Add a "picklist mode" to the example app

@@ -8,8 +8,9 @@ import 'package:mobile_scanner_example/barcode_scanner_simple.dart';
 import 'package:mobile_scanner_example/barcode_scanner_window.dart';
 import 'package:mobile_scanner_example/barcode_scanner_window.dart';
 import 'package:mobile_scanner_example/barcode_scanner_zoom.dart';
 import 'package:mobile_scanner_example/barcode_scanner_zoom.dart';
 import 'package:mobile_scanner_example/mobile_scanner_overlay.dart';
 import 'package:mobile_scanner_example/mobile_scanner_overlay.dart';
+import 'package:mobile_scanner_example/picklist/picklist_result.dart';
 
 
-void main() {
+void main() async {
   runApp(
   runApp(
     const MaterialApp(
     const MaterialApp(
       title: 'Mobile Scanner Example',
       title: 'Mobile Scanner Example',
@@ -91,6 +92,11 @@ class MyHome extends StatelessWidget {
               'Analyze image from file',
               'Analyze image from file',
               const BarcodeScannerAnalyzeImage(),
               const BarcodeScannerAnalyzeImage(),
             ),
             ),
+            _buildItem(
+              context,
+              'Picklist mode',
+              const PicklistResult(),
+            ),
           ],
           ],
         ),
         ),
       ),
       ),

+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:mobile_scanner/mobile_scanner.dart';
+import 'package:mobile_scanner_example/picklist/classes/barcode_at_center.dart';
+import 'package:mobile_scanner_example/picklist/widgets/crosshair.dart';
+import 'package:mobile_scanner_example/scanner_error_widget.dart';
+
+// This sample implements picklist functionality.
+// The scanning can temporarily be suspended by the user by touching the screen.
+// When the scanning is active, the crosshair turns red.
+// When the scanning is suspended, the crosshair turns green.
+// A barcode has to touch the center of viewfinder to be scanned.
+// Therefore the Crosshair widget needs to be placed at the center of the
+// MobileScanner widget to visually line up.
+class BarcodeScannerPicklist extends StatefulWidget {
+  const BarcodeScannerPicklist({super.key});
+
+  @override
+  State<BarcodeScannerPicklist> createState() => _BarcodeScannerPicklistState();
+}
+
+class _BarcodeScannerPicklistState extends State<BarcodeScannerPicklist> {
+  final _mobileScannerController = MobileScannerController(
+    // The controller is started from the initState method.
+    autoStart: false,
+    // The know the placing of the barcodes, we need to know the size of the
+    // canvas they are placed on. Unfortunately the only known reliable way
+    // to get the dimensions, is to receive the complete image from the native
+    // side.
+    // https://github.com/juliansteenbakker/mobile_scanner/issues/1183
+    returnImage: true,
+  );
+
+  // On this subscription the barcodes are received.
+  StreamSubscription<Object?>? _subscription;
+
+  // This boolean indicates if the detection of barcodes is enabled or
+  // temporarily suspended.
+  final _scannerEnabled = ValueNotifier(true);
+
+  // This boolean is used to prevent multiple pops.
+  var _validBarcodeFound = false;
+
+  @override
+  void initState() {
+    // Enable and disable scanning on the native side, so we don't get a stream
+    // of images when not needed. This also improves the behavior (false
+    // positives) when the user switches quickly to another barcode after
+    // enabling the scanner by releasing the finger.
+    _scannerEnabled.addListener(() {
+      _scannerEnabled.value
+          ? _mobileScannerController.updateScanWindow(null)
+          : _mobileScannerController.updateScanWindow(Rect.zero);
+    });
+    // Lock to portrait (may not work on iPad with multitasking).
+    SystemChrome.setPreferredOrientations([DeviceOrientation.portraitUp]);
+    // Get a stream subscription and listen to received barcodes.
+    _subscription = _mobileScannerController.barcodes.listen(_handleBarcodes);
+    super.initState();
+    // Start the controller to start scanning.
+    unawaited(_mobileScannerController.start());
+  }
+
+  @override
+  void dispose() {
+    // Cancel the stream subscription.
+    unawaited(_subscription?.cancel());
+    _subscription = null;
+    super.dispose();
+    // Dispose the controller.
+    _mobileScannerController.dispose();
+  }
+
+  // Check the list of barcodes only if scannerEnables is true.
+  // Only take the barcode that is at the center of the image.
+  // Return the barcode found to the calling page with the help of the
+  // navigator.
+  void _handleBarcodes(BarcodeCapture barcodeCapture) {
+    // Discard all events when the scanner is disabled or when already a valid
+    // barcode is found.
+    if (!_scannerEnabled.value || _validBarcodeFound) {
+      return;
+    }
+    final barcode = findBarcodeAtCenter(barcodeCapture);
+    if (barcode != null) {
+      _validBarcodeFound = true;
+      Navigator.of(context).pop(barcode);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return PopScope(
+      onPopInvokedWithResult: (didPop, result) {
+        // Reset the page orientation to the system default values, when this page is popped
+        if (!didPop) {
+          return;
+        }
+        SystemChrome.setPreferredOrientations(<DeviceOrientation>[]);
+      },
+      child: Scaffold(
+        appBar: AppBar(title: const Text('Picklist scanner')),
+        backgroundColor: Colors.black,
+        body: Listener(
+          // Detect if the user touches the screen and disable/enable the scanner accordingly
+          behavior: HitTestBehavior.opaque,
+          onPointerDown: (_) => _scannerEnabled.value = false,
+          onPointerUp: (_) => _scannerEnabled.value = true,
+          onPointerCancel: (_) => _scannerEnabled.value = true,
+          // A stack containing the image feed and the crosshair
+          // The location of the crosshair must be at the center of the MobileScanner, otherwise the detection area and the visual representation do not line up.
+          child: Stack(
+            fit: StackFit.expand,
+            children: [
+              MobileScanner(
+                controller: _mobileScannerController,
+                errorBuilder: (context, error, child) =>
+                    ScannerErrorWidget(error: error),
+                fit: BoxFit.contain,
+              ),
+              Crosshair(_scannerEnabled),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}

+import 'package:flutter/material.dart';
+import 'package:mobile_scanner/mobile_scanner.dart';
+
+/// This function finds the barcode that touches the center of the
+/// image. If no barcode is found that touches the center, null is returned.
+/// See [_BarcodeScannerPicklistState] and the returnImage option for more info.
+///
+/// https://github.com/juliansteenbakker/mobile_scanner/issues/1183
+Barcode? findBarcodeAtCenter(BarcodeCapture barcodeCapture) {
+  final imageSize = barcodeCapture.size;
+  for (final barcode in barcodeCapture.barcodes) {
+    if (_isPolygonTouchingTheCenter(
+      imageSize: imageSize,
+      polygon: barcode.corners,
+    )) {
+      return barcode;
+    }
+  }
+  return null;
+}
+
+/// Check if the polygon, represented by a list of offsets, touches the center of
+/// an image when the size of the image is given.
+bool _isPolygonTouchingTheCenter({
+  required Size imageSize,
+  required List<Offset> polygon,
+}) {
+  final centerOfCameraOutput = Offset(
+    imageSize.width / 2,
+    imageSize.height / 2,
+  );
+  return _isPointInPolygon(
+    point: centerOfCameraOutput,
+    polygon: polygon,
+  );
+}
+
+/// Credits to chatGPT:
+/// Checks if a given [point] is inside the [polygon] boundaries.
+///
+/// Parameters:
+///   - [point]: The `Offset` (usually represents a point in 2D space) to check.
+///   - [polygon]: A List of `Offset` representing the vertices of the polygon.
+///
+/// Returns:
+///   - A boolean value: `true` if the point is inside the polygon, or `false` otherwise.
+///
+/// Uses the ray-casting algorithm based on the Jordan curve theorem.
+bool _isPointInPolygon({
+  required Offset point,
+  required List<Offset> polygon,
+}) {
+  // Initial variables:
+  int i; // Loop variable for current vertex
+  int j = polygon.length -
+      1; // Last vertex index, initialized to the last vertex of the polygon
+  bool inside = false; // Boolean flag initialized to false
+
+  // Loop through each edge of the polygon
+  for (i = 0; i < polygon.length; j = i++) {
+    // Check if point's y-coordinate is within the y-boundaries of the edge
+    if (((polygon[i].dy > point.dy) != (polygon[j].dy > point.dy)) &&
+        // Check if the point's x-coordinate is to the left of the edge
+        (point.dx <
+            (polygon[j].dx -
+                        polygon[i]
+                            .dx) * // Horizontal distance between the vertices of the edge
+                    (point.dy -
+                        polygon[i]
+                            .dy) / // Scale factor based on the y-distance of the point to the lower vertex
+                    (polygon[j].dy -
+                        polygon[i]
+                            .dy) + // Vertical distance between the vertices of the edge
+                polygon[i].dx)) {
+      // Horizontal position of the lower vertex
+      // If the ray intersects the polygon edge, invert the inside flag
+      inside = !inside;
+    }
+  }
+  // Return the status of the inside flag which tells if the point is inside the polygon or not
+  return inside;
+}

+import 'package:flutter/material.dart';
+import 'package:mobile_scanner/mobile_scanner.dart';
+import 'package:mobile_scanner_example/picklist/barcode_scanner_picklist.dart';
+
+class PicklistResult extends StatefulWidget {
+  const PicklistResult({super.key});
+
+  @override
+  State<PicklistResult> createState() => _PicklistResultState();
+}
+
+class _PicklistResultState extends State<PicklistResult> {
+  String barcode = 'Scan Something!';
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(title: const Text('Picklist result')),
+      body: SafeArea(
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 16.0),
+          child: Center(
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: [
+                Text(barcode),
+                ElevatedButton(
+                  onPressed: () async {
+                    final scannedBarcode =
+                        await Navigator.of(context).push<Barcode>(
+                      MaterialPageRoute(
+                        builder: (context) => const BarcodeScannerPicklist(),
+                      ),
+                    );
+                    setState(
+                      () {
+                        if (scannedBarcode == null) {
+                          barcode = 'Scan Something!';
+                          return;
+                        }
+                        if (scannedBarcode.displayValue == null) {
+                          barcode = '>>binary<<';
+                          return;
+                        }
+                        barcode = scannedBarcode.displayValue!;
+                      },
+                    );
+                  },
+                  child: const Text('Scan'),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}

+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+
+class Crosshair extends StatelessWidget {
+  const Crosshair(
+    this.scannerEnabled, {
+    super.key,
+  });
+
+  final ValueListenable<bool> scannerEnabled;
+
+  @override
+  Widget build(BuildContext context) {
+    return ValueListenableBuilder(
+      valueListenable: scannerEnabled,
+      builder: (context, value, child) {
+        return Center(
+          child: Icon(
+            Icons.close,
+            color: scannerEnabled.value ? Colors.red : Colors.green,
+          ),
+        );
+      },
+    );
+  }
+}