Updated the nested navigation NavigationBar example (#137788)

Updated the NavigationBar API doc that describes
examples/api/lib/material/navigation_bar/navigation_bar.2.dart and made
some cosmetic changes to the example to improve its appearance in
Material 3. Also did a little gratuitous reformatting.

Fixes #136125

examples/api/lib/material/navigation_bar/navigation_bar.2.dart

@@ -32,33 +32,47 @@ class _HomeState extends State<Home> with TickerProviderStateMixin<Home> {
   int selectedIndex = 0;
 
   AnimationController buildFaderController() {
-    final AnimationController controller =
-        AnimationController(vsync: this, duration: const Duration(milliseconds: 200));
-    controller.addStatusListener((AnimationStatus status) {
+    final AnimationController controller = AnimationController(
+      vsync: this,
+      duration: const Duration(milliseconds: 300),
+    );
+    controller.addStatusListener(
+      (AnimationStatus status) {
         if (status == AnimationStatus.dismissed) {
           setState(() {}); // Rebuild unselected destinations offstage.
         }
-    });
+      },
+    );
     return controller;
   }
 
   @override
   void initState() {
     super.initState();
-    navigatorKeys =
-        List<GlobalKey<NavigatorState>>.generate(allDestinations.length, (int index) => GlobalKey()).toList();
-    destinationFaders =
-        List<AnimationController>.generate(allDestinations.length, (int index) => buildFaderController()).toList();
+
+    navigatorKeys = List<GlobalKey<NavigatorState>>.generate(
+      allDestinations.length,
+      (int index) => GlobalKey(),
+    ).toList();
+
+    destinationFaders = List<AnimationController>.generate(
+      allDestinations.length,
+      (int index) => buildFaderController(),
+    ).toList();
     destinationFaders[selectedIndex].value = 1.0;
-    destinationViews = allDestinations.map((Destination destination) {
+
+    final CurveTween tween = CurveTween(curve: Curves.fastOutSlowIn);
+    destinationViews = allDestinations.map<Widget>(
+      (Destination destination) {
         return FadeTransition(
-        opacity: destinationFaders[destination.index].drive(CurveTween(curve: Curves.fastOutSlowIn)),
+          opacity: destinationFaders[destination.index].drive(tween),
           child: DestinationView(
             destination: destination,
             navigatorKey: navigatorKeys[destination.index],
           ),
         );
-    }).toList();
+      },
+    ).toList();
   }
 
   @override
@@ -81,7 +95,8 @@ class _HomeState extends State<Home> with TickerProviderStateMixin<Home> {
           top: false,
           child: Stack(
             fit: StackFit.expand,
-            children: allDestinations.map((Destination destination) {
+            children: allDestinations.map(
+              (Destination destination) {
                 final int index = destination.index;
                 final Widget view = destinationViews[index];
                 if (index == selectedIndex) {
@@ -94,7 +109,8 @@ class _HomeState extends State<Home> with TickerProviderStateMixin<Home> {
                   }
                   return Offstage(child: view);
                 }
-            }).toList(),
+              },
+            ).toList(),
           ),
         ),
         bottomNavigationBar: NavigationBar(
@@ -104,12 +120,14 @@ class _HomeState extends State<Home> with TickerProviderStateMixin<Home> {
               selectedIndex = index;
             });
           },
-          destinations: allDestinations.map((Destination destination) {
+          destinations: allDestinations.map<NavigationDestination>(
+            (Destination destination) {
               return NavigationDestination(
                 icon: Icon(destination.icon, color: destination.color),
                 label: destination.title,
               );
-          }).toList(),
+            },
+          ).toList(),
         ),
       ),
     );
@@ -148,6 +166,7 @@ class RootPage extends StatelessWidget {
     final TextStyle headlineSmall = Theme.of(context).textTheme.headlineSmall!;
     final ButtonStyle buttonStyle = ElevatedButton.styleFrom(
       backgroundColor: destination.color,
+      foregroundColor: Colors.white,
       visualDensity: VisualDensity.comfortable,
       padding: const EdgeInsets.symmetric(vertical: 12, horizontal: 16),
       textStyle: headlineSmall,
@@ -157,6 +176,7 @@ class RootPage extends StatelessWidget {
       appBar: AppBar(
         title: Text('${destination.title} RootPage - /'),
         backgroundColor: destination.color,
+        foregroundColor: Colors.white,
       ),
       backgroundColor: destination.color[50],
       body: Center(
@@ -236,15 +256,23 @@ class ListPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     const int itemCount = 50;
+    final ColorScheme colorScheme = Theme.of(context).colorScheme;
     final ButtonStyle buttonStyle = OutlinedButton.styleFrom(
+      shape: RoundedRectangleBorder(
+        borderRadius: BorderRadius.circular(8),
+        side: BorderSide(
+          color: colorScheme.onSurface.withOpacity(0.12),
+        ),
+      ),
       foregroundColor: destination.color,
-      fixedSize: const Size.fromHeight(128),
+      fixedSize: const Size.fromHeight(64),
       textStyle: Theme.of(context).textTheme.headlineSmall,
     );
     return Scaffold(
       appBar: AppBar(
         title: Text('${destination.title} ListPage - /list'),
         backgroundColor: destination.color,
+        foregroundColor: Colors.white,
       ),
       backgroundColor: destination.color[50],
       body: SizedBox.expand(
@@ -256,7 +284,11 @@ class ListPage extends StatelessWidget {
               child: OutlinedButton(
                 style: buttonStyle.copyWith(
                   backgroundColor: MaterialStatePropertyAll<Color>(
-                    Color.lerp(destination.color[100], Colors.white, index / itemCount)!,
+                    Color.lerp(
+                      destination.color[100],
+                      Colors.white,
+                      index / itemCount
+                    )!,
                   ),
                 ),
                 onPressed: () {
@@ -303,6 +335,7 @@ class _TextPageState extends State<TextPage> {
       appBar: AppBar(
         title: Text('${widget.destination.title} TextPage - /list/text'),
         backgroundColor: widget.destination.color,
+        foregroundColor: Colors.white,
       ),
       backgroundColor: widget.destination.color[50],
       body: Container(

packages/flutter/lib/src/material/navigation_bar.dart

@@ -62,12 +62,18 @@ const double _kIndicatorWidth = 64;
 /// {@end-tool}
 ///
 /// {@tool dartpad}
-/// This example shows a [NavigationBar] as it is used within a [Scaffold]
-/// widget when there are nested navigators that provide local navigation. The
-/// [NavigationBar] has four [NavigationDestination] widgets with different
-/// color schemes. The [onDestinationSelected] callback changes the selected
-/// item's index and displays a corresponding page with its own local navigator
-/// in the body of a [Scaffold].
+/// This example shows a [NavigationBar] within a main [Scaffold]
+/// widget that's used to control the visibility of destination pages.
+/// Each destination has its own scaffold and a nested navigator that
+/// provides local navigation. The example's [NavigationBar] has four
+/// [NavigationDestination] widgets with different color schemes. Its
+/// [onDestinationSelected] callback changes the selected
+/// destination's index and displays a corresponding page with its own
+/// local navigator and scaffold - all within the body of the main
+/// scaffold. The destination pages are organized in a [Stack] and
+/// switching destinations fades out the current page and
+/// fades in the new one. Destinations that aren't visible or animating
+/// are kept [Offstage].
 ///
 /// ** See code in examples/api/lib/material/navigation_bar/navigation_bar.2.dart **
 /// {@end-tool}