AudioPlayer播放没有声音

问题代码：

class CallManager: NSObject {
    static let shared = CallManager()
    ...
    func ring() { //响铃
        let audio = AVAudioSession.sharedInstance()
        do {
            try audio.setCategory(.ambient)
            try audio.setActive(true)
            guard let file = Bundle.main.path(forResource: "shake_sound.mp3", ofType: nil) else {
                return
            }
            let url = URL(string: file)
            let player = try? AVAudioPlayer(contentsOf: url!)
            player?.volume = 1
            player?.prepareToPlay()
            player?.play()
        } catch {
            print(error)
        }
    }
}

解决方案：
把player的定义从函数中提出来，定义成全局变量

class CallManager: NSObject {
    static let shared = CallManager()
    ...
    var player:AVAudioPlayer? = nil
    func ring() { //响铃
        let audio = AVAudioSession.sharedInstance()
        do {
            ...
            player = try? AVAudioPlayer(contentsOf: url!)
            ...
        } catch {
            print(error)
        }
    }
}