Barcode refactor (#363)

* Move barcode.dart

* Fix

* Refactoring barcode scanner code:

- Abstract the "controller" class (for future development)
- Break barcode scanning code out into multiple files
- Add CameraBarcodeController class (qr_code_scanner)

* Add await

* Make barcode scan delay configurable

* remove unused import

* Handle camera exceptions

* Improve sequencing for camera scanner

- Show loading overlay
- Prevent reload if view is no longer mounted

* Update docstring

* Update release notes

lib/barcode/controller.dart

@@ -0,0 +1,108 @@
+import "package:flutter/material.dart";
+import "package:one_context/one_context.dart";
+
+import "package:inventree/preferences.dart";
+
+import "package:inventree/barcode/handler.dart";
+
+import "package:inventree/widget/progress.dart";
+
+/*
+ * Generic class which provides a barcode scanner interface.
+ * 
+ * When the controller is instantiated, it is passed a "handler" class,
+ * which is used to process the scanned barcode.
+ */
+class InvenTreeBarcodeController extends StatefulWidget {
+
+  const InvenTreeBarcodeController(this.handler, {Key? key}) : super(key: key);
+
+  final BarcodeHandler handler;
+
+  @override
+  State<StatefulWidget> createState() => InvenTreeBarcodeControllerState();
+}
+
+
+/*
+ * Base state widget for the barcode controller.
+ * This defines the basic interface for the barcode controller.
+ */
+class InvenTreeBarcodeControllerState extends State<InvenTreeBarcodeController> {
+
+  InvenTreeBarcodeControllerState() : super();
+
+  final GlobalKey barcodeControllerKey = GlobalKey(debugLabel: "barcodeController");
+
+  // Internal state flag to test if we are currently processing a barcode
+  bool processingBarcode = false;
+
+  /*
+   * Method to handle scanned data.
+   * Any implementing class should call this method when a barcode is scanned.
+   * Barcode data should be passed as a string
+   */
+  Future<void> handleBarcodeData(String? data) async {
+    
+    // Check that the data is valid, and this view is still mounted
+    if (!mounted || data == null || data.isEmpty) {
+      return;
+    }
+
+    // Currently processing a barcode - ignore this one
+    if (processingBarcode) {
+      return;
+    }
+
+    setState(() {
+      processingBarcode = true;
+    });
+
+    BuildContext? context = OneContext().context;
+
+    showLoadingOverlay(context!);
+    await pauseScan();
+
+    await widget.handler.processBarcode(data);
+
+    // processBarcode may have popped the context
+    if (!mounted) {
+      hideLoadingOverlay();
+      return;
+    }
+
+    int delay = await InvenTreeSettingsManager().getValue(INV_BARCODE_SCAN_DELAY, 500) as int;
+
+    Future.delayed(Duration(milliseconds: delay), () {
+      hideLoadingOverlay();
+      if (mounted) {
+        resumeScan().then((_) {
+          if (mounted) {
+            setState(() {
+              processingBarcode = false;
+            });
+          }
+        });
+      }
+    });
+  }
+
+  // Hook function to "pause" the barcode scanner
+  Future<void> pauseScan() async {
+    // Implement this function in subclass
+  }
+
+  // Hook function to "resume" the barcode scanner
+  Future<void> resumeScan() async {
+    // Implement this function in subclass
+  }
+
+  /*
+   * Implementing classes are in control of building out the widget
+   */
+  @override
+  Widget build(BuildContext context) {
+    return Container();
+  }
+
+}