Flutter: How to pause and restart a `Stream` listener?

Question

I am developing a flutter app to read QR Codes. I am using qr_code_scanner: ^0.3.5 library. Below is my code.

import 'dart:io';

import 'package:flutter/material.dart';
import 'package:google_fonts/google_fonts.dart';
import 'package:qr_code_scanner/qr_code_scanner.dart';

class ScanQRCodeScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Colors.white,
        appBar: AppBar(
          title: Text("Scan QR Code"),
        ),
        body: _ScanQRCodeUI());
  }
}

class _ScanQRCodeUI extends StatefulWidget {
  @override
  State createState() {
    return _ScanQRCodeUIState();
  }
}

class _ScanQRCodeUIState extends State<_ScanQRCodeUI> {
  final GlobalKey qrKey = GlobalKey(debugLabel: 'QR');
  Barcode result;
  QRViewController controller;

  // In order to get hot reload to work we need to pause the camera if the platform
  // is android, or resume the camera if the platform is iOS.
  @override
  void reassemble() {
    super.reassemble();
    if (Platform.isAndroid) {
      controller.pauseCamera();
    } else if (Platform.isIOS) {
      controller.resumeCamera();
    }
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Expanded(flex: 4, child: _buildQrView(context)),
        Expanded(flex: 1, child: _dataDisplayUI())
      ],
    );
  }

  Widget _buildQrView(BuildContext context) {
    // For this example we check how width or tall the device is and change the scanArea and overlay accordingly.
    var scanArea = (MediaQuery.of(context).size.width < 400 ||
            MediaQuery.of(context).size.height < 400)
        ? 200.0
        : 400.0;
    // To ensure the Scanner view is properly sizes after rotation
    // we need to listen for Flutter SizeChanged notification and update controller
    return QRView(
      key: qrKey,
      onQRViewCreated: _onQRViewCreated,
      overlay: QrScannerOverlayShape(
          borderColor: Colors.red,
          borderRadius: 10,
          borderLength: 30,
          borderWidth: 10,
          cutOutSize: scanArea),
    );
  }

  void _onQRViewCreated(QRViewController controller) {
    setState(() {
      this.controller = controller;
    });

    controller.scannedDataStream.listen((scanData) async {
      print("Hello0");
      setState(() {
        result = scanData;
        print(result.code);
      });

      // await controller.pauseCamera();
    });
  }

  Widget _dataDisplayUI() {
    const yellowColor = const Color(0xffEDE132);

    return Column(
      children: [
        Row(
          children: [
            Expanded(
                flex: 7,
                child: Container(
                  margin:
                      EdgeInsets.only(top: 30, bottom: 30, left: 10, right: 10),
                  child:
                      Text("You have added 12 products. Click here to publish.",
                          style: GoogleFonts.poppins(
                              textStyle: TextStyle(
                            color: Colors.black,
                            fontWeight: FontWeight.normal,
                            fontSize: 14,
                          ))),
                )),
            Expanded(
                flex: 3,
                child: Container(
                  width: 60,
                  height: 60,
                  child: Center(
                      child: Text("12",
                          style: GoogleFonts.poppins(
                              textStyle: TextStyle(
                            color: Colors.black,
                            fontWeight: FontWeight.bold,
                            fontSize: 21,
                          )))),
                  decoration:
                      BoxDecoration(shape: BoxShape.circle, color: yellowColor),
                ))
          ],
        )
      ],
    );
  }

  @override
  void dispose() {
    controller?.dispose();
    super.dispose();
  }
}

I am using this app to scan products one by one, just like how the cashier does in a super marker using a barcode scanner.

The issue is, this scanner is listening to a stream and it is keep on running. Pay your attention to the _onQRViewCreated method. As a result, the same QR is being read multiple times before we even move the camera to the next QR code.

How can I make sure there is a delay between 2 scans? For an example, when I scan for a QR Code, I have to wait for another 2 seconds to scan the next QR.

If my idea of creating a delay between 2 scans is wrong, I am open for other ideas as well.

user_9074658 · Answer

You can use

controller.scannedDataStream.first

that stops listening to other events from the stream.

Another solution would be to set an internal state property like below:

bool QrBeingProcessed = false;

and when you scan the first qr set it to true, till you're done.

user_12425707 · Answer

Another way would be to use DateTime to check and see if the time difference between the current scan and the last scan is under a specified amount (say 3 seconds). If so, we don't set the state to the new scanData. In effect, this puts a 3-second delay between each QR code scan. See the code below.

     controller.scannedDataStream.listen((scanData) {
      final currentScan = DateTime.now();
      if (lastScan == null || currentScan.difference(lastScan!) > const Duration(seconds: 3)) 
      {
        lastScan = currentScan;
        print(scanData.code);
  
        setState(() {
          result = scanData;
          print('the qr just read was ' + scanData.code);
        });
     }
  });

}

Flutter: How to pause and restart a `Stream` listener?

2 Answers