如何从AVAudioPlayer的核心数据中播放NSData

Question

我将mp3文件作为NSData二进制数据保存到Core Data中。然后，我在NSFetchedResultsController的帮助下加载此文件。然后，我编写了以下代码，但出现了错误。我该如何解决？

致命错误：解开可选值时意外发现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()
    }
}

UPDATE我检查了一下，发现AVAudioPlayer为零，但我在本堂课开始时就将此球员初始化。

仍然我更改了代码，但出现了相同的错误第二种方法

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

我添加了文件类型提示，它对我有用。

  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

播放来自Core Data的mp3文件时遇到相同的问题。还有另一种解决方案，它不需要您创建资源，而仅需要文件管理器。在Swift 5中可以使用：

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
            }

        }

    } 

}