如何從 AVAudioPlayer 中的 Core Data 播放 NSData

Question

我將 mp3 文件作為 NSData 二進制數據保存到 Core Data 中。 然后我在NSFetchedResultsController幫助下加載這個文件。 然后我編寫了以下代碼，但出現錯誤。 我該如何解決？

致命錯誤：在解開 Optional 值時意外發現 nil

var audioPlayer = AVAudioPlayer()
func play() {
    if musicData != nil {
        audioPlayer = AVAudioPlayer(data: musicData, error: nil)
        AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback, error: nil)
        AVAudioSession.sharedInstance().setActive(true, error: nil)
        audioPlayer.prepareToPlay()
        audioPlayer.play()
    }
}

更新我檢查了它，我看到AVAudioPlayer為零，但我在這門課的開頭初始化了這個播放器。

我仍然更改了代碼，但出現相同的錯誤 第二種方法

var av = AVAudioPlayer()
func play() {
    let object = fetchedResultsController.fetchedObjects?.first as! Entity
    let songData = object.mp3

    var urlData: NSURL!
    songData.writeToURL(urlData, atomically: true)
    av = AVAudioPlayer(contentsOfURL: urlData, error: nil)
    av.prepareToPlay()
    av.play()
}

Answer 1

我添加了文件類型提示，它對我有用。

  func playMusic() {
        let url = NSBundle.mainBundle().URLForResource("Music", withExtension: "mp3")!
        let data = NSData(contentsOfURL: url)!
        AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayback, error: nil)
        AVAudioSession.sharedInstance().setActive(true, error: nil)
        audioPlayer = AVAudioPlayer(data: data, fileTypeHint: AVFileTypeMPEGLayer3, error: nil)
        audioPlayer.prepareToPlay()
        audioPlayer.play()
    }

Answer 2

從 Core Data 播放 mp3 文件時遇到了同樣的問題。 還有另一種解決方案，它不需要您制作資源，而只需要文件管理器。 適用於 Swift 5：

final class AudioPlayer: AVPlayer, ObservableObject {

    private func changeSong(with newSong: Song) { // struct Song has function findInDatabase, which can return saved data from CoreData (or error)

        // trying to find song in CoreData
        newSong.findInDatabase() { (result: Result<SavedSong, CoreDataError>) in

            switch result {
            case .success(let savedSong):
                // SavedSong struct has variable 'file' which is of type Data
                guard let songFile = savedSong.file, !songFile.isEmpty else {
                    // your error handling
                    return
                }

                guard var localURL = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask).first else { return } // or error handling
                localURL.appendPathComponent("song.mp3")
                try! songFile.write(to: localURL) // saving song data in url

                // now just playing from created URL
                let nextPlayerItem = AVPlayerItem(url: localURL)
                self.replaceCurrentItem(with: nextPlayerItem) // 'self' - is AVPlayer instance
                self.play()

            case .failure( _):
                // error handling, for example try to play from URL
            }

        }

    } 

}