如何在iOS上观察后台的输出音量

Question

我遇到了在后台观察输出音量的问题，显然是因为当应用输入后台时音频会话变为非活动状态。我已经在项目设置中打开音频的后台模式，它没有帮助。

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        let audioSession = AVAudioSession.sharedInstance()
        try? audioSession.setCategory(AVAudioSessionCategoryPlayback)
        try? audioSession.setMode(AVAudioSessionModeDefault)
        try? audioSession.setActive(true)
        audioSession.addObserver(self, forKeyPath: "outputVolume", options: .new, context: nil)
}

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        print(AVAudioSession.sharedInstance().outputVolume)
    }
}