handle tapping on previously played tiles

lib/main.dart

@@ -78,7 +78,10 @@ class _MyHomePageState extends State<MyHomePage> {
                     const TextStyle(fontSize: 30, fontWeight: FontWeight.bold),
               )),
               const Spacer(flex: 1),
-              game,
+              TTCGame(
+                turn: turn,
+                onClick: onTurnEnd,
+              ),
               const Spacer(flex: 5),
             ],
           )),
@@ -97,10 +100,10 @@ class TTCGame extends StatefulWidget {
   });
 
   final TTCState turn;
-  final Function? onClick;
   final List<TTCState>? data;
 
-  /// hook to retrieve the turn state of the game
+  /// hook into acction of tapping on square
+  final Function? onClick;
 
   @override
   State<StatefulWidget> createState() => _TTCGameState();
@@ -111,6 +114,8 @@ class _TTCGameState extends State<TTCGame> {
   late List<TTCState> data;
 
   void setCellState(int index) {
+    switch (data[index]) {
+      case TTCState.empty:
         setState(() {
           data[index] = turn;
 
@@ -121,6 +126,12 @@ class _TTCGameState extends State<TTCGame> {
           };
         });
         widget.onClick?.call();
+        break;
+      default:
+        ScaffoldMessenger.of(context)
+          ..clearSnackBars()
+          ..showSnackBar(const SnackBar(content: Text("Invalid Choice")));
+    }
   }
 
   Widget _genCell(int index, TTCState state) => Container(