Added onResume listener and updateDimensions call (#250)

lib/src/lifecycle_event_handler.dart

@@ -0,0 +1,30 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/foundation.dart';
+
+class LifecycleEventHandler extends WidgetsBindingObserver {
+  LifecycleEventHandler({
+    this.resumeCallBack,
+    this.suspendingCallBack,
+  });
+
+  final AsyncCallback resumeCallBack;
+  final AsyncCallback suspendingCallBack;
+
+  @override
+  Future<void> didChangeAppLifecycleState(AppLifecycleState state) async {
+    switch (state) {
+      case AppLifecycleState.resumed:
+        if (resumeCallBack != null) {
+          await resumeCallBack();
+        }
+        break;
+      case AppLifecycleState.inactive:
+      case AppLifecycleState.paused:
+      case AppLifecycleState.detached:
+        if (suspendingCallBack != null) {
+          await suspendingCallBack();
+        }
+        break;
+    }
+  }
+}

lib/src/qr_code_scanner.dart

@@ -4,6 +4,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
+import 'lifecycle_event_handler.dart';
 import 'qr_scanner_overlay_shape.dart';
 import 'types/barcode.dart';
 import 'types/barcode_format.dart';
@@ -58,6 +59,17 @@ class QRView extends StatefulWidget {
 class _QRViewState extends State<QRView> {
   var _channel;
 
+  @override
+  void initState() {
+    super.initState();
+
+    WidgetsBinding.instance.addObserver(
+        LifecycleEventHandler(resumeCallBack: () async =>
+            QRViewController.updateDimensions(widget.key, _channel,
+            overlay: widget.overlay)
+    ));
+  }
+
   @override
   Widget build(BuildContext context) {
     return NotificationListener(