model: Cache stream-color swatch on Subscription object

As mentioned in a TODO, I'm not sure it makes sense to deal with
package:flutter/painting.dart from within our lib/api/model/ code.
We might end up factoring that out somewhere else.

Next, we'll have our UnreadCountBadge widget optionally consume one
of these swatches.

Author: chrisbobbe

lib/api/model/model.dart

@@ -1,3 +1,5 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/painting.dart';
 import 'package:json_annotation/json_annotation.dart';
 
 import 'reaction.dart';
@@ -321,14 +323,25 @@ class Subscription {
   /// As an int that dart:ui's Color constructor will take:
   ///   <https://api.flutter.dev/flutter/dart-ui/Color/Color.html>
   @JsonKey(readValue: _readColor)
-  final int color;
-
+  final int color; // TODO(#135) Clear out _swatch cache on changes to this
   static Object? _readColor(Map 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);
   }
 
+  @JsonKey(includeToJson: false)
+  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(color);
+
+  @visibleForTesting
+  @JsonKey(includeToJson: false)
+  StreamColorSwatch? get debugCachedSwatchValue => _swatch;
+
   Subscription({
     required this.streamId,
     required this.name,
@@ -359,6 +372,29 @@ class Subscription {
   Map<String, dynamic> toJson() => _$SubscriptionToJson(this);
 }
 
+/// A [ColorSwatch] with colors related to a base stream color.
+///
+/// Use this in UI code for colors related to [Subscription.color],
+/// such as the background of an unread count badge.
+class StreamColorSwatch extends ColorSwatch<_StreamColorVariant> {
+  StreamColorSwatch(int base) : super(base, _compute(base));
+
+  Color get base => this[_StreamColorVariant.base]!;
+
+  static Map<_StreamColorVariant, Color> _compute(int base) {
+    final baseAsColor = Color(base);
+
+    return {
+      _StreamColorVariant.base: baseAsColor,
+    };
+  }
+}
+
+enum _StreamColorVariant {
+  base,
+  // TODO more, like the unread-count badge background color
+}
+
 /// As in the get-messages response.
 ///
 /// https://zulip.com/api/get-messages#response

test/api/model/model_checks.dart

@@ -1,6 +1,12 @@
+import 'dart:ui';
+
 import 'package:checks/checks.dart';
 import 'package:zulip/api/model/model.dart';
 
+extension StreamColorSwatchChecks on Subject<StreamColorSwatch> {
+  Subject<Color> get base => has((s) => s.base, 'base');
+}
+
 extension MessageChecks on Subject<Message> {
   Subject<String> get content => has((e) => e.content, 'content');
   Subject<bool> get isMeMessage => has((e) => e.isMeMessage, 'isMeMessage');

test/api/model/model_test.dart

@@ -1,4 +1,5 @@
 import 'dart:convert';
+import 'dart:ui';
 
 import 'package:checks/checks.dart';
 import 'package:test/scaffolding.dart';
@@ -77,6 +78,19 @@ void main() {
       check(subWithColor('#ffffff').color).equals(0xffffffff);
       check(subWithColor('#000000').color).equals(0xff000000);
     });
+
+    test('colorSwatch caching', () {
+      final sub = eg.subscription(stream: eg.stream(), color: 0xffffffff);
+      check(sub.debugCachedSwatchValue).isNull();
+      sub.colorSwatch();
+      check(sub.debugCachedSwatchValue).isNotNull().base.equals(const Color(0xffffffff));
+    });
+
+    group('StreamColorSwatch', () {
+      test('base', () {
+        check(StreamColorSwatch(0xffffffff)).base.equals(const Color(0xffffffff));
+      });
+    });
   });
 
   group('Message', () {