I'm developing a photo backup app. To detect newly added or edited photos since the app launched, I keep a local dictionary in the format [localIdentifier: modification_date]. However, PHAsset.modificationDate is not reliable. It often changes unexpectedly, possibly due to system operations like iCloud metadata updates. Is there a more reliable way to detect whether a photo has been modified by user since the last app launch? I'm thinking about using content hash instead, but I'm not sure how heavy this operation is in terms of performance.