ui: Provide [StreamColorSwatch]es via DesignVariables, not api/model/

Instead of putting them in Subscription model objects.

This helps toward zulip#95 by bundling this with our other
ThemeExtensions that will switch together between light and dark
variants when the theme changes. It's also just nicer to separate
this very UI-focused code out of api/model/ (zulip#393).

Not marking [nfc] just because of differences in caching logic,
although I don't expect a user-visible perf effect.

Related: zulip#95
Fixes: zulip#393

lib/api/model/model.dart

@@ -1,7 +1,5 @@
-import 'package:flutter/foundation.dart';
 import 'package:json_annotation/json_annotation.dart';
 
-import '../../widgets/stream_colors.dart';
 import 'events.dart';
 import 'initial_snapshot.dart';
 import 'reaction.dart';
@@ -403,29 +401,13 @@ class Subscription extends ZulipStream {
   /// As an int that dart:ui's Color constructor will take:
   ///   <https://api.flutter.dev/flutter/dart-ui/Color/Color.html>
   @JsonKey(readValue: _readColor)
-  int get color => _color;
-  int _color;
-  set color(int value) {
-    _color = value;
-    _swatch = null;
-  }
+  int color;
   static Object? _readColor(Map<dynamic, dynamic> json, String key) {
     final str = (json[key] as String);
     assert(RegExp(r'^#[0-9a-f]{6}$').hasMatch(str));
     return 0xff000000 | int.parse(str.substring(1), radix: 16);
   }
 
-  StreamColorSwatch? _swatch;
-  /// A [StreamColorSwatch] for the subscription, memoized.
-  // TODO I'm not sure this is the right home for this; it seems like we might
-  //   instead have chosen to put it in more UI-centered code, like in a custom
-  //   material [ColorScheme] class or something. But it works for now.
-  StreamColorSwatch colorSwatch() => _swatch ??= StreamColorSwatch.light(color);
-
-  @visibleForTesting
-  @JsonKey(includeToJson: false)
-  StreamColorSwatch? get debugCachedSwatchValue => _swatch;
-
   Subscription({
     required super.streamId,
     required super.name,
@@ -447,8 +429,8 @@ class Subscription extends ZulipStream {
     required this.audibleNotifications,
     required this.pinToTop,
     required this.isMuted,
-    required int color,
-  }) : _color = color;
+    required this.color,
+  });
 
   factory Subscription.fromJson(Map<String, dynamic> json) =>
     _$SubscriptionFromJson(json);

lib/widgets/inbox.dart

@@ -10,6 +10,7 @@ import 'page.dart';
 import 'sticky_header.dart';
 import 'store.dart';
 import 'text.dart';
+import 'theme.dart';
 import 'unread_count_badge.dart';
 
 class InboxPage extends StatefulWidget {
@@ -423,12 +424,14 @@ class _StreamHeaderItem extends _HeaderItem {
 
   @override String get title => subscription.name;
   @override IconData get icon => iconDataForStream(subscription);
-  @override Color collapsedIconColor(context) => subscription.colorSwatch().iconOnPlainBackground;
-  @override Color uncollapsedIconColor(context) => subscription.colorSwatch().iconOnBarBackground;
+  @override Color collapsedIconColor(context) =>
+    colorSwatchFor(context, subscription).iconOnPlainBackground;
+  @override Color uncollapsedIconColor(context) =>
+    colorSwatchFor(context, subscription).iconOnBarBackground;
   @override Color uncollapsedBackgroundColor(context) =>
-    subscription.colorSwatch().barBackground;
+    colorSwatchFor(context, subscription).barBackground;
   @override Color? unreadCountBadgeBackgroundColor(context) =>
-    subscription.colorSwatch().unreadCountBadgeBackground;
+    colorSwatchFor(context, subscription).unreadCountBadgeBackground;
 
   @override Future<void> onCollapseButtonTap() async {
     await super.onCollapseButtonTap();
@@ -525,7 +528,8 @@ class _TopicItem extends StatelessWidget {
             const SizedBox(width: 12),
             if (hasMention) const _AtMentionMarker(),
             Padding(padding: const EdgeInsetsDirectional.only(end: 16),
-              child: UnreadCountBadge(backgroundColor: subscription.colorSwatch(),
+              child: UnreadCountBadge(
+                backgroundColor: colorSwatchFor(context, subscription),
                 count: count)),
           ]))));
   }

lib/widgets/message_list.dart

@@ -22,6 +22,7 @@ import 'profile.dart';
 import 'sticky_header.dart';
 import 'store.dart';
 import 'text.dart';
+import 'theme.dart';
 
 class MessageListPage extends StatefulWidget {
   const MessageListPage({super.key, required this.narrow});
@@ -67,8 +68,9 @@ class _MessageListPageState extends State<MessageListPage> {
       case StreamNarrow(:final streamId):
       case TopicNarrow(:final streamId):
         final subscription = store.subscriptions[streamId];
-        appBarBackgroundColor = subscription?.colorSwatch().barBackground
-          ?? _kUnsubscribedStreamRecipientHeaderColor;
+        appBarBackgroundColor = subscription != null
+          ? colorSwatchFor(context, subscription).barBackground
+          : _kUnsubscribedStreamRecipientHeaderColor;
         // All recipient headers will match this color; remove distracting line
         // (but are recipient headers even needed for topic narrows?)
         removeAppBarBottomBorder = true;
@@ -664,7 +666,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     final Color backgroundColor;
     final Color iconColor;
     if (subscription != null) {
-      final swatch = subscription.colorSwatch();
+      final swatch = colorSwatchFor(context, subscription);
       backgroundColor = swatch.barBackground;
       iconColor = swatch.iconOnBarBackground;
     } else {

lib/widgets/stream_colors.dart

@@ -6,6 +6,74 @@ import 'package:flutter_color_models/flutter_color_models.dart';
 import '../api/model/model.dart';
 import 'color.dart';
 
+/// A lazily-computed map from a stream's base color to a
+/// corresponding [StreamColorSwatch].
+abstract class StreamColorSwatches {
+  /// The [StreamColorSwatches] for the light theme.
+  static final StreamColorSwatches light = _StreamColorSwatchesLight();
+
+  /// The [StreamColorSwatches] for the dark theme.
+  static final StreamColorSwatches dark = _StreamColorSwatchesDark();
+
+  final Map<int, StreamColorSwatch> _cache = {};
+
+  /// Gives the [StreamColorSwatch] for a [subscription.color].
+  StreamColorSwatch forBaseColor(int base) =>
+    _cache[base] ??= _computeForBaseColor(base);
+
+  StreamColorSwatch _computeForBaseColor(int base);
+
+  /// Gives a [StreamColorSwatches], lerped to [other] at [t].
+  ///
+  /// If [this] and [other] are [identical], returns [this].
+  ///
+  /// Else returns an instance whose [forBaseColor] will call
+  /// [this.forBaseColor] and [other.forBaseColor]
+  /// and return [StreamColorSwatch.lerp]'s result on those.
+  /// This computation is cached on the instance
+  /// in order to save work building [t]'s animation frame when there are
+  /// multiple UI elements using the same [subscription.color].
+  StreamColorSwatches lerp(StreamColorSwatches? other, double t) {
+    // This short-circuit helps when [a] and [b]
+    // are both [StreamColorSwatches.light]
+    // or both [StreamColorSwatches.dark].
+    // Empirically, [lerp] is called even when the theme hasn't changed,
+    // so this is an important optimization.
+    if (identical(this, other)) {
+      return this;
+    }
+
+    return _StreamColorSwatchesLerped(this, other, t);
+  }
+}
+
+class _StreamColorSwatchesLight extends StreamColorSwatches {
+  _StreamColorSwatchesLight();
+
+  @override
+  StreamColorSwatch _computeForBaseColor(int base) => StreamColorSwatch.light(base);
+}
+
+class _StreamColorSwatchesDark extends StreamColorSwatches {
+  _StreamColorSwatchesDark();
+
+  @override
+  StreamColorSwatch _computeForBaseColor(int base) => StreamColorSwatch.dark(base);
+}
+
+class _StreamColorSwatchesLerped extends StreamColorSwatches {
+  _StreamColorSwatchesLerped(this.a, this.b, this.t);
+
+  final StreamColorSwatches a;
+  final StreamColorSwatches? b;
+  final double t;
+
+  @override
+  StreamColorSwatch _computeForBaseColor(int base) =>
+    StreamColorSwatch.lerp(a.forBaseColor(base), b?.forBaseColor(base), t)!;
+}
+
+
 /// A [ColorSwatch] with colors related to a base stream color.
 ///
 /// Use this in UI code for colors related to [Subscription.color],

lib/widgets/subscription_list.dart

@@ -9,6 +9,7 @@ import 'message_list.dart';
 import 'page.dart';
 import 'store.dart';
 import 'text.dart';
+import 'theme.dart';
 import 'unread_count_badge.dart';
 
 /// Scrollable listing of subscribed streams.
@@ -199,7 +200,7 @@ class SubscriptionItem extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final swatch = subscription.colorSwatch();
+    final swatch = colorSwatchFor(context, subscription);
     final hasUnreads = (unreadCount > 0);
     return Material(
       // TODO(#95) need dark-theme color

lib/widgets/theme.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 
+import '../api/model/model.dart';
 import 'content.dart';
+import 'stream_colors.dart';
 import 'text.dart';
 
 ThemeData zulipThemeData(BuildContext context) {
@@ -78,14 +80,16 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
     bgTopBar = const Color(0xfff5f5f5),
     borderBar = const Color(0x33000000),
     icon = const Color(0xff666699),
-    title = const Color(0xff1a1a1a);
+    title = const Color(0xff1a1a1a),
+    streamColorSwatches = StreamColorSwatches.light;
 
   DesignVariables._({
     required this.bgMain,
     required this.bgTopBar,
     required this.borderBar,
     required this.icon,
     required this.title,
+    required this.streamColorSwatches,
   });
 
   /// The [DesignVariables] from the context's active theme.
@@ -104,20 +108,25 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
   final Color icon;
   final Color title;
 
+  // Not exactly from the Figma design, but from Vlad anyway.
+  final StreamColorSwatches streamColorSwatches;
+
   @override
   DesignVariables copyWith({
     Color? bgMain,
     Color? bgTopBar,
     Color? borderBar,
     Color? icon,
     Color? title,
+    StreamColorSwatches? streamColorSwatches,
   }) {
     return DesignVariables._(
       bgMain: bgMain ?? this.bgMain,
       bgTopBar: bgTopBar ?? this.bgTopBar,
       borderBar: borderBar ?? this.borderBar,
       icon: icon ?? this.icon,
       title: title ?? this.title,
+      streamColorSwatches: streamColorSwatches ?? this.streamColorSwatches,
     );
   }
 
@@ -132,6 +141,15 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       borderBar: Color.lerp(borderBar, other?.borderBar, t)!,
       icon: Color.lerp(icon, other?.icon, t)!,
       title: Color.lerp(title, other?.title, t)!,
+      streamColorSwatches: streamColorSwatches.lerp(other?.streamColorSwatches, t),
     );
   }
 }
+
+/// The theme-appropriate [StreamColorSwatch] based on [subscription.color].
+///
+/// For how this value is cached, see [StreamColorSwatches.forBaseColor].
+StreamColorSwatch colorSwatchFor(BuildContext context, Subscription subscription) {
+  return DesignVariables.of(context)
+    .streamColorSwatches.forBaseColor(subscription.color);
+}

test/api/model/model_test.dart

@@ -1,13 +1,11 @@
 import 'dart:convert';
-import 'dart:ui';
 
 import 'package:checks/checks.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/model.dart';
 
 import '../../example_data.dart' as eg;
 import '../../stdlib_checks.dart';
-import '../../widgets/stream_colors_checks.dart';
 import 'model_checks.dart';
 
 void main() {
@@ -117,17 +115,6 @@ void main() {
       check(subWithColor('#ffffff').color).equals(0xffffffff);
       check(subWithColor('#000000').color).equals(0xff000000);
     });
-
-    test('colorSwatch caching', () {
-      final sub = eg.subscription(eg.stream(), color: 0xffffffff);
-      check(sub.debugCachedSwatchValue).isNull();
-      sub.colorSwatch();
-      check(sub.debugCachedSwatchValue).isNotNull().base.equals(const Color(0xffffffff));
-      sub.color = 0xffff0000;
-      check(sub.debugCachedSwatchValue).isNull();
-      sub.colorSwatch();
-      check(sub.debugCachedSwatchValue).isNotNull().base.equals(const Color(0xffff0000));
-    });
   });
 
   group('Message', () {

test/widgets/inbox_test.dart

@@ -423,9 +423,10 @@ void main() {
           final collapseIcon = findHeaderCollapseIcon(tester, headerRow!);
           check(collapseIcon).icon.equals(ZulipIcons.arrow_down);
           final streamIcon = findStreamHeaderIcon(tester, streamId);
-          check(streamIcon).color.equals(subscription.colorSwatch().iconOnBarBackground);
+          check(streamIcon).color.equals(
+            StreamColorSwatch.light(subscription.color).iconOnBarBackground);
           check(streamHeaderBackgroundColor(tester, streamId))
-            .isNotNull().equals(subscription.colorSwatch().barBackground);
+            .isNotNull().equals(StreamColorSwatch.light(subscription.color).barBackground);
           check(tester.widgetList(findSectionContent)).isNotEmpty();
         }
 
@@ -445,7 +446,8 @@ void main() {
           final collapseIcon = findHeaderCollapseIcon(tester, headerRow!);
           check(collapseIcon).icon.equals(ZulipIcons.arrow_right);
           final streamIcon = findStreamHeaderIcon(tester, streamId);
-          check(streamIcon).color.equals(subscription.colorSwatch().iconOnPlainBackground);
+          check(streamIcon).color.equals(
+            StreamColorSwatch.light(subscription.color).iconOnPlainBackground);
           check(streamHeaderBackgroundColor(tester, streamId))
             .isNotNull().equals(Colors.white);
           check(tester.widgetList(findSectionContent)).isEmpty();

test/widgets/message_list_test.dart

@@ -19,6 +19,7 @@ import 'package:zulip/widgets/content.dart';
 import 'package:zulip/widgets/icons.dart';
 import 'package:zulip/widgets/message_list.dart';
 import 'package:zulip/widgets/store.dart';
+import 'package:zulip/widgets/stream_colors.dart';
 import 'package:zulip/widgets/theme.dart';
 
 import '../api/fake_api.dart';
@@ -293,7 +294,7 @@ void main() {
 
       testWidgets('color of recipient header background', (tester) async {
         final subscription = eg.subscription(stream, color: Colors.red.value);
-        final swatch = subscription.colorSwatch();
+        final swatch = StreamColorSwatch.light(subscription.color);
         await setupMessageListPage(tester,
           messages: [eg.streamMessage(stream: subscription)],
           subscriptions: [subscription]);
@@ -308,7 +309,7 @@ void main() {
       testWidgets('color of stream icon', (tester) async {
         final stream = eg.stream(isWebPublic: true);
         final subscription = eg.subscription(stream, color: Colors.red.value);
-        final swatch = subscription.colorSwatch();
+        final swatch = StreamColorSwatch.light(subscription.color);
         await setupMessageListPage(tester,
           messages: [eg.streamMessage(stream: subscription)],
           subscriptions: [subscription]);