[platform_channels] adds implementation of BasicMessageChannel for iOS (#610)

platform_channels/lib/main.dart

@@ -25,6 +25,11 @@ class PlatformChannelSample extends StatelessWidget {
         '/addPetDetails': (context) => AddPetDetails(),
       },
       title: 'Platform Channel Sample',
+      theme: ThemeData(
+        snackBarTheme: SnackBarThemeData(
+          backgroundColor: Colors.blue[500],
+        ),
+      ),
       home: HomePage(),
     );
   }

platform_channels/lib/src/pet_list_screen.dart

@@ -15,6 +15,7 @@ class PetListScreen extends StatefulWidget {
 
 class _PetListScreenState extends State<PetListScreen> {
   PetListModel petListModel;
+  final scaffoldKey = GlobalKey<ScaffoldState>();
 
   @override
   void initState() {
@@ -23,9 +24,17 @@ class _PetListScreenState extends State<PetListScreen> {
     // to PetModel.
     BasicMessageChannel('stringCodecDemo', StringCodec())
         .setMessageHandler((message) async {
-      setState(() {
-        petListModel = PetListModel.fromJson(message);
-      });
+      if (message == null) {
+        scaffoldKey.currentState.showSnackBar(
+          SnackBar(
+              content:
+                  const Text('An error occurred while adding pet details.')),
+        );
+      } else {
+        setState(() {
+          petListModel = PetListModel.fromJson(message);
+        });
+      }
       return;
     });
   }
@@ -33,6 +42,7 @@ class _PetListScreenState extends State<PetListScreen> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
+      key: scaffoldKey,
       appBar: AppBar(
         title: Text('Pet List'),
       ),
@@ -84,7 +94,6 @@ class BuildPetList extends StatelessWidget {
 
   void showSnackBar(String message, BuildContext context) {
     Scaffold.of(context).showSnackBar(SnackBar(
-      backgroundColor: Theme.of(context).primaryColor,
       content: Text(message),
     ));
   }