api: Add DmMessage.allRecipientIds, to avoid displayRecipients

Author: gnprice

lib/api/model/model.dart

@@ -393,9 +393,29 @@ class DmMessage extends Message {
   String get type => 'private';
 
   /// The `display_recipient` from the server, sorted by user ID numerically.
+  ///
+  /// This lists the sender as well as all (other) recipients, and it
+  /// lists each user just once.  In particular the self-user is always
+  /// included.
+  ///
+  /// Note the data here is not updated on changes to the users, so everything
+  /// other than the user IDs may be stale.
+  /// Consider using [allRecipientIds] instead, and getting user details
+  /// from the store.
+  // TODO(server): Document that it's all users.  That statement is based on
+  //   reverse-engineering notes in zulip-mobile:src/api/modelTypes.js at PmMessage.
   @DmRecipientListConverter()
   final List<DmRecipient> displayRecipient;
 
+  /// The user IDs of all users in the thread, sorted numerically.
+  ///
+  /// This lists the sender as well as all (other) recipients, and it
+  /// lists each user just once.  In particular the self-user is always
+  /// included.
+  ///
+  /// This is a result of [List.map], so it has an efficient `length`.
+  Iterable<int> get allRecipientIds => displayRecipient.map((e) => e.id);
+
   DmMessage({
     super.avatarUrl,
     required super.client,

test/api/model/model_test.dart

@@ -90,5 +90,22 @@ void main() {
       check(parse(withRecipients([user11, user2, user3])).displayRecipient)
         .deepEquals(asRecipients([user2, user3, user11]));
     });
+
+    test('allRecipientIds', () {
+      check(parse(withRecipients([user2])).allRecipientIds)
+        .deepEquals([2]);
+
+      check(parse(withRecipients([user2, user3])).allRecipientIds)
+        .deepEquals([2, 3]);
+      check(parse(withRecipients([user3, user2])).allRecipientIds)
+        .deepEquals([2, 3]);
+
+      check(parse(withRecipients([user2, user3, user11])).allRecipientIds)
+        .deepEquals([2, 3, 11]);
+      check(parse(withRecipients([user3, user11, user2])).allRecipientIds)
+        .deepEquals([2, 3, 11]);
+      check(parse(withRecipients([user11, user2, user3])).allRecipientIds)
+        .deepEquals([2, 3, 11]);
+    });
   });
 }