[Swift] AVFoundation 이용하기

import Cocoa
import AVFoundation
import AVKit

//[url] => [AVPlayerItem]: AVPlayerItem.AVAsset.[AVMetadataItem]: AVMetadataItem.AVMetadataKey.String? => [Track] => [Album]

struct Track {
    var title: String?
    var albumName: String?
    var singer: String?
    
    init(aTitle: String?, aAlbumName: String?, aSinger: String?) {
        self.title = aTitle
        self.albumName = aAlbumName
        self.singer = aSinger
    }
}

struct Album {
    var albumName: String?
    var tracks: [Track]
    
    init(aAlbumName: String?, aTracks: [Track]) {
        self.albumName = aAlbumName
        self.tracks = aTracks
    }
}

class TrackManager {
    var avPlayerItems: [AVPlayerItem] = []
    var albums: [Album] = []
    
    init() {
        avPlayerItems = LoadAVPlayerItems() //[AVPlyerItems]
        let tracks = LoadTracks()//[Track]
        albums = LoadAlbums(tracks: tracks)//[Album]
        albums.count
    }
    
    func LoadAVPlayerItems() -> [AVPlayerItem] {
        let urls = Bundle.main.urls(forResourcesWithExtension: "mp3", subdirectory: nil) ?? []
        let playerItems = urls.map{ AVPlayerItem(url: $0) }
        return playerItems
    }
    
    func LoadTracks() -> [Track] {
        let tracks = avPlayerItems.compactMap( {$0.converToTrack()} )
        return tracks
    }
    
    func LoadAlbums(tracks: [Track]) -> [Album] {
        let trackDic = Dictionary(grouping: tracks) { track in
            track.albumName
        }
        var albums: [Album] = []
        
        for track in trackDic {
            let album: Album = Album.init(aAlbumName: track.key, aTracks: track.value)
            albums.append(album)
        }
        
        return albums
    }
}

extension AVPlayerItem {
    func converToTrack() -> Track {
        let metadatas = asset.metadata
        
        var metaTitle: String?
        var metaAlbumName: String?
        var metaSinger: String?
        
        for meta in metadatas {
            if let title = meta.title {
                metaTitle = title
            }
            
            if let albumName = meta.albumName {
                metaAlbumName = albumName
            }
            if let singer = meta.singer {
                metaSinger = singer
            }
        }
        
        return Track(aTitle: metaTitle, aAlbumName: metaAlbumName, aSinger: metaSinger)
    }
}

extension AVMetadataItem {
    var title: String? {
        guard let title = commonKey?.rawValue, title == "title" else {
            return nil
        }
        return stringValue
    }
    
    var albumName: String? {
        guard let albumName = commonKey?.rawValue, albumName == "albumName" else {
            return nil
        }
        return stringValue
    }
    
    var singer: String? {
        guard let singer = commonKey?.rawValue, singer == "singer" else {
            return nil
        }
        return stringValue
    }
}