Centralize status polling with last-known-good, stale, and error tiers

The four polling status cards (ARP, mDNS, scanners summary, devices
summary) each rolled their own Timer/_status/_error/_isLoading state and
flipped to a red 'Error' UI on the first failed poll, throwing away
last-known-good data that was still in memory. A momentary backend blip
made every card flash red.

Introduce PolledValue<T>, a ChangeNotifier that owns the periodic timer,
in-flight CancelToken, last value, last error, and a freshness
classification (initialLoading / fresh / stale / error). 'stale' kicks in
on the first failure but keeps the value visible; 'error' only takes over
after staleErrorAfter elapses without a success (30s for the 5s scanner
polls, 3min for the 1min device-summary poll). All four cards now branch
on freshness and show a small live-updating warning icon with a tooltip
('Last updated N ago — <error>') while stale, instead of flipping red.

Also threads CancelToken? through the three polled GET endpoints.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/utils/oott_api.dart

@@ -235,18 +235,31 @@ class BackendAPI {
         .toList();
   }
 
-  Future<DeviceSummary> getDeviceSummary() async {
+  Future<DeviceSummary> getDeviceSummary({CancelToken? cancelToken}) async {
-    final response = await _dio.get('/devices/summary');
+    final response = await _dio.get(
+      '/devices/summary',
+      cancelToken: cancelToken,
+    );
     return DeviceSummary.fromJson(response.data as Map<String, dynamic>);
   }
 
-  Future<ArpScannerStatus> getArpScannerStatus() async {
+  Future<ArpScannerStatus> getArpScannerStatus({
-    final response = await _dio.get('/arp_scanner/status');
+    CancelToken? cancelToken,
+  }) async {
+    final response = await _dio.get(
+      '/arp_scanner/status',
+      cancelToken: cancelToken,
+    );
     return ArpScannerStatus.fromJson(response.data as Map<String, dynamic>);
   }
 
-  Future<MdnsScannerStatus> getMdnsScannerStatus() async {
+  Future<MdnsScannerStatus> getMdnsScannerStatus({
-    final response = await _dio.get('/mdns_scanner/status');
+    CancelToken? cancelToken,
+  }) async {
+    final response = await _dio.get(
+      '/mdns_scanner/status',
+      cancelToken: cancelToken,
+    );
     return MdnsScannerStatus.fromJson(response.data as Map<String, dynamic>);
   }
 

frontend/lib/utils/polled_value.dart

@@ -0,0 +1,76 @@
+import 'dart:async';
+
+import 'package:dio/dio.dart';
+import 'package:flutter/foundation.dart';
+
+import 'oott_api.dart';
+
+enum PolledFreshness { initialLoading, fresh, stale, error }
+
+class PolledValue<T> extends ChangeNotifier {
+  PolledValue({
+    required Future<T> Function({CancelToken? cancelToken}) fetch,
+    required Duration pollInterval,
+    required Duration staleErrorAfter,
+  }) : _fetch = fetch,
+       _staleErrorAfter = staleErrorAfter {
+    _load();
+    _pollTimer = Timer.periodic(pollInterval, (_) => _load());
+  }
+
+  final Future<T> Function({CancelToken? cancelToken}) _fetch;
+  final Duration _staleErrorAfter;
+  Timer? _pollTimer;
+  CancelToken? _cancelToken;
+  bool _disposed = false;
+
+  T? _value;
+  DateTime? _lastSuccessAt;
+  String? _lastErrorMessage;
+  bool _everCompleted = false;
+
+  T? get value => _value;
+  DateTime? get lastSuccessAt => _lastSuccessAt;
+  String? get lastErrorMessage => _lastErrorMessage;
+
+  PolledFreshness get freshness {
+    if (!_everCompleted && _value == null) {
+      return PolledFreshness.initialLoading;
+    }
+    final last = _lastSuccessAt;
+    if (_value == null || last == null) return PolledFreshness.error;
+    if (_lastErrorMessage == null) return PolledFreshness.fresh;
+    return DateTime.now().difference(last) >= _staleErrorAfter
+        ? PolledFreshness.error
+        : PolledFreshness.stale;
+  }
+
+  Future<void> _load() async {
+    _cancelToken?.cancel();
+    final token = CancelToken();
+    _cancelToken = token;
+    try {
+      final result = await _fetch(cancelToken: token);
+      if (_disposed || token != _cancelToken) return;
+      _value = result;
+      _lastSuccessAt = DateTime.now();
+      _lastErrorMessage = null;
+      _everCompleted = true;
+      notifyListeners();
+    } catch (e) {
+      if (_disposed || token != _cancelToken) return;
+      if (e is DioException && e.type == DioExceptionType.cancel) return;
+      _lastErrorMessage = dioErrorToUserMessage(e);
+      _everCompleted = true;
+      notifyListeners();
+    }
+  }
+
+  @override
+  void dispose() {
+    _disposed = true;
+    _pollTimer?.cancel();
+    _cancelToken?.cancel();
+    super.dispose();
+  }
+}

frontend/lib/widgets/arp_scanner_card.dart

@@ -6,6 +6,8 @@ import 'package:go_router/go_router.dart';
 import '../model/arp_scanner_status.dart';
 import '../utils/duration_formatter.dart';
 import '../utils/oott_api.dart';
+import '../utils/polled_value.dart';
+import 'polled_stale_indicator.dart';
 
 class ArpScannerCard extends StatefulWidget {
   const ArpScannerCard({super.key});
@@ -15,18 +17,18 @@ class ArpScannerCard extends StatefulWidget {
 }
 
 class _ArpScannerCardState extends State<ArpScannerCard> {
-  ArpScannerStatus? _status;
+  late final PolledValue<ArpScannerStatus> _polled;
-  DateTime? _statusReceivedAt;
-  bool _isLoading = true;
-  String? _error;
-  Timer? _refreshTimer;
   Timer? _tickTimer;
 
   @override
   void initState() {
     super.initState();
-    _load();
+    _polled = PolledValue<ArpScannerStatus>(
-    _refreshTimer = Timer.periodic(const Duration(seconds: 5), (_) => _load());
+      fetch: ({cancelToken}) =>
+          BackendAPI.instance.getArpScannerStatus(cancelToken: cancelToken),
+      pollInterval: const Duration(seconds: 5),
+      staleErrorAfter: const Duration(seconds: 30),
+    );
     _tickTimer = Timer.periodic(const Duration(seconds: 1), (_) {
       if (mounted) setState(() {});
     });
@@ -34,103 +36,106 @@ class _ArpScannerCardState extends State<ArpScannerCard> {
 
   @override
   void dispose() {
-    _refreshTimer?.cancel();
     _tickTimer?.cancel();
+    _polled.dispose();
     super.dispose();
   }
 
-  Future<void> _load() async {
-    try {
-      final status = await BackendAPI.instance.getArpScannerStatus();
-      if (!mounted) return;
-      setState(() {
-        _status = status;
-        _statusReceivedAt = DateTime.now();
-        _error = null;
-        _isLoading = false;
-      });
-    } catch (e) {
-      if (!mounted) return;
-      setState(() {
-        _error = e.toString();
-        _isLoading = false;
-      });
-    }
-  }
-
   @override
   Widget build(BuildContext context) {
-    if (_isLoading) {
+    return ListenableBuilder(
-      return const Card(
+      listenable: _polled,
-        child: Padding(
+      builder: (context, _) {
-          padding: EdgeInsets.all(16),
+        if (_polled.freshness == PolledFreshness.initialLoading) {
-          child: Center(child: CircularProgressIndicator()),
+          return const Card(
-        ),
+            child: Padding(
-      );
+              padding: EdgeInsets.all(16),
-    }
+              child: Center(child: CircularProgressIndicator()),
+            ),
+          );
+        }
 
-    final (color, label, sublabel) = _resolveState(context);
+        final (color, label, sublabel) = _resolveState();
+        final isStale = _polled.freshness == PolledFreshness.stale;
 
-    return Card(
+        return Card(
-      clipBehavior: Clip.antiAlias,
+          clipBehavior: Clip.antiAlias,
-      child: InkWell(
+          child: InkWell(
-        onTap: () => context.go('/status'),
+            onTap: () => context.go('/status'),
-        child: Padding(
+            child: Padding(
-          padding: const EdgeInsets.all(16),
+              padding: const EdgeInsets.all(16),
-          child: Row(
+              child: Row(
-            children: [
+                children: [
-              Icon(Icons.circle, color: color, size: 14),
+                  Icon(Icons.circle, color: color, size: 14),
-              const SizedBox(width: 12),
+                  const SizedBox(width: 12),
-              Expanded(
+                  Expanded(
-                child: Column(
+                    child: Column(
-                  crossAxisAlignment: CrossAxisAlignment.start,
+                      crossAxisAlignment: CrossAxisAlignment.start,
-                  children: [
+                      children: [
-                    Text(
+                        Row(
-                      'ARP Scanner',
+                          children: [
-                      style: Theme.of(context).textTheme.titleMedium,
+                            Text(
-                    ),
+                              'ARP Scanner',
-                    const SizedBox(height: 2),
+                              style: Theme.of(context).textTheme.titleMedium,
-                    Text(label, style: Theme.of(context).textTheme.bodyMedium),
+                            ),
-                    if (sublabel != null) ...[
+                            if (isStale) ...[
-                      const SizedBox(height: 2),
+                              const SizedBox(width: 6),
-                      Text(
+                              PolledStaleIndicator(polled: _polled),
-                        sublabel,
+                            ],
-                        style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                          ],
-                          color: Theme.of(context).colorScheme.outline,
                         ),
-                      ),
+                        const SizedBox(height: 2),
-                    ],
+                        Text(
-                  ],
+                          label,
-                ),
+                          style: Theme.of(context).textTheme.bodyMedium,
+                        ),
+                        if (sublabel != null) ...[
+                          const SizedBox(height: 2),
+                          Text(
+                            sublabel,
+                            style: Theme.of(context).textTheme.bodySmall
+                                ?.copyWith(
+                                  color: Theme.of(context).colorScheme.outline,
+                                ),
+                          ),
+                        ],
+                      ],
+                    ),
+                  ),
+                ],
               ),
-            ],
+            ),
           ),
-        ),
+        );
-      ),
+      },
     );
   }
 
-  (Color, String, String?) _resolveState(BuildContext context) {
+  (Color, String, String?) _resolveState() {
-    if (_error != null || _status == null) {
+    if (_polled.freshness == PolledFreshness.error) {
       return (
         Colors.red,
         'Error',
-        'Unable to reach the server or a server-side error occurred. Check the logs for details.',
+        _polled.lastErrorMessage ??
+            'Unable to reach the server. Check the logs for details.',
       );
     }
 
-    final elapsed = _statusReceivedAt != null
+    final status = _polled.value!;
-        ? DateTime.now().difference(_statusReceivedAt!).inSeconds.toDouble()
+    final lastSuccessAt = _polled.lastSuccessAt!;
-        : 0.0;
+    final elapsed = DateTime.now()
+        .difference(lastSuccessAt)
+        .inSeconds
+        .toDouble();
 
-    if (_status!.isRunning) {
+    if (status.isRunning) {
-      final sub = _status!.runningForSeconds != null
+      final sub = status.runningForSeconds != null
-          ? 'Running for ${formatSeconds(_status!.runningForSeconds! + elapsed)}'
+          ? 'Running for ${formatSeconds(status.runningForSeconds! + elapsed)}'
           : null;
       return (Colors.green, 'Running', sub);
     }
-    if (_status!.nextRunInSeconds != null) {
+    if (status.nextRunInSeconds != null) {
-      final remaining = (_status!.nextRunInSeconds! - elapsed).clamp(
+      final remaining = (status.nextRunInSeconds! - elapsed).clamp(
         0.0,
         double.infinity,
       );

frontend/lib/widgets/device_summary_card.dart

@@ -1,10 +1,10 @@
-import 'dart:async';
-
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 
 import '../model/device_summary.dart';
 import '../utils/oott_api.dart';
+import '../utils/polled_value.dart';
+import 'polled_stale_indicator.dart';
 
 class DeviceSummaryCard extends StatefulWidget {
   const DeviceSummaryCard({super.key});
@@ -14,42 +14,25 @@ class DeviceSummaryCard extends StatefulWidget {
 }
 
 class _DeviceSummaryCardState extends State<DeviceSummaryCard> {
-  DeviceSummary? _summary;
+  late final PolledValue<DeviceSummary> _polled;
-  bool _isLoading = true;
-  String? _error;
-  Timer? _timer;
 
   @override
   void initState() {
     super.initState();
-    _load();
+    _polled = PolledValue<DeviceSummary>(
-    _timer = Timer.periodic(const Duration(minutes: 1), (_) => _load());
+      fetch: ({cancelToken}) =>
+          BackendAPI.instance.getDeviceSummary(cancelToken: cancelToken),
+      pollInterval: const Duration(minutes: 1),
+      staleErrorAfter: const Duration(minutes: 3),
+    );
   }
 
   @override
   void dispose() {
-    _timer?.cancel();
+    _polled.dispose();
     super.dispose();
   }
 
-  Future<void> _load() async {
-    try {
-      final summary = await BackendAPI.instance.getDeviceSummary();
-      if (!mounted) return;
-      setState(() {
-        _summary = summary;
-        _error = null;
-        _isLoading = false;
-      });
-    } catch (e) {
-      if (!mounted) return;
-      setState(() {
-        _error = e.toString();
-        _isLoading = false;
-      });
-    }
-  }
-
   @override
   Widget build(BuildContext context) {
     return Card(
@@ -58,64 +41,91 @@ class _DeviceSummaryCardState extends State<DeviceSummaryCard> {
         onTap: () => context.go('/devices'),
         child: Padding(
           padding: const EdgeInsets.all(16),
-          child: Column(
+          child: ListenableBuilder(
-            crossAxisAlignment: CrossAxisAlignment.start,
+            listenable: _polled,
-            children: [
+            builder: (context, _) {
-              Text('Devices', style: Theme.of(context).textTheme.titleLarge),
+              return Column(
-              const SizedBox(height: 12),
+                crossAxisAlignment: CrossAxisAlignment.start,
-              if (_isLoading)
+                children: [
-                const Center(child: CircularProgressIndicator())
+                  Row(
-              else if (_error != null)
+                    children: [
-                Text(
+                      Text(
-                  'Error loading device summary',
+                        'Devices',
-                  style: TextStyle(color: Theme.of(context).colorScheme.error),
+                        style: Theme.of(context).textTheme.titleLarge,
-                )
+                      ),
-              else if (_summary != null) ...[
+                      if (_polled.freshness == PolledFreshness.stale) ...[
-                _SummaryRow(
+                        const SizedBox(width: 6),
-                  label: 'Registered in the system',
+                        PolledStaleIndicator(polled: _polled),
-                  value: '${_summary!.totalRegistered}',
+                      ],
-                ),
+                    ],
-                const Divider(height: 20),
-                Text(
-                  'Seen in the last 24 hours',
-                  style: Theme.of(context).textTheme.bodyMedium?.copyWith(
-                    color: Theme.of(context).colorScheme.onSurface,
-                    fontWeight: FontWeight.w600,
                   ),
-                ),
+                  const SizedBox(height: 12),
-                const SizedBox(height: 6),
+                  ..._buildBody(context),
-                _SummaryRow(
+                ],
-                  label: 'Registered',
+              );
-                  value: '${_summary!.seenLastDayRegistered}',
+            },
-                ),
-                _SummaryRow(
-                  label: 'Unregistered',
-                  value: '${_summary!.seenLastDayUnregistered}',
-                ),
-                const Divider(height: 20),
-                Text(
-                  'Seen in the last 7 days',
-                  style: Theme.of(context).textTheme.bodyMedium?.copyWith(
-                    color: Theme.of(context).colorScheme.onSurface,
-                    fontWeight: FontWeight.w600,
-                  ),
-                ),
-                const SizedBox(height: 6),
-                _SummaryRow(
-                  label: 'Registered',
-                  value: '${_summary!.seenLastWeekRegistered}',
-                ),
-                _SummaryRow(
-                  label: 'Unregistered',
-                  value: '${_summary!.seenLastWeekUnregistered}',
-                ),
-              ],
-            ],
           ),
         ),
       ),
     );
   }
+
+  List<Widget> _buildBody(BuildContext context) {
+    switch (_polled.freshness) {
+      case PolledFreshness.initialLoading:
+        return const [Center(child: CircularProgressIndicator())];
+      case PolledFreshness.error:
+        return [
+          Text(
+            _polled.lastErrorMessage ?? 'Error loading device summary',
+            style: TextStyle(color: Theme.of(context).colorScheme.error),
+          ),
+        ];
+      case PolledFreshness.fresh:
+      case PolledFreshness.stale:
+        final summary = _polled.value!;
+        return [
+          _SummaryRow(
+            label: 'Registered in the system',
+            value: '${summary.totalRegistered}',
+          ),
+          const Divider(height: 20),
+          Text(
+            'Seen in the last 24 hours',
+            style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+              color: Theme.of(context).colorScheme.onSurface,
+              fontWeight: FontWeight.w600,
+            ),
+          ),
+          const SizedBox(height: 6),
+          _SummaryRow(
+            label: 'Registered',
+            value: '${summary.seenLastDayRegistered}',
+          ),
+          _SummaryRow(
+            label: 'Unregistered',
+            value: '${summary.seenLastDayUnregistered}',
+          ),
+          const Divider(height: 20),
+          Text(
+            'Seen in the last 7 days',
+            style: Theme.of(context).textTheme.bodyMedium?.copyWith(
+              color: Theme.of(context).colorScheme.onSurface,
+              fontWeight: FontWeight.w600,
+            ),
+          ),
+          const SizedBox(height: 6),
+          _SummaryRow(
+            label: 'Registered',
+            value: '${summary.seenLastWeekRegistered}',
+          ),
+          _SummaryRow(
+            label: 'Unregistered',
+            value: '${summary.seenLastWeekUnregistered}',
+          ),
+        ];
+    }
+  }
 }
 
 class _SummaryRow extends StatelessWidget {

frontend/lib/widgets/mdns_scanner_card.dart

@@ -6,6 +6,8 @@ import 'package:go_router/go_router.dart';
 import '../model/mdns_scanner_status.dart';
 import '../utils/duration_formatter.dart';
 import '../utils/oott_api.dart';
+import '../utils/polled_value.dart';
+import 'polled_stale_indicator.dart';
 
 class MdnsScannerCard extends StatefulWidget {
   const MdnsScannerCard({super.key});
@@ -15,18 +17,18 @@ class MdnsScannerCard extends StatefulWidget {
 }
 
 class _MdnsScannerCardState extends State<MdnsScannerCard> {
-  MdnsScannerStatus? _status;
+  late final PolledValue<MdnsScannerStatus> _polled;
-  DateTime? _statusReceivedAt;
-  bool _isLoading = true;
-  String? _error;
-  Timer? _refreshTimer;
   Timer? _tickTimer;
 
   @override
   void initState() {
     super.initState();
-    _load();
+    _polled = PolledValue<MdnsScannerStatus>(
-    _refreshTimer = Timer.periodic(const Duration(seconds: 5), (_) => _load());
+      fetch: ({cancelToken}) =>
+          BackendAPI.instance.getMdnsScannerStatus(cancelToken: cancelToken),
+      pollInterval: const Duration(seconds: 5),
+      staleErrorAfter: const Duration(seconds: 30),
+    );
     _tickTimer = Timer.periodic(const Duration(seconds: 1), (_) {
       if (mounted) setState(() {});
     });
@@ -34,109 +36,112 @@ class _MdnsScannerCardState extends State<MdnsScannerCard> {
 
   @override
   void dispose() {
-    _refreshTimer?.cancel();
     _tickTimer?.cancel();
+    _polled.dispose();
     super.dispose();
   }
 
-  Future<void> _load() async {
-    try {
-      final status = await BackendAPI.instance.getMdnsScannerStatus();
-      if (!mounted) return;
-      setState(() {
-        _status = status;
-        _statusReceivedAt = DateTime.now();
-        _error = null;
-        _isLoading = false;
-      });
-    } catch (e) {
-      if (!mounted) return;
-      setState(() {
-        _error = e.toString();
-        _isLoading = false;
-      });
-    }
-  }
-
   @override
   Widget build(BuildContext context) {
-    if (_isLoading) {
+    return ListenableBuilder(
-      return const Card(
+      listenable: _polled,
-        child: Padding(
+      builder: (context, _) {
-          padding: EdgeInsets.all(16),
+        if (_polled.freshness == PolledFreshness.initialLoading) {
-          child: Center(child: CircularProgressIndicator()),
+          return const Card(
-        ),
+            child: Padding(
-      );
+              padding: EdgeInsets.all(16),
-    }
+              child: Center(child: CircularProgressIndicator()),
+            ),
+          );
+        }
 
-    final (color, label, sublabels) = _resolveState(context);
+        final (color, label, sublabels) = _resolveState();
+        final isStale = _polled.freshness == PolledFreshness.stale;
 
-    return Card(
+        return Card(
-      clipBehavior: Clip.antiAlias,
+          clipBehavior: Clip.antiAlias,
-      child: InkWell(
+          child: InkWell(
-        onTap: () => context.go('/status'),
+            onTap: () => context.go('/status'),
-        child: Padding(
+            child: Padding(
-          padding: const EdgeInsets.all(16),
+              padding: const EdgeInsets.all(16),
-          child: Row(
+              child: Row(
-            children: [
+                children: [
-              Icon(Icons.circle, color: color, size: 14),
+                  Icon(Icons.circle, color: color, size: 14),
-              const SizedBox(width: 12),
+                  const SizedBox(width: 12),
-              Expanded(
+                  Expanded(
-                child: Column(
+                    child: Column(
-                  crossAxisAlignment: CrossAxisAlignment.start,
+                      crossAxisAlignment: CrossAxisAlignment.start,
-                  children: [
+                      children: [
-                    Text(
+                        Row(
-                      'mDNS Scanner',
+                          children: [
-                      style: Theme.of(context).textTheme.titleMedium,
+                            Text(
-                    ),
+                              'mDNS Scanner',
-                    const SizedBox(height: 2),
+                              style: Theme.of(context).textTheme.titleMedium,
-                    Text(label, style: Theme.of(context).textTheme.bodyMedium),
+                            ),
-                    for (final sublabel in sublabels) ...[
+                            if (isStale) ...[
-                      const SizedBox(height: 2),
+                              const SizedBox(width: 6),
-                      Text(
+                              PolledStaleIndicator(polled: _polled),
-                        sublabel,
+                            ],
-                        style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                          ],
-                          color: Theme.of(context).colorScheme.outline,
                         ),
-                      ),
+                        const SizedBox(height: 2),
-                    ],
+                        Text(
-                  ],
+                          label,
-                ),
+                          style: Theme.of(context).textTheme.bodyMedium,
+                        ),
+                        for (final sublabel in sublabels) ...[
+                          const SizedBox(height: 2),
+                          Text(
+                            sublabel,
+                            style: Theme.of(context).textTheme.bodySmall
+                                ?.copyWith(
+                                  color: Theme.of(context).colorScheme.outline,
+                                ),
+                          ),
+                        ],
+                      ],
+                    ),
+                  ),
+                ],
               ),
-            ],
+            ),
           ),
-        ),
+        );
-      ),
+      },
     );
   }
 
-  (Color, String, List<String>) _resolveState(BuildContext context) {
+  (Color, String, List<String>) _resolveState() {
-    if (_error != null || _status == null) {
+    if (_polled.freshness == PolledFreshness.error) {
       return (
         Colors.red,
         'Error',
         [
-          'Unable to reach the server or a server-side error occurred. Check the logs for details.',
+          _polled.lastErrorMessage ??
+              'Unable to reach the server. Check the logs for details.',
         ],
       );
     }
 
-    final elapsed = _statusReceivedAt != null
+    final status = _polled.value!;
-        ? DateTime.now().difference(_statusReceivedAt!).inSeconds.toDouble()
+    final lastSuccessAt = _polled.lastSuccessAt!;
-        : 0.0;
+    final elapsed = DateTime.now()
+        .difference(lastSuccessAt)
+        .inSeconds
+        .toDouble();
 
-    if (_status!.isListening) {
+    if (status.isListening) {
       final sublabels = <String>[];
-      if (_status!.listeningForSeconds != null) {
+      if (status.listeningForSeconds != null) {
         sublabels.add(
-          'Listening for ${formatSeconds(_status!.listeningForSeconds! + elapsed)} · ${_status!.devicesSeen} devices seen',
+          'Listening for ${formatSeconds(status.listeningForSeconds! + elapsed)} · ${status.devicesSeen} devices seen',
         );
       } else {
-        sublabels.add('${_status!.devicesSeen} devices seen');
+        sublabels.add('${status.devicesSeen} devices seen');
       }
-      if (_status!.lastDeviceSeenSecondsAgo != null) {
+      if (status.lastDeviceSeenSecondsAgo != null) {
         sublabels.add(
-          'Last device ${formatSeconds(_status!.lastDeviceSeenSecondsAgo! + elapsed)} ago',
+          'Last device ${formatSeconds(status.lastDeviceSeenSecondsAgo! + elapsed)} ago',
         );
       }
       return (Colors.green, 'Listening', sublabels);

frontend/lib/widgets/polled_stale_indicator.dart

@@ -0,0 +1,54 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+
+import '../utils/duration_formatter.dart';
+import '../utils/polled_value.dart';
+
+class PolledStaleIndicator extends StatefulWidget {
+  const PolledStaleIndicator({super.key, required this.polled});
+
+  final PolledValue polled;
+
+  @override
+  State<PolledStaleIndicator> createState() => _PolledStaleIndicatorState();
+}
+
+class _PolledStaleIndicatorState extends State<PolledStaleIndicator> {
+  Timer? _ticker;
+
+  @override
+  void initState() {
+    super.initState();
+    _ticker = Timer.periodic(const Duration(seconds: 1), (_) {
+      if (mounted) setState(() {});
+    });
+  }
+
+  @override
+  void dispose() {
+    _ticker?.cancel();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final lastSuccessAt = widget.polled.lastSuccessAt;
+    final ago = lastSuccessAt != null
+        ? formatSeconds(
+            DateTime.now().difference(lastSuccessAt).inSeconds.toDouble(),
+          )
+        : 'a while';
+    final message = widget.polled.lastErrorMessage != null
+        ? 'Last updated $ago ago — ${widget.polled.lastErrorMessage}'
+        : 'Last updated $ago ago';
+    return Tooltip(
+      message: message,
+      child: Icon(
+        Icons.warning_amber_rounded,
+        size: 14,
+        color: Theme.of(context).colorScheme.onSurfaceVariant,
+      ),
+    );
+  }
+}

frontend/lib/widgets/scanners_status_card.dart

@@ -7,6 +7,8 @@ import '../model/arp_scanner_status.dart';
 import '../model/mdns_scanner_status.dart';
 import '../utils/duration_formatter.dart';
 import '../utils/oott_api.dart';
+import '../utils/polled_value.dart';
+import 'polled_stale_indicator.dart';
 
 class ScannersStatusCard extends StatefulWidget {
   const ScannersStatusCard({super.key});
@@ -16,20 +18,25 @@ class ScannersStatusCard extends StatefulWidget {
 }
 
 class _ScannersStatusCardState extends State<ScannersStatusCard> {
-  ArpScannerStatus? _arpStatus;
+  late final PolledValue<ArpScannerStatus> _arp;
-  String? _arpError;
+  late final PolledValue<MdnsScannerStatus> _mdns;
-  MdnsScannerStatus? _mdnsStatus;
-  String? _mdnsError;
-  DateTime? _statusReceivedAt;
-  bool _isLoading = true;
-  Timer? _refreshTimer;
   Timer? _tickTimer;
 
   @override
   void initState() {
     super.initState();
-    _load();
+    _arp = PolledValue<ArpScannerStatus>(
-    _refreshTimer = Timer.periodic(const Duration(seconds: 5), (_) => _load());
+      fetch: ({cancelToken}) =>
+          BackendAPI.instance.getArpScannerStatus(cancelToken: cancelToken),
+      pollInterval: const Duration(seconds: 5),
+      staleErrorAfter: const Duration(seconds: 30),
+    );
+    _mdns = PolledValue<MdnsScannerStatus>(
+      fetch: ({cancelToken}) =>
+          BackendAPI.instance.getMdnsScannerStatus(cancelToken: cancelToken),
+      pollInterval: const Duration(seconds: 5),
+      staleErrorAfter: const Duration(seconds: 30),
+    );
     _tickTimer = Timer.periodic(const Duration(seconds: 1), (_) {
       if (mounted) setState(() {});
     });
@@ -37,75 +44,53 @@ class _ScannersStatusCardState extends State<ScannersStatusCard> {
 
   @override
   void dispose() {
-    _refreshTimer?.cancel();
     _tickTimer?.cancel();
+    _arp.dispose();
+    _mdns.dispose();
     super.dispose();
   }
 
-  Future<void> _load() async {
-    ArpScannerStatus? arp;
-    String? arpError;
-    try {
-      arp = await BackendAPI.instance.getArpScannerStatus();
-    } catch (e) {
-      arpError = dioErrorToUserMessage(e);
-    }
-
-    MdnsScannerStatus? mdns;
-    String? mdnsError;
-    try {
-      mdns = await BackendAPI.instance.getMdnsScannerStatus();
-    } catch (e) {
-      mdnsError = dioErrorToUserMessage(e);
-    }
-
-    if (!mounted) return;
-    setState(() {
-      _arpStatus = arp;
-      _arpError = arpError;
-      _mdnsStatus = mdns;
-      _mdnsError = mdnsError;
-      _statusReceivedAt = DateTime.now();
-      _isLoading = false;
-    });
-  }
-
-  double get _elapsed => _statusReceivedAt != null
-      ? DateTime.now().difference(_statusReceivedAt!).inSeconds.toDouble()
-      : 0.0;
-
   @override
   Widget build(BuildContext context) {
-    if (_isLoading) {
+    return ListenableBuilder(
-      return const Card(
+      listenable: Listenable.merge([_arp, _mdns]),
-        child: Padding(
+      builder: (context, _) {
-          padding: EdgeInsets.all(16),
+        if (_arp.freshness == PolledFreshness.initialLoading ||
-          child: Center(child: CircularProgressIndicator()),
+            _mdns.freshness == PolledFreshness.initialLoading) {
-        ),
+          return const Card(
-      );
+            child: Padding(
-    }
+              padding: EdgeInsets.all(16),
+              child: Center(child: CircularProgressIndicator()),
+            ),
+          );
+        }
 
-    final (arpColor, arpText) = _resolveArp();
+        final (arpColor, arpText) = _resolveArp();
-    final (mdnsColor, mdnsText) = _resolveMdns();
+        final (mdnsColor, mdnsText) = _resolveMdns();
 
-    return Card(
+        return Card(
-      clipBehavior: Clip.antiAlias,
+          clipBehavior: Clip.antiAlias,
-      child: InkWell(
+          child: InkWell(
-        onTap: () => context.go('/status'),
+            onTap: () => context.go('/status'),
-        child: Padding(
+            child: Padding(
-          padding: const EdgeInsets.all(16),
+              padding: const EdgeInsets.all(16),
-          child: Column(
+              child: Column(
-            crossAxisAlignment: CrossAxisAlignment.start,
+                crossAxisAlignment: CrossAxisAlignment.start,
-            children: [
+                children: [
-              Text('Status', style: Theme.of(context).textTheme.titleMedium),
+                  Text(
-              const SizedBox(height: 12),
+                    'Status',
-              _scannerRow(context, arpColor, 'ARP', arpText, _arpError),
+                    style: Theme.of(context).textTheme.titleMedium,
-              const SizedBox(height: 8),
+                  ),
-              _scannerRow(context, mdnsColor, 'mDNS', mdnsText, _mdnsError),
+                  const SizedBox(height: 12),
-            ],
+                  _scannerRow(context, arpColor, 'ARP', arpText, _arp),
+                  const SizedBox(height: 8),
+                  _scannerRow(context, mdnsColor, 'mDNS', mdnsText, _mdns),
+                ],
+              ),
+            ),
           ),
-        ),
+        );
-      ),
+      },
     );
   }
 
@@ -114,18 +99,26 @@ class _ScannersStatusCardState extends State<ScannersStatusCard> {
     Color color,
     String name,
     String statusText,
-    String? errorMessage,
+    PolledValue polled,
   ) {
     Widget dot = Icon(Icons.circle, color: color, size: 12);
-    if (errorMessage != null) {
+    if (polled.freshness == PolledFreshness.error) {
-      dot = Tooltip(message: errorMessage, child: dot);
+      dot = Tooltip(message: polled.lastErrorMessage ?? 'Error', child: dot);
     }
     return Row(
       children: [
         dot,
         const SizedBox(width: 10),
         Expanded(
-          child: Text(name, style: Theme.of(context).textTheme.bodyMedium),
+          child: Row(
+            children: [
+              Text(name, style: Theme.of(context).textTheme.bodyMedium),
+              if (polled.freshness == PolledFreshness.stale) ...[
+                const SizedBox(width: 6),
+                PolledStaleIndicator(polled: polled),
+              ],
+            ],
+          ),
         ),
         const SizedBox(width: 8),
         Text(
@@ -139,15 +132,20 @@ class _ScannersStatusCardState extends State<ScannersStatusCard> {
   }
 
   (Color, String) _resolveArp() {
-    if (_arpError != null || _arpStatus == null) {
+    if (_arp.freshness == PolledFreshness.error) {
       return (Colors.red, 'Error');
     }
-    if (_arpStatus!.isRunning) {
+    final status = _arp.value!;
-      final secs = (_arpStatus!.runningForSeconds ?? 0) + _elapsed;
+    final elapsed = DateTime.now()
+        .difference(_arp.lastSuccessAt!)
+        .inSeconds
+        .toDouble();
+    if (status.isRunning) {
+      final secs = (status.runningForSeconds ?? 0) + elapsed;
       return (Colors.green, 'Running for ${formatSeconds(secs)}');
     }
-    if (_arpStatus!.nextRunInSeconds != null) {
+    if (status.nextRunInSeconds != null) {
-      final remaining = (_arpStatus!.nextRunInSeconds! - _elapsed).clamp(
+      final remaining = (status.nextRunInSeconds! - elapsed).clamp(
         0.0,
         double.infinity,
       );
@@ -157,12 +155,17 @@ class _ScannersStatusCardState extends State<ScannersStatusCard> {
   }
 
   (Color, String) _resolveMdns() {
-    if (_mdnsError != null || _mdnsStatus == null) {
+    if (_mdns.freshness == PolledFreshness.error) {
       return (Colors.red, 'Error');
     }
-    if (_mdnsStatus!.isListening) {
+    final status = _mdns.value!;
-      if (_mdnsStatus!.lastDeviceSeenSecondsAgo != null) {
+    final elapsed = DateTime.now()
-        final secs = _mdnsStatus!.lastDeviceSeenSecondsAgo! + _elapsed;
+        .difference(_mdns.lastSuccessAt!)
+        .inSeconds
+        .toDouble();
+    if (status.isListening) {
+      if (status.lastDeviceSeenSecondsAgo != null) {
+        final secs = status.lastDeviceSeenSecondsAgo! + elapsed;
         return (Colors.green, 'Last device seen ${formatSeconds(secs)} ago');
       }
       return (Colors.green, 'No devices seen yet');