PHAsset (or ALAsset) cross-device identifier

Question

I am actually creating an iOS app, and want to store some details about Photo Library pictures in an SQLite database. I also would like to be able to share this data across the different devices the user has (imagine someone wanted to reorganize pictures on his iPhone, and wants to retrieve the same order in his iPad).

I have searched across the Apple documentation, but did not find any PHAsset identifier that is shared across devices. I also tried to check if the PHImageFileURLKey, hashValue, or localIdentifier of the PHAsset where shared, but none of them are.

Has someone already done something like this?

I realized that I have to find this question again and make comment. Apple recently announced PHCloudIdentifier ! Please check official document https://developer.apple.com/documentation/photokit/phcloudidentifier — Kyle Yi, Dec 09 '21 at 19:14
You should change the accepted answer to the @falco-winkler answer. — Grimxn, Mar 05 '22 at 20:04

score 5 · Accepted Answer · answered Mar 04 '15 at 14:19

5

There is none at the moment. PHAsset had a shared identifier in the first iOS 8 Betas, but it was removed later on.

answered Mar 04 '15 at 14:19

holtmann

6,043
32
44

Falco Winkler · Answer 2 · 2022-02-13T12:00:26.430

Since iOS 15 there is an identifier that is available across all devices: https://developer.apple.com/documentation/photokit/phcloudidentifier

The API is pretty simple. The idea is that you store and sync cloud identifiers and then use them to retrieve local identifiers, which you then can use with existing API's like you are used to. You can request the cloud identifiers in batches, like so:

let localIdentifiers = <your-list-of-local-identifiers>
let mappings: [String: Result<PHCloudIdentifier, Error>] = 
  PHPhotoLibrary.shared().cloudIdentifierMappings(forLocalIdentifiers: localIdentifiers)

You can store such identifiers by using the stringValue property.

Vice versa (say you have a list of cloudIdentifiers with the string values of the identifiers):

let localIdentifiers: [PHCloudIdentifier: Result<String, Error>]
   = PHPhotoLibrary
        .shared()
        .localIdentifierMappings(
          for: cloudIdentifiers.map(PHCloudIdentifier.init(stringValue:)))

What errors can happen?

No identifier was found: Either you passed in an invalid identifier or the asset has been deleted
Multiple local identifiers for a cloud identifier where found (According to WWDC this happens if "Cloud state isn't in sync or the library relies on image content to find a match") More info about the error handling here

Additional insights that i derived through experimentation:

You seem to be able to get cloud identifiers without internet, with iCloud disabled for your app even without being signed into an iCloud account at all.
Even though the doc states it's an expensive operation, the overhead for loading some hundreds of identifiers at once is not noticeable for me

This should now be the accepted answer. It works perfectly. – Grimxn Mar 05 '22 at 20:03 — Grimxn, Mar 05 '22 at 20:03

score -2 · Answer 3 · answered Dec 02 '15 at 20:44

It looks like, that for all PHAssets that have been synced with iTunes, the localIdentifier is identical, even across devices. Means: if you import images from one device and sync them to another device, the localIdentifier of the PHAsset is identical.

PHAsset (or ALAsset) cross-device identifier

3 Answers3

Linked