NSDocument unique identifier

Question

Q: Is there any way to acquire unique identifier of NSDocument in Document-based™ application which is persistent during app restarts?

Note1: there definitely should be some kind of id which is not just fileURL, but something else because app works well with autosaved and even not yet saved documents.

Note2: for some reasons I do not want to generate and persist this id to document's content file.

Is your document a single file or a package? Does it have a place to store metadata? — Michael Dautermann, Jan 18 '17 at 04:30
Related question: [NSFileSystemFileNumber is changed after file is edited/updated in objective c](http://stackoverflow.com/q/30187163/4244136) — Willeke, Jan 20 '17 at 12:21

score 3 · Answer 1 · answered Jan 21 '17 at 13:43

Even I did not try it, I think that extended attributes can do the job:

uuid_t uuid;
[[NSUUID new] getUUIDBytes:uuid];

int result = setxattr( path, "com.yourcompany.yourapp.yourattr", &uuid, sizeof(uuid), 0, 0);

Since extended attributes are nor part of the file itself neither a standard to all file systems, they can be lost, when the file is copied on storage with a file system that does not support such attributes.

score 0 · Accepted Answer · answered Apr 08 '17 at 18:05

I ended up saving data into coder in a method encodeRestorableStateWithCoder during document closing and restoring that data in restoreDocumentWindowWithIdentifier when document is re-opened.

- (void) encodeRestorableStateWithCoder: (NSCoder *) coder {
    [super encodeRestorableStateWithCoder: coder];
    [coder encodeObject: @{@"color":@"red"} forKey: @"OBJECT_KEY"];
}

- (void) restoreDocumentWindowWithIdentifier: (NSString *) identifier
                                       state: (NSCoder *) state
                           completionHandler: (void (^) (NSWindow *, NSError *)) completionHandler {
    NSDictionary * restoredInfo = [state decodeObjectForKey: @"OBJECT_KEY"];
    [super restoreDocumentWindowWithIdentifier: identifier state: state completionHandler: completionHandler];
}

I think it is possible to save a generated UID there, but this storage might not be very reliable.

NSDocument unique identifier

2 Answers2