store: Add RecentDmConversationsView view-model

lib/api/model/initial_snapshot.dart

@@ -24,6 +24,8 @@ class InitialSnapshot {
 
   // TODO etc., etc.
 
+  final List<RecentDmConversation> recentPrivateConversations;
+
   final List<Subscription> subscriptions;
 
   final List<ZulipStream> streams;
@@ -66,6 +68,7 @@ class InitialSnapshot {
     this.zulipMergeBase,
     required this.alertWords,
     required this.customProfileFields,
+    required this.recentPrivateConversations,
     required this.subscriptions,
     required this.streams,
     required this.maxFileUploadSizeMib,
@@ -79,3 +82,23 @@ class InitialSnapshot {
 
   Map<String, dynamic> toJson() => _$InitialSnapshotToJson(this);
 }
+
+/// An item in `recent_private_conversations`.
+///
+/// For docs, search for "recent_private_conversations:"
+/// in <https://zulip.com/api/register-queue>.
+@JsonSerializable(fieldRename: FieldRename.snake)
+class RecentDmConversation {
+  final int maxMessageId;
+  final List<int> userIds;
+
+  RecentDmConversation({
+    required this.maxMessageId,
+    required this.userIds,
+  });
+
+  factory RecentDmConversation.fromJson(Map<String, dynamic> json) =>
+    _$RecentDmConversationFromJson(json);
+
+  Map<String, dynamic> toJson() => _$RecentDmConversationToJson(this);
+}

lib/model/narrow.dart

@@ -1,4 +1,5 @@
 
+import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
 import '../api/model/narrow.dart';
 import '../api/route/messages.dart';
@@ -70,6 +71,9 @@ class StreamNarrow extends Narrow {
   @override
   ApiNarrow apiEncode() => [ApiNarrowStream(streamId)];
 
+  @override
+  String toString() => 'StreamNarrow($streamId)';
+
   @override
   bool operator ==(Object other) {
     if (other is! StreamNarrow) return false;
@@ -102,6 +106,9 @@ class TopicNarrow extends Narrow implements SendableNarrow {
   @override
   StreamDestination get destination => StreamDestination(streamId, topic);
 
+  @override
+  String toString() => 'TopicNarrow($streamId, $topic)';
+
   @override
   bool operator ==(Object other) {
     if (other is! TopicNarrow) return false;
@@ -146,6 +153,14 @@ 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(),
+      selfUserId: selfUserId,
+    );
+  }
+
   /// The user IDs of everyone in the conversation, sorted.
   ///
   /// Each message in the conversation is sent by one of these users
@@ -207,6 +222,9 @@ class DmNarrow extends Narrow implements SendableNarrow {
   @override
   ApiNarrow apiEncode() => [ApiNarrowDm(allRecipientIds)];
 
+  @override
+  String toString() => 'DmNarrow(allRecipientIds: $allRecipientIds)';
+
   @override
   bool operator ==(Object other) {
     if (other is! DmNarrow) return false;

lib/model/recent_dm_conversations.dart

@@ -0,0 +1,126 @@
+import 'package:collection/collection.dart';
+import 'package:flutter/foundation.dart';
+
+import '../api/model/initial_snapshot.dart';
+import '../api/model/model.dart';
+import '../api/model/events.dart';
+import 'narrow.dart';
+
+/// A view-model for the recent-DM-conversations UI.
+///
+/// This maintains the list of recent DM conversations,
+/// plus additional data in order to efficiently maintain the list.
+class RecentDmConversationsView extends ChangeNotifier {
+  factory RecentDmConversationsView({
+    required List<RecentDmConversation> initial,
+    required selfUserId,
+  }) {
+    final entries = initial.map((conversation) => MapEntry(
+        DmNarrow.ofRecentDmConversation(conversation, selfUserId: selfUserId),
+        conversation.maxMessageId,
+      )).toList()..sort((a, b) => -a.value.compareTo(b.value));
+    return RecentDmConversationsView._(
+      map: Map.fromEntries(entries),
+      sorted: QueueList.from(entries.map((e) => e.key)),
+      selfUserId: selfUserId,
+    );
+  }
+
+  RecentDmConversationsView._({
+    required this.map,
+    required this.sorted,
+    required this.selfUserId,
+  });
+
+  /// The latest message ID in each conversation.
+  final Map<DmNarrow, int> map;
+
+  /// The [DmNarrow] keys of the map, sorted by latest message descending.
+  final QueueList<DmNarrow> sorted;
+
+  final int selfUserId;
+
+  /// Insert the key at the proper place in [sorted].
+  ///
+  /// Optimized, taking O(1) time, for the case where that place is the start,
+  /// because that's the common case for a new message.
+  /// May take O(n) time in general.
+  void _insertSorted(DmNarrow key, int msgId) {
+    final i = sorted.indexWhere((k) => map[k]! < msgId);
+    // QueueList is a deque, with O(1) to add at start or end.
+    switch (i) {
+      case == 0:
+        sorted.addFirst(key);
+      case < 0:
+        sorted.addLast(key);
+      default:
+        sorted.insert(i, key);
+    }
+  }
+
+  /// Handle [MessageEvent], updating [map] and [sorted].
+  ///
+  /// Can take linear time in general.  That sounds inefficient...
+  /// but it's what the webapp does, so must not be catastrophic. 🤷
+  /// (In fact the webapp calls `Array#sort`,
+  /// which takes at *least* linear time, and may be 𝛳(N log N).)
+  ///
+  /// The point of the event is that we're learning about the message
+  /// in real time immediately after it was sent --
+  /// so the overwhelmingly common case is that the message
+  /// is newer than any existing message we know about. (*)
+  /// That's therefore the case we optimize for,
+  /// particularly in the helper _insertSorted.
+  ///
+  /// (*) In fact at present that's the only possible case.
+  /// The alternative will become possible when we have the analogue of
+  /// zulip-mobile's FETCH_MESSAGES_COMPLETE, with fetches done for a
+  /// [MessageListView] reporting their messages back via the [PerAccountStore]
+  /// to all our central data structures.
+  /// Then that can race with a new-message event: for example,
+  /// say we get a fetch-messages result that includes the just-sent
+  /// message 1002, and only after that get the event about message 1001,
+  /// sent moments earlier.  The event queue always delivers events in order, so
+  /// even the race is possible only because we fetch messages outside of the
+  /// event queue.
+  void handleMessageEvent(MessageEvent event) {
+    final message = event.message;
+    if (message is! DmMessage) {
+      return;
+    }
+    final key = DmNarrow.ofMessage(message, selfUserId: selfUserId);
+    final prev = map[key];
+    if (prev == null) {
+      // The conversation is new.  Add to both `map` and `sorted`.
+      map[key] = message.id;
+      _insertSorted(key, message.id);
+    } else if (prev >= message.id) {
+      // The conversation already has a newer message.
+      // This should be impossible as long as we only listen for messages coming
+      // through the event system, which sends events in order.
+      // Anyway, do nothing.
+    } else {
+      // The conversation needs to be (a) updated in `map`...
+      map[key] = message.id;
+
+      // ... and (b) possibly moved around in `sorted` to keep the list sorted.
+      final i = sorted.indexOf(key);
+      assert(i >= 0, 'key in map should be in sorted');
+      if (i == 0) {
+        // The conversation was already the latest, so no reordering needed.
+        // (This is likely a common case in practice -- happens every time
+        // the user gets several DMs in a row in the same thread -- so good to
+        // optimize.)
+      } else {
+        // It wasn't the latest.  Just handle the general case.
+        sorted.removeAt(i); // linear time, ouch
+        _insertSorted(key, message.id);
+      }
+    }
+    notifyListeners();
+  }
+
+  // TODO update from messages loaded in message lists. When doing so,
+  //   review handleMessageEvent so it acknowledges the subtle races that can
+  //   happen when taking data from outside the event system.
+}

lib/model/store.dart

@@ -16,6 +16,7 @@ import '../log.dart';
 import 'autocomplete.dart';
 import 'database.dart';
 import 'message_list.dart';
+import 'recent_dm_conversations.dart';
 
 export 'package:drift/drift.dart' show Value;
 export 'database.dart' show Account, AccountsCompanion;
@@ -149,6 +150,7 @@ class PerAccountStore extends ChangeNotifier {
     required this.connection,
     required InitialSnapshot initialSnapshot,
   }) : zulipVersion = initialSnapshot.zulipVersion,
+       maxFileUploadSizeMib = initialSnapshot.maxFileUploadSizeMib,
        users = Map.fromEntries(
          initialSnapshot.realmUsers
          .followedBy(initialSnapshot.realmNonActiveUsers)
@@ -158,20 +160,30 @@ class PerAccountStore extends ChangeNotifier {
          (stream) => MapEntry(stream.streamId, stream))),
        subscriptions = Map.fromEntries(initialSnapshot.subscriptions.map(
          (subscription) => MapEntry(subscription.streamId, subscription))),
-       maxFileUploadSizeMib = initialSnapshot.maxFileUploadSizeMib;
+       recentDmConversationsView = RecentDmConversationsView(
+         initial: initialSnapshot.recentPrivateConversations, selfUserId: account.userId);
 
   final Account account;
   final ApiConnection connection; // TODO(#135): update zulipFeatureLevel with events
 
   // TODO(#135): Keep all this data updated by handling Zulip events from the server.
+
+  // Data attached to the realm or the server.
   final String zulipVersion; // TODO get from account; update there on initial snapshot
+  final int maxFileUploadSizeMib; // No event for this.
+
+  // Users and data about them.
   final Map<int, User> users;
+
+  // Streams, topics, and stuff about them.
   final Map<int, ZulipStream> streams;
   final Map<int, Subscription> subscriptions;
-  final int maxFileUploadSizeMib; // No event for this.
 
   // TODO lots more data.  When adding, be sure to update handleEvent too.
 
+  // TODO call [RecentDmConversationsView.dispose] in [dispose]
+  final RecentDmConversationsView recentDmConversationsView;
+
   final Set<MessageListView> _messageListViews = {};
 
   void registerMessageList(MessageListView view) {
@@ -254,6 +266,7 @@ class PerAccountStore extends ChangeNotifier {
       notifyListeners();
     } else if (event is MessageEvent) {
       assert(debugLog("server event: message ${jsonEncode(event.message.toJson())}"));
+      recentDmConversationsView.handleMessageEvent(event);
       for (final view in _messageListViews) {
         view.maybeAddMessage(event.message);
       }

pubspec.lock

@@ -178,7 +178,7 @@ packages:
     source: hosted
     version: "4.5.0"
   collection:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: collection
       sha256: f092b211a4319e98e5ff58223576de6c2803db36221657b46c82574721240687

pubspec.yaml

@@ -55,6 +55,7 @@ dependencies:
   app_settings: ^4.2.0
   image_picker: ^0.8.7+3
   package_info_plus: ^4.0.1
+  collection: ^1.17.2
 
 dev_dependencies:
   flutter_test: