How to Build an Expo Barcode Scanner

Nov 24, 2022

In the previous article, we demonstrated how to build a React Native QR code scanner using Vision Camera and the Dynamsoft Barcode Reader frame processor plugin. In this article, we are going to talk about how to build a barcode scanner using Expo.

Expo is an open-source platform for making universal native apps for Android, iOS, and the web with JavaScript and React. It is basically a set of tools built on top of React Native, which makes it easy to develop and distribute apps.

PS: The article uses bare React Native projects. If you need to use Expo in managed workflow, check out this article: How to Start a QR Code Scanner in React Native WebView

Build an Expo Barcode Scanner

Let’s do this in steps.

New Project

Install the Expo cli tool:

npm i -g expo-cli

Create a new project:

npx create-expo-app expo-barcode-scanner

Add Dependencies

Install React Native Vision Camera

expo install react-native-vision-camera

Install React Native Reanimated to enable Frame Processor
```
expo install react-native-reanimated
```

Install the Dynamsoft Barcode Reader Frame Processor plugin

expo install vision-camera-dynamsoft-barcode-reader

We also have to update the babel.config.js to register the plugin.

 module.exports = function(api) {
   api.cache(true);
   return {
     presets: ['babel-preset-expo'],
+     plugins: [
+     [
+       'react-native-reanimated/plugin',
+       {
+         globals: ['__decode'],
+       },
+     ],
+    ],
   };
 };

Install react-native-svg to draw barcode overlays
```
expo install react-native-svg
```
Install react-native-safe-area-context to provide the SafeAreaView component
```
expo install react-native-safe-area-context
```

Add Camera Permission

Add the following to app.json to add camera permission.

"plugins": [
  [
    "react-native-vision-camera",
    {
      "cameraPermissionText": "$(PRODUCT_NAME) needs access to your Camera."
    }
  ]
]

Build the Project

We can then run expo prebuild to generate the native projects for Android and iOS.

Then we can open Android Studio or Xcode to build and run the project.

We can run the following to open the metro bundler in an Expo project as well.

npx react-native start

Expo also provides a service called EAS, which is a hosted service for building app binaries for your Expo and React Native projects. Using EAS, we can build and publish the app without having an Android or iOS development environment on our own devices.

We can run the following command to use the EAS service:

eas build

Make the App Work as a Barcode Scanner

Next, let’s modify the app to make it work as a barcode scanner.

Add the Camera Component

Replace App.js with the following content. After we start the app, we can see the camera preview taking up the whole screen.

export default function App() {
  const [hasPermission, setHasPermission] = React.useState(false);
  const devices = useCameraDevices();
  const device = devices.back;
  
  React.useEffect(() => {
    (async () => {
      const status = await Camera.requestCameraPermission();
      setHasPermission(status === 'authorized');
    })();
  }, []);

  return (
    <SafeAreaView style={styles.container}>
      {device != null &&
      hasPermission && (
      <>
          <Camera
          style={StyleSheet.absoluteFill}
          device={device}
          isActive={true}
          />
          ))}
      </>)}
    </SafeAreaView>
  );
}

const styles = StyleSheet.create({
  container: {
    flex:1,
  },
});

Enable Barcode Reading

We can use the frame processor feature of Vision Camera to enable barcode reading.

Define the frame processor function:

import * as REA from 'react-native-reanimated';
   
const [barcodeResults, setBarcodeResults] = React.useState([]);
const frameProcessor = useFrameProcessor((frame) => {
    'worklet'
    const config = {};
    config.rotateImage = true;
    const results = decode(frame,config)
    REA.runOnJS(setBarcodeResults)(results);
  }, [])

The function will try to read barcodes from the camera frame and then update the barcodeResults state.

Set the frame processor for the camera component.

 <Camera
   style={StyleSheet.absoluteFill}
   device={device}
   isActive={true}
+  frameProcessor={frameProcessor}
+  frameProcessorFps={5}
 />

Display the barcode results using Text component.

{barcodeResults.map((barcode, idx) => (
  <Text key={idx} style={styles.barcodeText}>
      {barcode.barcodeFormat +": "+ barcode.barcodeText}
  </Text>
))}

Draw Barcode Overlays using SVG

We can highlight the detected barcodes using SVG.

Add the SVG component with Polygon elements.

<Svg style={[StyleSheet.absoluteFill]} 
  viewBox={getViewBox()}
  preserveAspectRatio="xMidYMid slice">

  {barcodeResults.map((barcode, idx) => (
    <Polygon key={idx}
      points={getPointsData(barcode)}
      fill="lime"
      stroke="green"
      opacity="0.5"
      strokeWidth="1"
    />
  ))}
</Svg>

Add two states frameWidth and frameHeight. We can get the values from the frame processor.

const [frameWidth, setFrameWidth] = React.useState(720);
const [frameHeight, setFrameHeight] = React.useState(1280);
const frameProcessor = useFrameProcessor((frame) => {
 'worklet'
  //...
  REA.runOnJS(setFrameWidth)(frame.width);
  REA.runOnJS(setFrameHeight)(frame.height);
}, [])

We can get the viewBox attribute based on frameWidth and frameHeight. For Android, because the image sensor’s direction is landscape, we need to switch the width and height if the device is portrait.

const getViewBox = () => {
  const frameSize = getFrameSize();
  const viewBox = "0 0 "+frameSize[0]+" "+frameSize[1];
  return viewBox;
}
   
const getFrameSize = () => {
  let width, height;
  if (Platform.OS === 'android') {
    if (frameWidth>frameHeight && Dimensions.get('window').width>Dimensions.get('window').height){
      width = frameWidth;
      height = frameHeight;
    }else {
      console.log("Has rotation");
      width = frameHeight;
      height = frameWidth;
    }
  } else {
    width = frameWidth;
    height = frameHeight;
  }
  return [width, height];
}

Get the point attribute for the polygons based on the barcode location info.

const getPointsData = (lr) => {
  let pointsData = lr.x1 + "," + lr.y1 + " ";
  pointsData = pointsData+lr.x2 + "," + lr.y2 +" ";
  pointsData = pointsData+lr.x3 + "," + lr.y3 +" ";
  pointsData = pointsData+lr.x4 + "," + lr.y4;
  return pointsData;
}

All right, we’ve now finished the barcode scanner. Here is a screenshot of the result.

Expo Barcode Scanner

Source Code

Here is the link to the Expo barcode scanner demo:

https://github.com/tony-xlh/expo-barcode-scanner

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