update barcode scanner window sample

@@ -16,16 +16,75 @@ class BarcodeScannerWithScanWindow extends StatefulWidget {
 
 class _BarcodeScannerWithScanWindowState
     extends State<BarcodeScannerWithScanWindow> {
-  late MobileScannerController controller = MobileScannerController();
-  Barcode? barcode;
-  BarcodeCapture? capture;
+  final MobileScannerController controller = MobileScannerController();
 
-  Future<void> onDetect(BarcodeCapture barcode) async {
-    capture = barcode;
-    setState(() => this.barcode = barcode.barcodes.first);
+  @override
+  void initState() {
+    super.initState();
+
+    controller.start();
+  }
+
+  Widget _buildBarcodeOverlay() {
+    return ValueListenableBuilder(
+      valueListenable: controller,
+      builder: (context, value, child) {
+        // Not ready.
+        if (!value.isInitialized || !value.isRunning || value.error != null) {
+          return const SizedBox();
+        }
+
+        return StreamBuilder<BarcodeCapture>(
+          stream: controller.barcodes,
+          builder: (context, snapshot) {
+            final BarcodeCapture? barcodeCapture = snapshot.data;
+
+            // No barcode.
+            if (barcodeCapture == null || barcodeCapture.barcodes.isEmpty) {
+              return const SizedBox();
             }
 
-  MobileScannerArguments? arguments;
+            final scannedBarcode = barcodeCapture.barcodes.first;
+
+            // No barcode corners, or size, or no camera preview size.
+            if (scannedBarcode.corners.isEmpty ||
+                value.size.isEmpty ||
+                barcodeCapture.size.isEmpty) {
+              return const SizedBox();
+            }
+
+            return CustomPaint(
+              painter: BarcodeOverlay(
+                barcodeCorners: scannedBarcode.corners,
+                barcodeSize: barcodeCapture.size,
+                boxFit: BoxFit.contain,
+                cameraPreviewSize: value.size,
+              ),
+            );
+          },
+        );
+      },
+    );
+  }
+
+  Widget _buildScanWindow(Rect scanWindowRect) {
+    return ValueListenableBuilder(
+      valueListenable: controller,
+      builder: (context, value, child) {
+        // Not ready.
+        if (!value.isInitialized ||
+            !value.isRunning ||
+            value.error != null ||
+            value.size.isEmpty) {
+          return const SizedBox();
+        }
+
+        return CustomPaint(
+          painter: ScannerOverlay(scanWindowRect),
+        );
+      },
+    );
+  }
 
   @override
   Widget build(BuildContext context) {
@@ -34,77 +93,69 @@ class _BarcodeScannerWithScanWindowState
       width: 200,
       height: 200,
     );
+
     return Scaffold(
       appBar: AppBar(title: const Text('With Scan window')),
       backgroundColor: Colors.black,
-      body: Builder(
-        builder: (context) {
-          return Stack(
+      body: Stack(
         fit: StackFit.expand,
         children: [
           MobileScanner(
             fit: BoxFit.contain,
             scanWindow: scanWindow,
             controller: controller,
-                onScannerStarted: (arguments) {
-                  setState(() {
-                    this.arguments = arguments;
-                  });
-                },
             errorBuilder: (context, error, child) {
               return ScannerErrorWidget(error: error);
             },
-                onDetect: onDetect,
-              ),
-              if (barcode != null &&
-                  barcode?.corners != null &&
-                  arguments != null)
-                CustomPaint(
-                  painter: BarcodeOverlay(
-                    barcode: barcode!,
-                    arguments: arguments!,
-                    boxFit: BoxFit.contain,
-                    capture: capture!,
-                  ),
-                ),
-              CustomPaint(
-                painter: ScannerOverlay(scanWindow),
           ),
+          _buildBarcodeOverlay(),
+          _buildScanWindow(scanWindow),
           Align(
             alignment: Alignment.bottomCenter,
             child: Container(
-                  alignment: Alignment.bottomCenter,
+              alignment: Alignment.center,
+              padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
               height: 100,
               color: Colors.black.withOpacity(0.4),
-                  child: Row(
-                    mainAxisAlignment: MainAxisAlignment.spaceEvenly,
-                    children: [
-                      Center(
-                        child: SizedBox(
-                          width: MediaQuery.of(context).size.width - 120,
-                          height: 50,
-                          child: FittedBox(
-                            child: Text(
-                              barcode?.displayValue ?? 'Scan something!',
+              child: StreamBuilder<BarcodeCapture>(
+                stream: controller.barcodes,
+                builder: (context, snapshot) {
+                  final barcodeCapture = snapshot.data;
+
+                  String displayValue = 'Scan something!';
+
+                  if (barcodeCapture != null &&
+                      barcodeCapture.barcodes.isNotEmpty) {
+                    final String? value =
+                        barcodeCapture.barcodes.first.displayValue;
+
+                    if (value != null) {
+                      displayValue = value;
+                    }
+                  }
+
+                  return Text(
+                    displayValue,
                     overflow: TextOverflow.fade,
                     style: Theme.of(context)
                         .textTheme
                         .headlineMedium!
                         .copyWith(color: Colors.white),
-                            ),
-                          ),
-                        ),
-                      ),
-                    ],
+                  );
+                },
               ),
             ),
           ),
         ],
-          );
-        },
       ),
     );
   }
+
+  @override
+  Future<void> dispose() async {
+    await controller.dispose();
+    super.dispose();
+  }
 }
 
 class ScannerOverlay extends CustomPainter {
@@ -114,6 +165,8 @@ class ScannerOverlay extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
+    // TODO: use `Offset.zero & size` instead of Rect.largest
+    // we need to pass the size to the custom paint widget
     final backgroundPath = Path()..addRect(Rect.largest);
     final cutoutPath = Path()..addRect(scanWindow);
 
@@ -151,7 +204,9 @@ class BarcodeOverlay extends CustomPainter {
 
   @override
   void paint(Canvas canvas, Size size) {
-    if (barcodeCorners.isEmpty) {
+    if (barcodeCorners.isEmpty ||
+        barcodeSize.isEmpty ||
+        cameraPreviewSize.isEmpty) {
       return;
     }