Keep a reference to a file after it has moved in objective-c?

Question

I have a Cocoa application that stores a reference to multimedia files (images, videos, etc) on the user's computer. I'm wondering if there is a way to get a reference to that file other that using a file path so that if the user moves that file to a different folder on their computer, I would still know where it is. I'm currently storing the array of file paths that are passed back from the standard Cocoa open dialogue:

-(void)addMultimediaDidEnd:(NSOpenPanel*)sheet
           returnCode:(int)returnCode
          contextInfo:(NSString *)contextInfo 
{   
    if(returnCode == NSOKButton) {
        [sheet orderOut:nil];
        [self saveFiles:[sheet filenames]];
    }
}

Answer 1

In OS X 10.6 (Snow Leopard), an NSURL can be converted to a file reference URL (using -[NSURL fileReferenceURL]) which references a file across moves while your application is running. If you want to persist this file reference, use +[NSURL writeBookmarkData:toURL:options:error:] passing the bookmark data generated with -[NSURL bookmarkDataWithOptions:includingResourceValuesForKeys:relativeToURL:error]. The bookmark can be resolved later with +[NSURL URLByResolvingBookmarkData:options:relativeToURL:bookmarkDataIsStale:error:] passing the bookmark data returned from +[NSURL bookmarkDataWithContentsOfURL:error:].

Prior to OS X 10.6, the same functionality (minus some network aware niceties) is available via the AliasManager, a Carbon-era interface to the OS X file alias system. There are a couple of Objective-C wrappers on top of the Alias Manager that make using it from Cocoa much nicer. My favorite is Wolf Rentzsch's additions to Chris Hanson's BDAlias (available on github).

Answer 2

Here's a quick example of using bookmarks to track files across moves:

- (NSData *)bookmarkFromURL:(NSURL *)url {
    NSData *bookmark = [url bookmarkDataWithOptions:NSURLBookmarkCreationMinimalBookmark
                     includingResourceValuesForKeys:NULL
                                      relativeToURL:NULL
                                              error:NULL];
    return bookmark;
}

- (NSURL *)urlFromBookmark:(NSData *)bookmark {
    NSURL *url = [NSURL URLByResolvingBookmarkData:bookmark
                                           options:NSURLBookmarkResolutionWithoutUI
                                     relativeToURL:NULL
                               bookmarkDataIsStale:NO
                                             error:NULL];
    return url;
}

From https://github.com/ptrsghr/FileWatcher/blob/master/FileWatcherExample/Classes/FileWatcher.m