Dark/bright: Auto (Android's dark theme), Dark or Bright, kept across restarts

Settings has a segmented Auto / Dark / Bright. Auto follows platformBrightness, so
Android's schedule too. The Dash's quick button holds the other variant; a long press
is auto again. The mode is kept with shared_preferences.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/README.md

@@ -16,7 +16,7 @@ pirate-station-aide's (main views, then the rarely used ones grouped as Tools).
 | `pages/` | One file per view: `dash`, `signals` (values and raw traffic), `checklist` (Captures), `camera`, `link_test`, `settings` |
 | `actions/` | Things done from anywhere, as dialogs and sheets: `marks` (free-form mark), `orin_power` (power off / reboot), `server_url` (gateway address) |
 | `link/` | Talking to the Orin: `boundary` (the protocol client), `net` (binding to the USB link), `reading` (one signal's state as a display needs it) |
-| `theme/` | `palette.dart`: the palette roles and `hudFont` |
+| `theme/` | `palette.dart`: the palette roles, `hudFont`, and `Themes` (which variant shows: auto or held, kept across restarts) |
 | `widgets/` | Reusable display widgets, one per file, each fed values, never the boundary (`link_status` takes it whole: it shows the link) |
 
 ## Widgets
@@ -40,6 +40,9 @@ pirate-station-aide's (main views, then the rarely used ones grouped as Tools).
   and the state gradient `good` → `caution` → `alarm`; plus the derived `surface` (raised bars) and
   `onFill(colour)` (text on a fill). No widget names a colour. Themes are JSON in `assets/themes/`, `rei` by
   default; smart-serow's four-key files still load (their `highlight` is `alarm`).
+- **Dark or bright.** `Themes.mode` is `auto` (Android's dark theme, schedule included), `dark` or
+  `bright`, chosen in Settings and kept across restarts. The Dash's quick button holds the other variant;
+  a long press there is auto again.
 - **Cosmetic fonts are opt-in.** A widget that wants the HUD face names `fontFamily: hudFont` in its own
   `TextStyle`; everything else keeps the basic font, which covers CJK where DIN 1451 does not.
 - **Missing is a state** (app guideline 2): a widget given no value draws "—" or subdued, never an error.

lib/pages/dash.dart

@@ -164,7 +164,7 @@ class DashPage extends StatelessWidget {
   }
 }
 
-/// The dash's own thin top line: the menu, link state, and the dark/bright switch.
+/// The dash's own thin top line: the menu, link state, and the dark/bright quick switch.
 class _DashBar extends StatelessWidget {
   const _DashBar({required this.boundary, required this.themes});
   final Boundary boundary;
@@ -185,12 +185,18 @@ class _DashBar extends StatelessWidget {
           MenuButton(color: p.subdued),
           Text(text, style: TextStyle(color: color, letterSpacing: 2)),
           const Spacer(),
-          IconButton(
-            tooltip: themes.isDark ? 'Bright' : 'Dark',
-            onPressed: themes.toggle,
-            icon: Icon(
-              themes.isDark ? Icons.light_mode : Icons.dark_mode,
-              color: p.subdued,
+          // Tap: hold the other variant. Long press: back to auto (Settings has both).
+          GestureDetector(
+            onLongPress: () => themes.mode = VariantMode.auto,
+            child: IconButton(
+              tooltip: themes.isDark ? 'Bright' : 'Dark',
+              onPressed: themes.toggle,
+              icon: Icon(
+                themes.mode == VariantMode.auto
+                    ? Icons.brightness_auto
+                    : (themes.isDark ? Icons.light_mode : Icons.dark_mode),
+                color: p.subdued,
+              ),
             ),
           ),
         ],

lib/pages/settings.dart

@@ -52,14 +52,38 @@ class SettingsPage extends StatelessWidget {
               onTap: () => editUrl(context, boundary, usb),
             ),
             section('Display'),
-            SwitchListTile(
-              secondary: Icon(
-                themes.isDark ? Icons.dark_mode : Icons.light_mode,
+            ListTile(
+              leading: Icon(themes.isDark ? Icons.dark_mode : Icons.light_mode),
+              title: const Text('Dark or bright'),
+              subtitle: const Text(
+                "Auto follows Android's dark theme and its schedule. "
+                "The Dash's button holds one; a long press there is auto again.",
+              ),
+            ),
+            Padding(
+              padding: const EdgeInsets.fromLTRB(72, 0, 16, 8),
+              child: SegmentedButton<VariantMode>(
+                segments: const [
+                  ButtonSegment(
+                    value: VariantMode.auto,
+                    icon: Icon(Icons.brightness_auto),
+                    label: Text('Auto'),
+                  ),
+                  ButtonSegment(
+                    value: VariantMode.dark,
+                    icon: Icon(Icons.dark_mode),
+                    label: Text('Dark'),
+                  ),
+                  ButtonSegment(
+                    value: VariantMode.bright,
+                    icon: Icon(Icons.light_mode),
+                    label: Text('Bright'),
+                  ),
+                ],
+                selected: {themes.mode},
+                showSelectedIcon: false,
+                onSelectionChanged: (s) => themes.mode = s.single,
               ),
-              title: const Text('Dark variant'),
-              subtitle: const Text('Bright for daylight, dark for night'),
-              value: themes.isDark,
-              onChanged: (v) => themes.isDark = v,
             ),
             ListTile(
               leading: const Icon(Icons.palette),

lib/theme/palette.dart

@@ -20,6 +20,7 @@ import 'dart:math' show max, min;
 
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
+import 'package:shared_preferences/shared_preferences.dart';
 
 /// The HUD face (smart-serow's DIN 1451). Opt-in: a widget names it in its own
 /// TextStyle; everything else keeps the basic font, which covers CJK where DIN does not.
@@ -116,29 +117,70 @@ class Palette {
   );
 }
 
-/// A theme: both variants, and which one is showing. Listen to it to follow a switch.
-class Themes extends ChangeNotifier {
-  Themes(this.name, this.dark, this.bright);
+/// Which variant shows: Android's own dark theme (auto), or one held fixed.
+enum VariantMode { auto, dark, bright }
+
+/// A theme: both variants, and which one is showing. Listen to it to follow a switch,
+/// and, in auto, Android's dark theme turning on or off (by hand or on its schedule).
+/// The mode is kept across restarts.
+class Themes extends ChangeNotifier with WidgetsBindingObserver {
+  Themes(this.name, this.dark, this.bright, [this._prefs]) {
+    _mode =
+        VariantMode.values.asNameMap()[_prefs?.getString(_key)] ??
+        VariantMode.auto;
+    WidgetsBinding.instance.addObserver(this);
+  }
+
+  static const _key = 'variant_mode';
 
   final String name;
   final Palette dark;
   final Palette bright;
-  bool _isDark = true;
+  final SharedPreferences? _prefs;
+  late VariantMode _mode;
 
-  bool get isDark => _isDark;
-  Palette get current => _isDark ? dark : bright;
+  VariantMode get mode => _mode;
 
-  set isDark(bool v) {
-    if (v == _isDark) return;
-    _isDark = v;
+  set mode(VariantMode v) {
+    if (v == _mode) return;
+    _mode = v;
+    _prefs?.setString(_key, v.name);
     notifyListeners();
   }
 
-  void toggle() => isDark = !_isDark;
+  bool get isDark => switch (_mode) {
+    VariantMode.auto =>
+      WidgetsBinding.instance.platformDispatcher.platformBrightness ==
+          Brightness.dark,
+    VariantMode.dark => true,
+    VariantMode.bright => false,
+  };
+
+  Palette get current => isDark ? dark : bright;
+
+  /// The quick switch: hold the other variant fixed.
+  void toggle() => mode = isDark ? VariantMode.bright : VariantMode.dark;
+
+  @override
+  void didChangePlatformBrightness() {
+    if (_mode == VariantMode.auto) notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
 
   /// [themeName], falling back to `default`, then to the hard-coded palette
   /// (smart-serow's chain).
   static Future<Themes> load([String name = themeName]) async {
+    SharedPreferences? prefs;
+    try {
+      prefs = await SharedPreferences.getInstance();
+    } catch (_) {
+      // No storage: the mode starts at auto each time.
+    }
     for (final n in {name, 'default'}) {
       try {
         final j = jsonDecode(
@@ -148,19 +190,25 @@ class Themes extends ChangeNotifier {
           n,
           Palette._fromJson(j['dark'] as Map<String, dynamic>),
           Palette._fromJson(j['bright'] as Map<String, dynamic>),
+          prefs,
         );
       } catch (_) {
         // Missing or malformed: try the next one.
       }
     }
-    return Themes('fallback', Palette.fallbackDark, Palette.fallbackBright);
+    return Themes(
+      'fallback',
+      Palette.fallbackDark,
+      Palette.fallbackBright,
+      prefs,
+    );
   }
 
   /// Material's theme for the rest of the app, from the same roles: buttons and
   /// selection in accent, errors in alarm, raised surfaces in surface.
   ThemeData material() {
     final p = current;
-    final brightness = _isDark ? Brightness.dark : Brightness.light;
+    final brightness = isDark ? Brightness.dark : Brightness.light;
     return ThemeData(
       useMaterial3: true,
       brightness: brightness,