I create new Song NSManagedObjects in a background managed object context since I expect the user to select as many as 10k to 100k songs.

AudioFile is a core data entity in my app that models any files that are of type audio. Song is the “middleman”/”intermediate” entity that has a “many to one” relationship to a playlist.

When the user presses “Done” in the navigation bar of my SwiftUI view, it gets passed onto my view model array to reflect the changes in a list view.

At the moment, I am not sure how to properly pass the NSManagedObjects from the background context to the main queue’s managed object context without causing performance issues.

class PlaylistSongsSelectionVM: ObservableObject {        
    private var songsToBeAdded = [Song]()
    
    private let moc = DataController.shared.container.viewContext
    
    private let backgroundContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
    
    private init() {
        backgroundContext.parent = moc
    }

    func addAudioFileToPlaylist(audioFile: AudioFile) {
        backgroundContext.perform { [weak self] in
            guard let self else { return }
//Creates dummy songs for testing
            for _ in 1...10000 {
                let newSong = Song(moc: backgroundContext, audioFile: audioFile)
                songsToBeAdded.append(newSong)
            }
        }
    }

    
    ///Call this method when user presses done to add the songs to the respective playlist
    func addSongsToPlaylist() {
        try? backgroundContext.save()
        
        Task {
            var ids: [NSManagedObjectID]? = nil
            
            await backgroundContext.perform { [weak self] in
                guard let self else { return }
                ids = backgroundContext.registeredObjects.map {
                    $0.objectID
                }
                
                songsToBeAdded.removeAll()
                backgroundContext.reset()
            }
            
            //Only add songs that needs to be added to the playlist
            if let ids {
                PlaylistCreateViewModel.shared.addSongsFromSongAddingView(for: ids)
            }
        }
    }
}

class PlaylistCreateViewModel: ObservableObject { 
    static let shared = PlaylistCreateViewModel()
    
    private let moc = DataController.shared.container.viewContext
    
    @Published var playlistSongs: [Song] = []

    func addSongsFromSongAddingView(for objectIds: [NSManagedObjectID]) async {
        objectIds.forEach { id in
            if let songToBeAdded = moc.object(with: id) as? Song {
                playlistSongs.append(songToBeAdded)
            }
        }
    }
}