https://stackoverflow.com/q/59054146/1271826

Example.swift

let group = DispatchGroup()

for album in albums { 
    let formattedAlbum = AlbumFormatted(context: self.persistenceManager.context) 
    formattedAlbum.albumName = album.name 
    group.enter()
    self.getTracklist(album: album.id!) { data in 
        // do whatever you need to add this `data` to your temporary result set
        group.leave()
    }
}

group.notify(queue: .main) {
    // now everything should be there
}