Add a Material/Cupertino adaptive application example (#69)

platform_design/lib/song_detail_tab.dart

@@ -0,0 +1,99 @@
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+
+import 'widgets.dart';
+
+// Page shown when a card in the songs tab is tapped.
+//
+// On Android, this page sits at the top of your app. On iOS, this page is on
+// top of the songs tab's content but is below the tab bar itself.
+class SongDetailTab extends StatelessWidget {
+  const SongDetailTab({ this.id, this.song, this.color });
+
+  final int id;
+  final String song;
+  final Color color;
+
+  Widget _buildBody() {
+    return SafeArea(
+      bottom: false,
+      left: false,
+      right: false,
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          Hero(
+            tag: id,
+            child: HeroAnimatingSongCard(
+              song: song,
+              color: color,
+              heroAnimation: AlwaysStoppedAnimation(1),
+            ),
+            // This app uses a flightShuttleBuilder to specify the exact widget
+            // to build while the hero transition is mid-flight.
+            //
+            // It could either be specified here or in SongsTab.
+            flightShuttleBuilder: (context, animation, flightDirection, fromHeroContext, toHeroContext) {
+              return HeroAnimatingSongCard(
+                song: song,
+                color: color,
+                heroAnimation: animation,
+              );
+            },
+          ),
+          Divider(
+            height: 0,
+            color: Colors.grey,
+          ),
+          Expanded(
+            child: ListView.builder(
+              itemCount: 10,
+              itemBuilder: (context, index) {
+                if (index == 0) {
+                  return Padding(
+                    padding: const EdgeInsets.only(left: 15, top: 16, bottom: 8),
+                    child: Text('You might also like:', style: TextStyle(
+                      fontSize: 16,
+                      fontWeight: FontWeight.w500,
+                    )),
+                  );
+                }
+                // Just a bunch of boxes that simulates loading song choices.
+                return SongPlaceholderTile();
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+
+  // ===========================================================================
+  // Non-shared code below because we're using different scaffolds.
+  // ===========================================================================
+
+  Widget _buildAndroid(context) {
+    return Scaffold(
+      appBar: AppBar(title: Text(song)),
+      body: _buildBody(),
+    );
+  }
+
+  Widget _buildIos(context) {
+    return CupertinoPageScaffold(
+      navigationBar: CupertinoNavigationBar(
+        middle: Text(song),
+        previousPageTitle: 'Songs',
+      ),
+      child: _buildBody(),
+    );
+  }
+
+  @override
+  Widget build(context) {
+    return PlatformWidget(
+      androidBuilder: _buildAndroid,
+      iosBuilder: _buildIos,
+    );
+  }
+}