Updated widgets to listen to ChangeNotifier events

lib/counter_manager.dart

@@ -1,12 +1,13 @@
 import 'package:cloud_firestore/cloud_firestore.dart';
 import 'package:firebasecounter/app_state.dart';
+import 'package:flutter/material.dart';
 
 /// Interface that defines the functions required to manipulate
 /// the app state.
 ///
 /// Defined as an abstract class so that tests can operate on a
 /// version that does not communicate with Firebase.
-abstract class ICounterManager {
+abstract class ICounterManager extends ChangeNotifier {
   /// Any `CounterManager` must have an instance of the state
   /// object.
   AppState state;
@@ -17,12 +18,21 @@ abstract class ICounterManager {
   void increment();
 }
 
-class CounterManager implements ICounterManager {
+class CounterManager extends ChangeNotifier implements ICounterManager {
   AppState state = AppState();
-  void increment() => state = state.copyWith(DateTime.now());
+
+  /// Copies the state object with the timestamp of the most
+  /// recent click and tells the stream to update.
+  void increment() {
+    state = state.copyWith(DateTime.now());
+    // Adding this line is how `ChangeNotifier` tells widgets to
+    // re-render themselves.
+    notifyListeners();
+  }
 }
 
-class FirestoreCounterManager implements ICounterManager {
+class FirestoreCounterManager extends ChangeNotifier
+    implements ICounterManager {
   AppState state;
   final FirebaseFirestore _firestore;
   FirestoreCounterManager()
@@ -51,6 +61,7 @@ class FirestoreCounterManager implements ICounterManager {
           .toList();
       // Part 7
       state = AppState(_clicks);
+      notifyListeners();
     });
   }
 

lib/dependencies.dart

@@ -9,7 +9,7 @@ class DependenciesProvider extends StatelessWidget {
   Widget build(BuildContext context) {
     return MultiProvider(
       providers: [
-        Provider<ICounterManager>(
+        ChangeNotifierProvider<ICounterManager>(
             create: (context) => FirestoreCounterManager()),
       ],
       child: child,

lib/main.dart

@@ -67,7 +67,7 @@ class _MyHomePageState extends State<MyHomePage> {
       ),
       floatingActionButton: FloatingActionButton(
         // Reference `widget.manager` instead of `manager` directly
-        onPressed: () => setState(() => widget.manager.increment()),
+        onPressed: () => widget.manager.increment(),
         tooltip: 'Increment',
         child: Icon(Icons.add),
       ),