user [nfc]: Introduce senderDisplayName

Author: gnprice

lib/model/user.dart

@@ -43,10 +43,26 @@ mixin UserStore {
   ///
   /// This is the user's [User.fullName] if the user is known,
   /// and otherwise a translation of "(unknown user)".
+  ///
+  /// When a [Message] is available which the user sent,
+  /// use [senderDisplayName] instead for a better-informed fallback.
   String userDisplayName(int userId) {
     return users[userId]?.fullName
       ?? GlobalLocalizations.zulipLocalizations.unknownUserName;
   }
+
+  /// The name to show for the given message's sender in the UI.
+  ///
+  /// If the user is known (see [users]), this is their current [User.fullName].
+  /// If unknown, this uses the fallback value conveniently provided on the
+  /// [Message] object itself, namely [Message.senderFullName].
+  ///
+  /// For a user who isn't the sender of some known message,
+  /// see [userDisplayName].
+  String senderDisplayName(Message message) {
+    return users[message.senderId]?.fullName
+      ?? message.senderFullName;
+  }
 }
 
 /// The implementation of [UserStore] that does the work.

test/model/user_test.dart

@@ -22,6 +22,36 @@ void main() {
     });
   });
 
+  group('senderDisplayName', () {
+    test('on a known user', () async {
+      final store = eg.store();
+      final user = eg.user(fullName: 'Old Name');
+      await store.addUser(user);
+      final message = eg.streamMessage(sender: user);
+      await store.addMessage(message);
+      check(store.senderDisplayName(message)).equals('Old Name');
+
+      // If the user's name changes, `store.senderDisplayName` should update...
+      await store.handleEvent(RealmUserUpdateEvent(id: 1,
+        userId: user.userId, fullName: 'New Name'));
+      check(store.senderDisplayName(message)).equals('New Name');
+      // ... even though the Message object itself still has the old name.
+      check(store.messages[message.id]!).senderFullName.equals('Old Name');
+    });
+
+    test('on an unknown user', () async {
+      final store = eg.store();
+      final message = eg.streamMessage(sender: eg.user(fullName: 'Some User'));
+      await store.addMessage(message);
+      // If the user is unknown, `store.senderDisplayName` should fall back
+      // to the name in the message...
+      check(store.senderDisplayName(message)).equals('Some User');
+      // ... even though `store.userDisplayName` (with no message available
+      // for fallback) only has a generic fallback name.
+      check(store.userDisplayName(message.senderId)).equals('(unknown user)');
+    });
+  });
+
   group('RealmUserUpdateEvent', () {
     // TODO write more tests for handling RealmUserUpdateEvent