快速清晰的相机预览图层

Question

我有一个应用程序，可以在AVCaptureVideoPreviewLayer上显示相机预览。 当应用程序进入后台会话时停止。问题是，当我再次打开应用程序时，然后在初始化新会话之前，从我在图层上停止会话的时间开始预览。如何在AVCaptureVideoPreviewLayer上清除此预览？我希望它只包含黑色背景，直到新会话开始工作。

代码的某些部分是下面的，但不是全部：

private var displayVideo = false {
    didSet {
        guard displayVideo != oldValue else { return }
        if displayVideo {
            // Configure video capture session.
            captureSession = {
                guard let captureDevice = self.captureDevice else { return nil }
                let captureSession = AVCaptureSession()
                captureSession.sessionPreset = AVCaptureSessionPresetHigh
                let videoDeviceInput = try! AVCaptureDeviceInput(device: captureDevice)
                guard captureSession.canAddInput(videoDeviceInput) else {
                    print("Warning. Failed to add input to capture session.")
                    return nil
                }
                captureSession.addInput(videoDeviceInput)
                configureVideoDevice()
                return captureSession
            }()
            configureVideoOrientation()
        }
        else { captureSession = nil }
    }
}

private var captureSession: AVCaptureSession? {
    didSet {
        if oldValue != captureSession { oldValue?.stopRunning() }
        cameraPreviewLayer.session = captureSession
        captureSession?.startRunning()
    }
}
private lazy var captureDevice: AVCaptureDevice? = {
    guard let videoDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo) else {
        print("Warning. Failed to get capture device.")
        return nil
    }
    return videoDevice
}()
private lazy var cameraPreviewLayer: AVCaptureVideoPreviewLayer = {
    let layer = self.layer as! AVCaptureVideoPreviewLayer
    layer.videoGravity = AVLayerVideoGravityResizeAspectFill
    return layer
}()