How to Implement a Flutter QR Code Scanner Plugin from Scratch

8 min readOct 28, 2021

If you want to create a Flutter camera application, a quick way is to install the official Camera plugin. However, the plugin is still under development, and there are no vision APIs available yet. This article will show you how to implement a custom Flutter camera view widget by wrapping an Android native camera view which supports QR code scanning.

Installation

Dynamsoft Barcode Reader:

allprojects {
      repositories {
          google()
          jcenter()
          maven {
              url "https://download2.dynamsoft.com/maven/dbr/aar"
          }
      }
  }
    
  dependencies {
      implementation 'com.dynamsoft:dynamsoftbarcodereader:8.8.0@aar'
  }

Dynamsoft Camera Enhancer:

allprojects {
      repositories {
          google()
          jcenter()
          maven {
              url "https://download.dynamsoft.com/maven/dce/aar"
          }
      }
  }
  dependencies {
      implementation 'com.dynamsoft:dynamsoftcameraenhancer:2.0.0@aar'
  }

Creating A Simple Flutter Widget from Android TextView

In this section, let’s follow Flutter’s tutorial to host an Android TextView in Flutter widget with PlatformView.

Create a new Flutter plugin project:

flutter create --org com.dynamsoft --template=plugin --platforms=android -a java flutter_qrcode_scanner

On the Dart side, we can see the following code in lib/flutter_qrcode_scanner.dart:

import 'dart:async';
    
 import 'package:flutter/services.dart';
    
 class FlutterQrcodeScanner {
   static const MethodChannel _channel = MethodChannel('flutter_qrcode_scanner');
    
   static Future<String?> get platformVersion async {
     final String? version = await _channel.invokeMethod('getPlatformVersion');
     return version;
   }
 }

Delete the code snippet above, and define a stateful widget as follows:

class ScannerView extends StatefulWidget {
   const ScannerView({Key? key}) : super(key: key);
   @override
   State<StatefulWidget> createState() => _ScannerViewState();
 }
    
 class _ScannerViewState extends State<ScannerView> {
   @override
   void initState() {
     super.initState();
   }
    
   @override
   Widget build(BuildContext context) {
     const String viewType = 'com.dynamsoft.flutter_qrcode_scanner/nativeview';
     final Map<String, dynamic> creationParams = <String, dynamic>{};
    
     return AndroidView(
       viewType: viewType,
       creationParams: creationParams,
       creationParamsCodec: const StandardMessageCodec(),
     );
   }
 }

On the platform side, we create NativeView class which extends io.flutter.plugin.platform.PlatformView to provide a reference to TextView:

import android.content.Context;
 import android.graphics.Color;
 import android.view.View;
 import android.widget.TextView;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import io.flutter.plugin.platform.PlatformView;
 import java.util.Map;
    
 class NativeView implements PlatformView {
     @NonNull private final TextView textView;
     
      NativeView(@NonNull Context context, int id, @Nullable Map<String, Object> creationParams) {
          textView = new TextView(context);
          textView.setTextSize(72);
          textView.setBackgroundColor(Color.rgb(255, 255, 255));
          textView.setText("Rendered on a native Android view (id: " + id + ")");
      }
     
      @NonNull
      @Override
      public View getView() {
          return textView;
      }
     
      @Override
      public void dispose() {}
 }

Then create a NativeViewFactory class to initialize NativeView:

import android.content.Context;
 import android.view.View;
 import androidx.annotation.Nullable;
 import androidx.annotation.NonNull;
 import io.flutter.plugin.common.BinaryMessenger;
 import io.flutter.plugin.common.StandardMessageCodec;
 import io.flutter.plugin.platform.PlatformView;
 import io.flutter.plugin.platform.PlatformViewFactory;
 import java.util.Map;

 class NativeViewFactory extends PlatformViewFactory {
   @NonNull private final BinaryMessenger messenger;

   NativeViewFactory(@NonNull BinaryMessenger messenger) {
     super(StandardMessageCodec.INSTANCE);
     this.messenger = messenger;
   }

   @NonNull
   @Override
   public PlatformView create(@NonNull Context context, int id, @Nullable Object args) {
     final Map<String, Object> creationParams = (Map<String, Object>) args;
     return new NativeView(context, id, creationParams);
   }
 }

In FlutterQrcodeScannerPlugin.java file, we register the factory class:

@Override
 public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
   channel = new MethodChannel(flutterPluginBinding.getBinaryMessenger(), "flutter_qrcode_scanner");
   channel.setMethodCallHandler(this);
   flutterPluginBinding.getPlatformViewRegistry().registerViewFactory(
       "com.dynamsoft.flutter_qrcode_scanner/nativeview",
       new NativeViewFactory(flutterPluginBinding.getBinaryMessenger()));
 }

Now, the plugin is ready to be used. We change the code of example/lib/main.dart to make it work:

import 'package:flutter/material.dart';
import 'dart:async';

import 'package:flutter/services.dart';
import 'package:flutter_qrcode_scanner/flutter_qrcode_scanner.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _platformVersion = 'Unknown';

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Plugin example app'),
        ),
        body: Center(
          child: ScannerView(),
        ),
      ),
    );
  }
}

Run the example:

flutter run

Implementing a Flutter Camera Widget from Android Camera View

If the code above can work for you, the next step is to replace the text view with a camera view. Here we use Dynamsoft Camera Enhancer as an example.

Configure android/build.gradle:

rootProject.allprojects {
     repositories {
         google()
         mavenCentral()
         maven {
             url "https://download.dynamsoft.com/maven/dce/aar"
         }
     }
 }
    
 android {
     compileSdkVersion 30

     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8

The minimum Android SDK version needs to be changed to 21 or higher.

In NativeView.java, import the camera package:

import com.dynamsoft.dce.*;

Change TextView to DCECameraView. With CameraEnhancer, we can start and stop the camera preview:

private final DCECameraView cameraView;
 private CameraEnhancer cameraEnhancer;

 NativeView(BinaryMessenger messenger, @NonNull Activity context, int id,
         @Nullable Map<String, Object> creationParams) {
     this.context = context;

     cameraEnhancer = new CameraEnhancer(context);
     cameraView = new DCECameraView(context);
     cameraEnhancer.setCameraView(cameraView);
     try {
         cameraEnhancer.open();
     } catch (Exception e) {
         e.printStackTrace();
     }
 }

In order to switch camera status along with activity lifecycle, we implement Application.ActivityLifecycleCallbacks:

class NativeView implements PlatformView, Application.ActivityLifecycleCallbacks {
     @Override
     public void onActivityResumed(Activity activity) {
         try {
             cameraEnhancer.open();
         } catch (Exception e) {
             e.printStackTrace();
         }
     }

     @Override
     public void onActivityPaused(Activity activity) {
         try {
             cameraEnhancer.close();
         } catch (Exception e) {
             e.printStackTrace();
         }
     }
 }

You may have noticed that we use Activity instead of Context in NativeView class. So the NativeViewFactory class needs to be changed as well:

class NativeViewFactory extends PlatformViewFactory {
   @NonNull private final BinaryMessenger messenger;
   @NonNull private Activity activity;

   NativeViewFactory(@NonNull BinaryMessenger messenger, Activity activity) {
     super(StandardMessageCodec.INSTANCE);
     this.messenger = messenger;
     this.activity = activity;
   }

   @NonNull
   @Override
   public PlatformView create(@NonNull Context context, int id, @Nullable Object args) {
     final Map<String, Object> creationParams = (Map<String, Object>) args;
     return new NativeView(messenger, activity, id, creationParams);
   }
 }

Finally, in FlutterQrcodeScannerPlugin class, we monitor activity status and pass the activity reference to NativeViewFactory instance:

public class FlutterQrcodeScannerPlugin implements FlutterPlugin, ActivityAware {
     private Activity activity;
     private FlutterPluginBinding flutterPluginBinding;

     @Override
     public void onAttachedToEngine(@NonNull FlutterPluginBinding flutterPluginBinding) {
       this.flutterPluginBinding = flutterPluginBinding;
     }

     @Override
     public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
       this.flutterPluginBinding = null;
     }

     @Override
     public void onAttachedToActivity(ActivityPluginBinding activityPluginBinding) {
       bind(activityPluginBinding);
     }

     @Override
     public void onReattachedToActivityForConfigChanges(ActivityPluginBinding activityPluginBinding) {
       bind(activityPluginBinding);
     }

     @Override
     public void onDetachedFromActivityForConfigChanges() {
       activity = null;
     }

     @Override
     public void onDetachedFromActivity() {
       activity = null;
     }

     private void bind(ActivityPluginBinding activityPluginBinding) {
       activity = activityPluginBinding.getActivity();
       flutterPluginBinding.getPlatformViewRegistry().registerViewFactory(
           "com.dynamsoft.flutter_qrcode_scanner/nativeview",
           new NativeViewFactory(flutterPluginBinding.getBinaryMessenger(), activity));
     }
 }

A Flutter camera view widget is done. But before running the example, we have to change the minimum SDK version to 21 in example/android/build.gradle.

Turning Flutter Camera Plugin into Flutter QR Code Scanner

Generally, we can take camera frames from preview callback function and do further image processing. To save developer’s time, a good camera widget should provide vision functionalities. In the following paragraph, you will see how to integrate Dynamsoft Barcode Reader SDK into the current Flutter camera view widget step by step.

The same to using the camera SDK, we add the barcode package to build.gradle:

rootProject.allprojects {
     repositories {
         google()
         mavenCentral()
         maven {
             url "https://download.dynamsoft.com/maven/dce/aar"
         }
         maven {
             url "https://download.dynamsoft.com/maven/dbr/aar"
         }
     }
 }
    
 android {
     compileSdkVersion 30

     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
     }

     defaultConfig {
         minSdkVersion 21
     }
 }

 dependencies {
     implementation 'com.dynamsoft:dynamsoftcameraenhancer:2.0.0@aar'
     implementation 'com.dynamsoft:dynamsoftbarcodereader:8.8.0@aar'
 }

Dynamsoft Camera Enhancer and Dynamsoft Barcode Reader SDK can co-work well. We put them into a QRCodeScanner.java file to implement real-time QR code scanning:

public class QRCodeScanner {
     private CameraEnhancer mCameraEnhancer;
     private BarcodeReader reader;
     private Activity context;
     private DCECameraView cameraView;
     private DetectionHandler handler;

     public interface DetectionHandler {
         public void onDetected(List<Map<String, Object>> data);
     }

     public QRCodeScanner(Activity context, DCECameraView cameraView) {
         this.context = context;
         this.cameraView = cameraView;
         mCameraEnhancer = new CameraEnhancer(context);
         mCameraEnhancer.setCameraView(cameraView);
         cameraView.setOverlayVisible(true);

         DCESettingParameters dceSettingParameters = new DCESettingParameters();
         dceSettingParameters.cameraInstance = mCameraEnhancer;
         dceSettingParameters.textResultCallback = mTextResultCallback;

         try {
             // mCameraEnhancer.open();
             reader = new BarcodeReader();
             reader.SetCameraEnhancerParam(dceSettingParameters);
             PublicRuntimeSettings settings = reader.getRuntimeSettings();
             settings.barcodeFormatIds = EnumBarcodeFormat.BF_QR_CODE;
             reader.updateRuntimeSettings(settings);
         } catch (Exception e) {
             // TODO: handle exception
         }
     }

     TextResultCallback mTextResultCallback = new TextResultCallback() {
         @Override
         public void textResultCallback(int i, TextResult[] results, Object userData) {
             if (results != null) {
                 final List<Map<String, Object>> ret = new ArrayList<Map<String, Object>>();
                 for (TextResult result: results) {
                     final Map<String, Object> data = new HashMap<>();
                     data.put("format", result.barcodeFormatString);
                     data.put("text", result.barcodeText);
                     data.put("x1", result.localizationResult.resultPoints[0].x);
                     data.put("y1", result.localizationResult.resultPoints[0].y);
                     data.put("x2", result.localizationResult.resultPoints[1].x);
                     data.put("y2", result.localizationResult.resultPoints[1].y);
                     data.put("x3", result.localizationResult.resultPoints[2].x);
                     data.put("y3", result.localizationResult.resultPoints[2].y);
                     data.put("x4", result.localizationResult.resultPoints[3].x);
                     data.put("y4", result.localizationResult.resultPoints[3].y);
                     data.put("angle", result.localizationResult.angle);
                     ret.add(data);
                 }

                 if (handler != null) {
                     handler.onDetected(ret);
                 }
             }
         }
     };

     public void setDetectionHandler(DetectionHandler handler) {
         this.handler = handler;
     }

     public void startScan() {
         try {
             // mCameraEnhancer.open();
             cameraView.setOverlayVisible(true);
             reader.StartCameraEnhancer();
         } catch (Exception e) {
             // TODO: handle exception
         }
     }

     public void stopScan() {
         try {
             // mCameraEnhancer.close();
             cameraView.setOverlayVisible(false);
             reader.StopCameraEnhancer();
         } catch (Exception e) {
             // TODO: handle exception
         }
     }

     public void setLicense(String license) {
         try {
             reader.initLicense(license);
         } catch (Exception e) {
             e.printStackTrace();
         }
     }
 }

To activate the QR code detection API, a valid license is required.

As a QR code detected, we need a way to send the results from Java to Dart. The MethodChannel class is used to send and receive messages between Dart and native code. Therefore, we add the following code to NativeView.java:

class NativeView implements PlatformView, MethodCallHandler, Application.ActivityLifecycleCallbacks, QRCodeScanner.DetectionHandler {
     @Override
     public void onMethodCall(@NonNull MethodCall call, @NonNull Result result) {
         switch (call.method) {
         case "startScanning":
             qrCodeScanner.startScan();
             result.success(null);
             break;
         case "stopScanning":
             qrCodeScanner.stopScan();
             result.success(null);
             break;
         case "setLicense":
             final String license = call.argument("license");
             qrCodeScanner.setLicense(license);
             result.success(null);
             break;
         case "setBarcodeFormats":
             final int formats = call.argument("formats");
             qrCodeScanner.setBarcodeFormats(formats);
             result.success(null);
             break;
         default:
             result.notImplemented();
         }
     }
 }

When QR code is detected, the onDetected callback function will be triggered. One thing you have to know is the function is called by a worker thread, whereas the method channel communication has to be invoked in the main thread. To achieve this, we use runOnUiThread to send messages to the main thread:

@Overridepublic void onDetected(List<Map<String, Object>> data) {
     context.runOnUiThread(new Runnable() {
         @Override
         public void run() {
             methodChannel.invokeMethod("onDetected", data);
         }
     });
 }

On the Dart side, we define a ScannerViewController in flutter_qrcode_scanner.dart to receive QR code detection results:

class _ScannerViewState extends State<ScannerView> {
   ScannerViewController? _controller;

   @override
   void initState() {
     super.initState();
   }

   @override
   Widget build(BuildContext context) {
     const String viewType = 'com.dynamsoft.flutter_qrcode_scanner/nativeview';
     final Map<String, dynamic> creationParams = <String, dynamic>{};

     return AndroidView(
       viewType: viewType,
       onPlatformViewCreated: _onPlatformViewCreated,
       creationParams: creationParams,
       creationParamsCodec: const StandardMessageCodec(),
     );
   }

   void _onPlatformViewCreated(int id) {
     _controller = ScannerViewController(id);
     widget.onScannerViewCreated(_controller!);
   }
 }

 class ScannerViewController {
   late MethodChannel _channel;
   final StreamController<List<BarcodeResult>> _streamController =
       StreamController<List<BarcodeResult>>();
   Stream<List<BarcodeResult>> get scannedDataStream => _streamController.stream;

   ScannerViewController(int id) {
     _channel =
         MethodChannel('com.dynamsoft.flutter_qrcode_scanner/nativeview_$id');
     _channel.setMethodCallHandler((call) async {
       switch (call.method) {
         case 'onDetected':
           if (call.arguments != null) {
             List<BarcodeResult> data = fromPlatformData(call.arguments as List);
             _streamController.sink.add(data);
           }
           break;
       }
     });
   }
 }

The functions for invoking native methods include:

Future<void> startScanning() async {
   await _channel.invokeMethod('startScanning');
 }

 Future<void> stopScanning() async {
   await _channel.invokeMethod('stopScanning');
 }

 /// Apply for a 30-day FREE trial license: https://www.dynamsoft.com/customer/license/trialLicense
 Future<void> setLicense(String license) async {
   await _channel.invokeMethod('setLicense', {'license': license});
 }

So far, the Flutter QR code scanner plugin has been completely finished. Here is the full example of a simple QR code scanner:

import 'package:flutter/material.dart';
import 'dart:async';

import 'package:flutter/services.dart';
import 'package:flutter_camera_qrcode_scanner/flutter_qrcode_scanner.dart';
import 'package:flutter_camera_qrcode_scanner/dynamsoft_barcode.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  ScannerViewController? controller;
  String _barcodeResults = '';

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('QR Code Scanner'),
        ),
        body: Stack(children: <Widget>[
          ScannerView(onScannerViewCreated: onScannerViewCreated),
          Column(
            mainAxisAlignment: MainAxisAlignment.end,
            children: [
              Container(
                height: 100,
                child: SingleChildScrollView(
                  child: Text(
                    _barcodeResults,
                    style: TextStyle(fontSize: 14, color: Colors.white),
                  ),
                ),
              ),
              Container(
                height: 100,
                child: Row(
                    mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                    children: <Widget>[
                      MaterialButton(
                          child: Text('Start Scan'),
                          textColor: Colors.white,
                          color: Colors.blue,
                          onPressed: () async {
                            controller!.startScanning();
                          }),
                      MaterialButton(
                          child: Text("Stop Scan"),
                          textColor: Colors.white,
                          color: Colors.blue,
                          onPressed: () async {
                            controller!.stopScanning();
                          })
                    ]),
              ),
            ],
          )
        ]),
      ),
    );
  }

  void onScannerViewCreated(ScannerViewController controller) {
    setState(() {
      this.controller = controller;
    });
    controller.setLicense('LICENSE-KEY');
    controller.startScanning(); // auto start scanning
    controller.scannedDataStream.listen((results) {
      setState(() {
        _barcodeResults = getBarcodeResults(results);
      });
    });
  }

  String getBarcodeResults(List<BarcodeResult> results) {
    StringBuffer sb = new StringBuffer();
    for (BarcodeResult result in results) {
      sb.write(result.format);
      sb.write("\n");
      sb.write(result.text);
      sb.write("\n\n");
    }
    if (results.isEmpty) sb.write("No QR Code Detected");
    return sb.toString();
  }