Swift - some mp3 file metadata return nil - swift

Question

I have code like below. For some file I got metadata like artist, title and other without any problem. For other files metadata list is nil but when I check metadata in editor like Tagger - title and other metadata exists. Furthermore when I change metadata in external editor for at least one key - my code starts work properly.
Could someone explain me where I make mistake ?
static func getBookInCatalog(url: URL) -> Book {
let book = Book(url: url)
let isDir: ObjCBool = false
var directoryContents = [URL]()
var totalTime: CMTime?
var size: UInt64 = 0
var chapters:Int = 0
do {
directoryContents = try FileManager.default.contentsOfDirectory(at: url, includingPropertiesForKeys: nil, options: [])
} catch let error as NSError {
print(error.localizedDescription)
return book
}
for item in directoryContents {
if !isDir.boolValue {
let result = appDelegate.fileTypes.filter { $0==item.pathExtension.lowercased() }
if !result.isEmpty {
chapters += 1
let fileSize = (try! FileManager.default.attributesOfItem(atPath: item.path)[FileAttributeKey.size] as! NSNumber).uint64Value
size += fileSize
let playerItem = AVPlayerItem(url: item)
let metadataList = playerItem.asset.commonMetadata
let asset = AVURLAsset(url: item, options: nil)
let audioDuration = asset.duration
if let _ = totalTime {
totalTime = totalTime! + audioDuration
} else {
totalTime = audioDuration
}
for metadata in metadataList {
guard let key = metadata.commonKey, let value = metadata.value else{
continue
}
switch key {
case "albumName":
if book.title == nil || book.title == "" {
book.title = (value as? String)!
}
case "artist":
if book.author == nil || book.author == "" {
book.author = (value as? String)!
}
case "artwork" where value is NSData:
if book.image == nil {
book.image = UIImage(data: (value as! NSData) as Data)
}
default:
continue
}
}
}
}
}
if let imageInsideCatalog = getImageFromFolder(url: url){
book.image = imageInsideCatalog
}
if book.title == nil {
book.title = url.deletingPathExtension().lastPathComponent
}
book.chapters = chapters
book.totalTime = totalTime
book.size = size
return book
}

Answer 1

MP3 meta data "standards" have gone through several major iterations over the years (see http://id3.org) . Your editor may be able to read older formats (that AVURLAsset may not support) and save them using the latest/current standard which would make them compatible after any change.