add: working alert

app/lib/pages/default_page.dart

@@ -19,12 +19,13 @@ class _DefaultPageState extends State<DefaultPage> {
   late Timer _timer;
   bool showBar = false;
   bool serverConnection = true;
+  bool dialogShown = false;
 
   late Future<List<Measurement>> markerListFuture;
   late Future<Uint8List> relationFuture;
 
-  /// Fetch measurement and relation data, check server connection, and
-  /// start a request timer
+  final GlobalKey<ScaffoldMessengerState> _scaffoldMessengerKey = GlobalKey<ScaffoldMessengerState>();
+
   @override
   void initState() {
     super.initState();
@@ -36,44 +37,55 @@ class _DefaultPageState extends State<DefaultPage> {
       // Return the measurements
       return measurements;
     }).catchError((error) {
+      serverConnection = false;
       throw Exception("Failed to fetch measurements: $error");
     });
 
-    if (!serverConnection) {
-      _showConnectionMessage();
-    }
-
     relationFuture = fetchRelation();
 
-    // Schedule fetchMarkerData to run periodically based on fetchInterval from consts
+    // Schedule fetchMarkerData to run periodically based on fetchInterval
     const Duration interval = Duration(minutes: fetchInterval); // NB fetchInterval value to be determined
     _timer = Timer.periodic(interval, (timer) {
       fetchMeasurements();
     });
   }
 
-  /// Display a message for 5 seconds informing the user
-  /// of lacking server connection
-  void _showConnectionMessage() {
-    const snackBar = SnackBar(
-      content: Text('Failed to connect to the server. Information about'
-          'ice safety could be outdated!'),
-      duration: Duration(seconds: 5),
-    );
-    ScaffoldMessenger.of(context).showSnackBar(snackBar);
-  }
-  /// Timer for resending requests to server
   @override
   void dispose() {
-    // Cancel timer on widget termination
     _timer.cancel();
     super.dispose();
   }
 
+  /// Display message to user
+  void showConnectionMessage() {
+    showDialog(
+      context: context,
+      builder: (context) => AlertDialog(
+        actions: [
+          TextButton(
+            onPressed: () {
+              Navigator.of(context).pop();
+              },
+            child: const Text("Ok"),
+          )
+        ],
+        title: const Center(
+          child: Text("No server connection")
+        ),
+        contentPadding: const EdgeInsets.all(10.0),
+        content: const Text(
+          "The app may display outdated information. Use with caution!",
+          textAlign: TextAlign.center, // Align text center
+        ),
+      ),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       home: Scaffold(
+        key: _scaffoldMessengerKey,
         appBar: AppBar(
           title: const Text('IceMap'),
           actions: [
@@ -90,7 +102,6 @@ class _DefaultPageState extends State<DefaultPage> {
         body: FutureBuilder(
           future: Future.wait([markerListFuture, relationFuture]),
           builder: (BuildContext context, AsyncSnapshot<List<dynamic>> snapshot) {
-            // Display loading screen until data is fetched
             if (snapshot.connectionState == ConnectionState.waiting) {
               return Container(
                 decoration: const BoxDecoration( // Background color for loading screen
@@ -107,12 +118,17 @@ class _DefaultPageState extends State<DefaultPage> {
             } else if (snapshot.hasError) {
               return Center(child: Text('Error: ${snapshot.error}'));
             } else {
-              if (!serverConnection) { // NB: implement dialogue box
-                print("Failed to connect to server"); // Here...
-              }
-              // Display default page once all data is loaded from server
+              // Alert the user after the build completes if there is no server connection
+              WidgetsBinding.instance.addPostFrameCallback((_) {
+                if (serverConnection && !dialogShown) {
+                  dialogShown = true;
+                  showConnectionMessage();
+                }
+              });
+
               List<Measurement> markerList = snapshot.data![0] as List<Measurement>;
               Uint8List relation = snapshot.data![1] as Uint8List;
+
               return Container( // Return container with list view and background color
                 decoration: const BoxDecoration( // Set background color
                   gradient: LinearGradient(
@@ -134,4 +150,3 @@ class _DefaultPageState extends State<DefaultPage> {
     );
   }
 }
-