.mycompany.mymacapp/FileStorage/Bn4VaCnCUAEJjLb.png-large.png
Is this API for getResourceValue not supposed to work with locally stored files? Or am I doing something wrong?

Answer 1

This is part of the API. Furthermore you should use a File coordinator while working with files coming from iCloud:
[url startAccessingSecurityScopedResource];
NSFileCoordinator *coordinator = [[NSFileCoordinator alloc] init];
__block NSError *error;
[coordinator coordinateReadingItemAtURL:url options:0 error:&error byAccessor:^(NSURL *newURL) {
[newURL getResourceValue:&image forKey:NSURLThumbnailDictionaryKey error:&error];
}];
[url stopAccessingSecurityScopedResource];