Keep iOS music library and core data in sync - ios

Question

I'm writing a player software and need to add tags to the songs. For this I've created a core data record.
How do I keep my library in sync with the iOS music library? My approach until now is to get the list of songs in the music library, and the list of songs in CoreData. Then use a dictionary and search for the uuid. If it exists I remove it from my list of CoreData items. After this loop I simply remove all that is left in my CoreData items list.
The problem with this approach, for some reason it crashes with NSFastEnumerationMutationHandler which basicly means I'm trying to access an object in an array in a loop just after I removed it.
- (void)updateMediaLibrary {
NSArray *mediaItems = [self getMediaItems];
NSMutableDictionary *coreDataItems = [self getCoreDataItems];
for (MPMediaItem *item in mediaItems) {
NSNumber *uuid = [item valueForProperty:MPMediaItemPropertyPersistentID];
Song *song = [coreDataItems valueForKey:[uuid stringValue]];
if (!song) {
[Song createSongFromMediaItem:item inContext:self.context];
}
else {
[coreDataItems removeObjectForKey:[uuid stringValue]];
}
}
for (Song *item in coreDataItems) {
[Song deleteSongWithUID:item.libKey inContext:self.threadContext];
}
}
I might just be blind but for some reason I can't see my error in this code.
Do you have alternative/better suggestions, how to keep the songs in sync?
Thanks for your help!

Answer 1

Rather than making a list of the actual coreDataItems, make a list of the persistent keys and do your work with that, something like:
- (void)updateMediaLibrary {
NSArray *mediaItems = [self getMediaItems];
NSMutableSet *coreDataItems = [NSMutableSet setWithArray:[[self getCoreDataItems] allKeys]];
for (MPMediaItem *item in mediaItems) {
NSNumber *uuid = [item valueForProperty:MPMediaItemPropertyPersistentID];
if (![coreDataItems containsObject:[uuid stringValue]]) {
[Song createSongFromMediaItem:item inContext:self.context];
}
else {
[coreDataItems removeObject:[uuid stringValue]];
}
}
for (NSString* key in coreDataItems) {
[Song deleteSongWithUUID:key];
}
}

Answer 2

To me, the problem is probably coming from that method:
[Song createSongFromMediaItem:item inContext:self.context]
Or from that method:
[Song deleteSongWithUID:item.libKey inContext:self.threadContext].
They probably modify the NSArray and/or the NSMutableDictionary during enumeration.
One thing that you could do is:
NSArray *mediaItems = [self getMediaItems].copy;
NSMutableDictionary *coreDataItems = [self getCoreDataItems].mutablecopy;