iOS 9.0的AVCaptureDevice发现会话

我正在开发一个二维码和矩阵码阅读器应用程序。我得到了 AVCaptureDeviceInput AvCaptureDevice.DiscoverySession . 我的问题是它只有在iOS 10.0之后才可用。我怎样才能得到它的回退版本?

// Get the back-facing camera for capturing videos
if #available(iOS 10.0, *) {
    let deviceDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInWideAngleCamera], mediaType: AVMediaType.video, position: .back)
    guard let captureDevice = deviceDiscoverySession.devices.first else {
        print("Failed to get the camera device")
        return
    }

} else {
    // Fallback on earlier versions
}
do {
    // Get an instance of the AVCaptureDeviceInput class using the previous device object.
    let input = try AVCaptureDeviceInput(device: captureDevice)

    // Set the input device on the capture session.
    captureSession.addInput(input)

    // Initialize a AVCaptureMetadataOutput object and set it as the output device to the capture session.
    let captureMetadataOutput = AVCaptureMetadataOutput()
    captureSession.addOutput(captureMetadataOutput)

    // Set delegate and use the default dispatch queue to execute the call back
    captureMetadataOutput.setMetadataObjectsDelegate(self, queue: DispatchQueue.main)

    //TODO: Decide the data types!
    captureMetadataOutput.metadataObjectTypes = [AVMetadataObject.ObjectType.dataMatrix]
    //captureMetadataOutput.metadataObjectTypes = [AVMetadataObject.ObjectType.qr]

} catch {
    // If any error occurs, simply print it out and don't continue any more.
    print(error)
    return
}