channel: Add ChannelStore.compareChannelFolders

It'll be useful to have this encapsulated for when we implement the
UI (#1765).

Author: chrisbobbe

lib/model/channel.dart

@@ -63,6 +63,21 @@ mixin ChannelStore on UserStore {
   // ignore: valid_regexps
   static final _startsWithEmojiRegex = RegExp(r'^\p{Emoji}', unicode: true);
 
+  /// A compare function for [ChannelFolder]s, using [ChannelFolder.order].
+  ///
+  /// Channels without [ChannelFolder.order] will come first,
+  /// sorted alphabetically.
+  // TODO(server-11) Once [ChannelFolder.order] is required,
+  //   remove alphabetical sorting.
+  static int compareChannelFolders(ChannelFolder a, ChannelFolder b) {
+    return switch ((a.order, b.order)) {
+      (null,   null) => a.name.toLowerCase().compareTo(b.name.toLowerCase()),
+      (null,  int()) => -1,
+      (int(),  null) => 1,
+      (int a, int b) => a.compareTo(b),
+    };
+  }
+
   /// The visibility policy that the self-user has for the given topic.
   ///
   /// This does not incorporate the user's channel-level policy,

test/model/channel_test.dart

@@ -1,4 +1,5 @@
 import 'package:checks/checks.dart';
+import 'package:collection/collection.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/events.dart';
 import 'package:zulip/api/model/initial_snapshot.dart';
@@ -91,6 +92,21 @@ void main() {
     });
   });
 
+  group('channelFolderComparator', () {
+    final folder1 = eg.channelFolder(id: 1, order: null, name: 'M');
+    final folder2 = eg.channelFolder(id: 2, order: null, name: 'n');
+    final folder3 = eg.channelFolder(id: 3, order: 2, name: 'a');
+    final folder4 = eg.channelFolder(id: 4, order: 0, name: 'b');
+    final folder5 = eg.channelFolder(id: 5, order: 1, name: 'c');
+
+    final store = eg.store(initialSnapshot: eg.initialSnapshot(
+      channelFolders: [folder1, folder2, folder3, folder4, folder5]));
+
+    final sorted = store.channelFolders.values.toList()
+      .sorted(ChannelStore.compareChannelFolders);
+    check(sorted).deepEquals([folder1, folder2, folder4, folder5, folder3]);
+  });
+
   group('SubscriptionEvent', () {
     final stream = eg.stream();