read method Null safety

Implementation

@override
Stream<Uint8List> read(PrinterDevice device, {Duration? timeout}) {
  StreamSubscription? subscription;
  StreamController<List<int>>? controller;
  bool cancelSubscription = false;
  controller = StreamController(onListen: () {
    if (timeout != null) {
      Future.delayed(timeout, () {
        if (!cancelSubscription) {
          cancelSubscription = true;
          controller?.close();
          subscription?.cancel();
        }
      });
    }
  }, onCancel: () {
    if (!cancelSubscription) {
      cancelSubscription = true;
      subscription?.cancel();
    }
  });

  if (device is BluetoothDevice) {
    subscription = bluetoothReadStream.listen((e) {
      var returnDevice = BluetoothDevice.fromProto(e.device);
      if (returnDevice.identify() == device.identify()) {
        controller?.add(e.data);
      }
    },
        onError: controller.addError,
        onDone: controller.close,
        cancelOnError: true);
  }
  if (device is UsbDevice) {
    subscription = usbReadStream.listen((e) {
      var returnDevice = UsbDevice.fromProto(e.device);
      if (returnDevice.identify() == device.identify()) {
        controller?.add(e.data);
      }
    },
        onError: controller.addError,
        onDone: controller.close,
        cancelOnError: true);
  }
  if (device is UsbAccessoryDevice) {
    subscription = usbAccessoryReadStream.listen((e) {
      var returnDevice = UsbAccessoryDevice.fromProto(e.device);
      if (returnDevice.identify() == device.identify()) {
        controller?.add(e.data);
      }
    },
        onError: controller.addError,
        onDone: controller.close,
        cancelOnError: true);
  }
  if (device is TcpDevice) {
    subscription = tcpReadStream.listen((e) {
      var returnDevice = TcpDevice.fromProto(e.device);
      if (returnDevice.identify() == device.identify()) {
        controller?.add(e.data);
      }
    },
        onError: controller.addError,
        onDone: controller.close,
        cancelOnError: true);
  }
  if (device is TtyDevice) {
    subscription = ttyReadStream.listen((e) {
      var returnDevice = TtyDevice.fromProto(e.device);
      if (returnDevice.identify() == device.identify()) {
        controller?.add(e.data);
      }
    },
        onError: controller.addError,
        onDone: controller.close,
        cancelOnError: true);
  }
  if (subscription != null) {
    return controller.stream.map((e) => Uint8List.fromList(e));
  }
  throw UnimplementedError();
}