Barcode scanning via camera in mobile app

Implementing Barcode Scanning via Camera in Mobile Applications

Real-time barcode scanning through video stream — a task where platform APIs differ significantly. iOS provides AVCaptureSession, Android — Camera2 API or CameraX. The goal is the same: capture a frame, pass to decoder, get result with minimal latency.

iOS: AVCaptureSession + AVCaptureMetadataOutput

Classical approach — pipeline via AVCaptureSession:

let session = AVCaptureSession()

guard let device = AVCaptureDevice.default(for: .video),
      let input = try? AVCaptureDeviceInput(device: device) else { return }

let metadataOutput = AVCaptureMetadataOutput()
session.addInput(input)
session.addOutput(metadataOutput)

metadataOutput.setMetadataObjectsDelegate(self, queue: .main)
metadataOutput.metadataObjectTypes = [.ean13, .ean8, .code128, .upce, .qr]

// Preview
let previewLayer = AVCaptureVideoPreviewLayer(session: session)
previewLayer.frame = view.bounds
previewLayer.videoGravity = .resizeAspectFill
view.layer.addSublayer(previewLayer)

DispatchQueue.global(qos: .userInitiated).async {
    session.startRunning()
}

session.startRunning() always on background thread — on main thread this blocks UI for 300-600ms at startup.

Delegate AVCaptureMetadataOutputObjectsDelegate receives result:

func metadataOutput(_ output: AVCaptureMetadataOutput,
                    didOutput metadataObjects: [AVMetadataObject],
                    from connection: AVCaptureConnection) {
    guard let object = metadataObjects.first as? AVMetadataMachineReadableCodeObject,
          let code = object.stringValue else { return }
    session.stopRunning()
    handleCode(code)
}

Scanning Zone

metadataOutput.rectOfInterest — restricts the area where codes are searched. Coordinates in normalized space (0.0-1.0), with axes inverted compared to UIKit. AVCaptureVideoPreviewLayer.metadataOutputRectConverted(fromLayerRect:) converts from UIKit coordinates to the required format.

Without rectOfInterest on crowded shelves with products, the camera may recognize a neighboring barcode instead of the intended one.

Android: CameraX + ML Kit

CameraX — recommended approach with API 21+:

val cameraProviderFuture = ProcessCameraProvider.getInstance(context)
cameraProviderFuture.addListener({
    val cameraProvider = cameraProviderFuture.get()

    val preview = Preview.Builder().build()
    preview.setSurfaceProvider(previewView.surfaceProvider)

    val imageAnalysis = ImageAnalysis.Builder()
        .setTargetResolution(Size(1280, 720))
        .setBackpressureStrategy(ImageAnalysis.STRATEGY_KEEP_ONLY_LATEST)
        .build()

    imageAnalysis.setAnalyzer(Executors.newSingleThreadExecutor(), BarcodeAnalyzer { barcode ->
        handleBarcode(barcode)
    })

    cameraProvider.bindToLifecycle(this, CameraSelector.DEFAULT_BACK_CAMERA, preview, imageAnalysis)
}, ContextCompat.getMainExecutor(context))

STRATEGY_KEEP_ONLY_LATEST — critically important. Without it, frames accumulate in queue and decoder lags by 1-2 seconds.

BarcodeAnalyzer — implementation of ImageAnalysis.Analyzer, internally ML Kit:

class BarcodeAnalyzer(private val onDetected: (String) -> Unit) : ImageAnalysis.Analyzer {
    private val scanner = BarcodeScanning.getClient(
        BarcodeScannerOptions.Builder().setBarcodeFormats(Barcode.FORMAT_ALL_FORMATS).build()
    )

    @androidx.camera.core.ExperimentalGetImage
    override fun analyze(imageProxy: ImageProxy) {
        val mediaImage = imageProxy.image ?: run { imageProxy.close(); return }
        val inputImage = InputImage.fromMediaImage(mediaImage, imageProxy.imageInfo.rotationDegrees)
        scanner.process(inputImage)
            .addOnSuccessListener { barcodes ->
                barcodes.firstOrNull()?.rawValue?.let { onDetected(it) }
            }
            .addOnCompleteListener { imageProxy.close() }
    }
}

imageProxy.close() in addOnCompleteListener — mandatory, otherwise CameraX stops delivering new frames.

Camera Permission and Autofocus

On budget Android devices (Realme C-series, Tecno), autofocus can work unstably. CameraControl.startFocusAndMetering() with FocusMeteringAction helps force focus to center every 2 seconds.

On iOS — AVCaptureDevice.focusMode = .continuousAutoFocus and autoFocusRangeRestriction = .near for close-range scanning.

Implementation timeline: 1-3 days. Cost calculated individually.