narrow: add DmNarrow.withUsers

Refactored other DmNarrow factories to use
this more generic interface

Author: sirpengi

lib/model/narrow.dart

@@ -155,15 +155,22 @@ class DmNarrow extends Narrow implements SendableNarrow {
 
   /// A [DmNarrow] from an item in [InitialSnapshot.recentPrivateConversations].
   factory DmNarrow.ofRecentDmConversation(RecentDmConversation conversation, {required int selfUserId}) {
-    return DmNarrow(
-      allRecipientIds: [...conversation.userIds, selfUserId]..sort(),
+    return DmNarrow.withUsers(
+      conversation.userIds,
       selfUserId: selfUserId,
     );
   }
 
   factory DmNarrow.withUser(int userId, {required int selfUserId}) {
+    return DmNarrow.withUsers(
+      [userId],
+      selfUserId: selfUserId,
+    );
+  }
+
+  factory DmNarrow.withUsers(List<int> userIds, {required int selfUserId}) {
     return DmNarrow(
-      allRecipientIds: {userId, selfUserId}.toList()..sort(),
+      allRecipientIds: {...userIds, selfUserId}.toList()..sort(),
       selfUserId: selfUserId,
     );
   }

test/model/narrow_test.dart

@@ -90,6 +90,20 @@ void main() {
           selfUserId: 5));
     });
 
+    test('withUsers: without selfUserId', () {
+      final actual = DmNarrow.withUsers([1, 2], selfUserId: 3);
+      check(actual).equals(DmNarrow(
+          allRecipientIds: [1, 2, 3],
+          selfUserId: 3));
+    });
+
+    test('withUsers: with selfUserId', () {
+      final actual = DmNarrow.withUsers([1, 2, 3], selfUserId: 3);
+      check(actual).equals(DmNarrow(
+          allRecipientIds: [1, 2, 3],
+          selfUserId: 3));
+    });
+
     test('otherRecipientIds', () {
       check(DmNarrow(allRecipientIds: [1, 2, 3], selfUserId: 2))
         .otherRecipientIds.deepEquals([1, 3]);