Hide volume HUD view in MPVolumeView

问题:

I use MPVolumeView and UISlider to control my device audio and below is my code:

extension MPVolumeView {
    var volumeSlider: UISlider {
        self.showsRouteButton = false
        self.showsVolumeSlider = true

        var slider = UISlider()
        for subview in self.subviews {
            if subview.isKind(of: UISlider.self){
                slider = subview as! UISlider
                slider.isContinuous = false
                slider.minimumTrackTintColor = UIColor.outlineStrokeColor
                slider.setThumbImage(#imageLiteral(resourceName: "volume"), for: .normal)
                slider.maximumTrackTintColor = UIColor.lightGray
                (subview as! UISlider).value = AVAudioSession.sharedInstance().outputVolume
                return slider
            }
        }
        return slider
    }
}


let mpVolumeView: MPVolumeView = {
    let view = MPVolumeView()
    return view
}()

override func viewDidLoad() { 
    _ = mpVolumeView.volumeSlider
    view.addSubview(mpVolumeView)
}

Everything works great but everytime when I open my app, the volume HUD appears and fades away after a few seconds. It won't show up again after that but I wish there would be an option to hide it all the time. I have tried to set MPVolumeView's frame to .zero but it still doesn't work. Is there a way that I can do so?