My sample project source code in GitHub

Using Google MLKit

Add dependencies

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dependencies {
    ...
    implementation 'androidx.camera:camera-lifecycle:1.2.1'
    implementation 'com.google.mlkit:barcode-scanning:17.0.3'

    def camerax_version = "1.2.1"
    implementation "androidx.camera:camera-core:${camerax_version}"
    implementation "androidx.camera:camera-camera2:${camerax_version}"
    implementation "androidx.camera:camera-view:${camerax_version}"
}

Request camera permission

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<uses-permission android:name="android.permission.CAMERA" />
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private const val REQUEST_CODE_PERMISSIONS = 10
private val REQUIRED_PERMISSIONS = arrayOf(Manifest.permission.CAMERA)
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override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    ...
    // Request camera permissions
    if (allPermissionsGranted()) {
        startPreview()
    } else {
        ActivityCompat.requestPermissions(
            this, REQUIRED_PERMISSIONS, REQUEST_CODE_PERMISSIONS)
    }
}

override fun onRequestPermissionsResult(
    requestCode: Int, permissions: Array<String>, grantResults: IntArray) {
    if (requestCode == REQUEST_CODE_PERMISSIONS) {
        if (allPermissionsGranted()) {
            startPreview()
        } else {
            Toast.makeText(this,
                "Permissions not granted by the user.",
                Toast.LENGTH_SHORT).show()
            finish()
        }
    }
}

private fun allPermissionsGranted() = REQUIRED_PERMISSIONS.all {
    ContextCompat.checkSelfPermission(
        baseContext, it) == PackageManager.PERMISSION_GRANTED
}

Bind preview and image analyzer

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private fun startPreview(){
    cameraProviderFuture = ProcessCameraProvider.getInstance(this)
    cameraProviderFuture.addListener(Runnable {
        bindPreview()
    }, ContextCompat.getMainExecutor(this))
}

private fun bindPreview() {
    var preview = Preview.Builder()
        .build()

    var cameraSelector = CameraSelector.Builder()
        .requireLensFacing(CameraSelector.LENS_FACING_BACK)
        .build()

    preview.setSurfaceProvider(previewView.surfaceProvider)

    val analyzer = ImageAnalysis.Builder()
        .setBackpressureStrategy(ImageAnalysis.STRATEGY_KEEP_ONLY_LATEST)
        .build()

    cameraExecutor = Executors.newSingleThreadExecutor()
    analyzer?.let {
        it.setAnalyzer(cameraExecutor) { imageProxy ->
            handleImage(imageProxy)
            imageProxy.close()
        }
    }

    try {
        val cameraProvider = cameraProviderFuture.get()
        cameraProvider.unbindAll()
        cameraProvider.bindToLifecycle(this, cameraSelector, preview, analyzer)
    } catch (e: Exception) {
        Log.e("ScanQRCode Test APP", "cameraProvider binding failed!", e)
    }
}

Handle QRCode image

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private fun handleImage(imageProxy: ImageProxy){
    val mediaImage = imageProxy.image
    if (mediaImage != null) {
        val image = InputImage.fromMediaImage(mediaImage, imageProxy.imageInfo.rotationDegrees)

        // Pass image to an ML Kit Vision API
        val options = BarcodeScannerOptions.Builder()
            .setBarcodeFormats(
                Barcode.FORMAT_QR_CODE,
                Barcode.FORMAT_AZTEC)
            .build()

        val scanner = BarcodeScanning.getClient(options)
        scanner.process(image).addOnSuccessListener { barcodes ->
            for (barcode in barcodes) {
                // Handle received barcodes...
                println("Scan result: ${barcode.rawValue}")

                Toast.makeText(
                    this,
                    "Scan result: ${barcode.rawValue}",
                    Toast.LENGTH_SHORT
                ).show()

                cameraExecutor.shutdown()
                finish()
            }
        }
    }
}

Reference

Other methods

Zxing

com.google.android.gms.vision.barcode