Add timer for the steps of each first aid situation

lib/screens/first_aid_steps_screen.dart

@@ -1,9 +1,36 @@
+import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:firstaid/notifiers/first_aid_steps_notifier.dart';
 import 'package:firstaid/providers/situation_provider.dart';
 import 'package:flutter_gen/gen_l10n/app_localizations.dart';
 
+final firstAidStepsStateProvider =
+    ChangeNotifierProvider.autoDispose((ref) => FirstAidStepsState());
+
+class FirstAidStepsState extends ChangeNotifier {
+  Stopwatch stopwatch = Stopwatch();
+  Timer? timer;
+
+  FirstAidStepsState() {
+    stopwatch.start();
+    timer =
+        Timer.periodic(Duration(seconds: 1), (Timer t) => notifyListeners());
+  }
+
+  void stop() {
+    stopwatch.stop();
+    timer?.cancel();
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    timer?.cancel();
+    super.dispose();
+  }
+}
+
 class FirstAidStepsScreen extends ConsumerWidget {
   final String id;
 
@@ -14,6 +41,7 @@ class FirstAidStepsScreen extends ConsumerWidget {
     final situation = ref.watch(situationProvider(id));
     final _checked = ref.watch(firstAidStepsProvider(id));
     final _stepsNotifier = ref.read(firstAidStepsProvider(id).notifier);
+    final _firstAidStepsState = ref.watch(firstAidStepsStateProvider);
 
     return Scaffold(
       appBar: AppBar(
@@ -30,13 +58,20 @@ class FirstAidStepsScreen extends ConsumerWidget {
                 value: _checked[index],
                 onChanged: (bool? value) {
                   _stepsNotifier.toggleStep(id, index);
+                  if (index == situation.steps.length - 1 && value == true) {
+                    _firstAidStepsState.stop();
+                  }
                 },
               ),
               title: Text(situation.steps[index]),
             ),
           );
         },
       ),
+      bottomNavigationBar: Text(
+        'Elapsed time: ${_firstAidStepsState.stopwatch.elapsed.inSeconds} seconds',
+        style: TextStyle(fontSize: 20),
+      ),
     );
   }
 }