unreads [nfc]: Factor out isUnread helper

And make it public, with dartdoc and tests, while we're at it.

Author: chrisbobbe

lib/model/unreads.dart

@@ -214,6 +214,18 @@ class Unreads extends ChangeNotifier {
     }
   }
 
+  /// The unread state for [messageId], or null if unknown.
+  ///
+  /// May be unknown only if [oldUnreadsMissing].
+  ///
+  /// This is inefficient; it iterates through [dms] and [channels].
+  // TODO implement efficiently
+  bool? isUnread(int messageId) {
+    final isPresent = _slowIsPresentInDms(messageId) || _slowIsPresentInStreams(messageId);
+    if (oldUnreadsMissing && !isPresent) return null;
+    return isPresent;
+  }
+
   void handleMessageEvent(MessageEvent event) {
     final message = event.message;
     if (message.flags.contains(MessageFlag.read)) {
@@ -327,10 +339,7 @@ class Unreads extends ChangeNotifier {
         switch (event) {
           case UpdateMessageFlagsAddEvent():
             mentions.addAll(
-              event.messages.where(
-                (messageId) => _slowIsPresentInStreams(messageId) || _slowIsPresentInDms(messageId),
-              ),
-            );
+              event.messages.where((messageId) => isUnread(messageId) == true));
 
           case UpdateMessageFlagsRemoveEvent():
             mentions.removeAll(event.messages);

test/model/unreads_test.dart

@@ -240,6 +240,49 @@ void main() {
     });
   });
 
+  group('isUnread', () {
+    final unreadDmMessage = eg.dmMessage(
+      from: eg.otherUser, to: [eg.selfUser], flags: []);
+    final readDmMessage = eg.dmMessage(
+      from: eg.otherUser, to: [eg.selfUser], flags: [MessageFlag.read]);
+    final unreadChannelMessage = eg.streamMessage(flags: []);
+    final readChannelMessage = eg.streamMessage(flags: [MessageFlag.read]);
+
+    final allMessages = [
+      unreadDmMessage, unreadChannelMessage,
+      readDmMessage,   readChannelMessage,
+    ];
+
+    void doTestCommon(String description, int messageId, bool expected) {
+      test(description, () {
+        prepare();
+        model.oldUnreadsMissing = false;
+        fillWithMessages(allMessages);
+        check(model.isUnread(messageId)).equals(expected);
+      });
+    }
+
+    void doTestOldUnreadsMissing(String description, int messageId, bool? expected) {
+      assert(expected == true || expected == null);
+      test('oldUnreadsMissing; $description', () {
+        prepare();
+        model.oldUnreadsMissing = true;
+        fillWithMessages(allMessages);
+        check(model.isUnread(messageId)).equals(expected);
+      });
+    }
+
+    doTestCommon('unread DM message',      unreadDmMessage.id,      true);
+    doTestCommon('read DM message',        readDmMessage.id,        false);
+    doTestCommon('unread channel message', unreadChannelMessage.id, true);
+    doTestCommon('read channel message',   readChannelMessage.id,   false);
+
+    doTestOldUnreadsMissing('unread DM message',      unreadDmMessage.id,      true);
+    doTestOldUnreadsMissing('read DM message',        readDmMessage.id,        null);
+    doTestOldUnreadsMissing('unread channel message', unreadChannelMessage.id, true);
+    doTestOldUnreadsMissing('read channel message',   readChannelMessage.id,   null);
+  });
+
   group('handleMessageEvent', () {
     for (final (isUnread, isStream, isDirectMentioned, isWildcardMentioned) in [
       (true,  true,  true,  true ),