msglist: Add and manage outbox message objects in message list view

This adds some overhead in magnitude of O(1) (where the constant is
the number of outbox messages in a view, expected to be small) on
message event handling.

We add outboxMessages as a list independent from messages on
_MessageSequence.  Because outbox messages are not rendered
(the raw content is shown as plain text), we leave the 1-1
relationship between `messages` and `contents` unchanged.

When computing `items`, we now start to look at `outboxMessages` as
well, with the guarantee that the items related to outbox messages
always come after those for other messages.  Look for places that call
`_processOutboxMessage(int index)` for references, and the changes to
`checkInvariants` on how this affects the message list invariants.

`addOutboxMessage` is similar to `handleMessage`.  However, outbox
messages do not rely on the fetched state, i.e. they can be
synchronously updated when the message list view was first
initialized.

This implements minimal support to display outbox message message item
widgets in the message list, without indicators for theirs states.
Retrieving content from failed sent requests and the full UI are
implemented in a later commit.

Author: PIG208

lib/model/message.dart

@@ -394,6 +394,8 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore, _OutboxMes
       }
     }
 
+    // TODO predict outbox message moves using propagateMode
+
     for (final view in _messageListViews) {
       view.messagesMoved(messageMove: messageMove, messageIds: event.messageIds);
     }

lib/model/message_list.dart

@@ -64,6 +64,22 @@ class MessageListMessageItem extends MessageListMessageBaseItem {
   });
 }
 
+/// An [OutboxMessage] to show in the message list.
+class MessageListOutboxMessageItem extends MessageListMessageBaseItem {
+  @override
+  final OutboxMessage message;
+  @override
+  final ZulipContent content;
+
+  MessageListOutboxMessageItem(
+    this.message, {
+    required super.showSender,
+    required super.isLastInBlock,
+  }) : content = ZulipContent(nodes: [
+    ParagraphNode(links: [], nodes: [TextNode(message.contentMarkdown)]),
+  ]);
+}
+
 /// The sequence of messages in a message list, and how to display them.
 ///
 /// This comprises much of the guts of [MessageListView].
@@ -140,14 +156,25 @@ mixin _MessageSequence {
   /// It exists as an optimization, to memoize the work of parsing.
   final List<ZulipMessageContent> contents = [];
 
+  /// The messages sent by the self-user, retrieved from
+  /// [MessageStore.outboxMessages].
+  ///
+  /// See also [items].
+  ///
+  /// Usually this should not have that many items, so we do not anticipate
+  /// performance issues with unoptimized O(N) iterations through this list.
+  final List<OutboxMessage> outboxMessages = [];
+
   /// The messages and their siblings in the UI, in order.
   ///
   /// This has a [MessageListMessageItem] corresponding to each element
-  /// of [messages], in order.  It may have additional items interspersed
-  /// before, between, or after the messages.
+  /// of [messages], then a [MessageListOutboxMessageItem] corresponding to each
+  /// element of [outboxMessages], in order.
+  /// It may have additional items interspersed before, between, or after the
+  /// messages.
   ///
-  /// This information is completely derived from [messages] and
-  /// the flags [haveOldest], [fetchingOlder] and [fetchOlderCoolingDown].
+  /// This information is completely derived from [messages], [outboxMessages]
+  /// and the flags [haveOldest], [fetchingOlder] and [fetchOlderCoolingDown].
   /// It exists as an optimization, to memoize that computation.
   ///
   /// See also [middleItem], an index which divides this list
@@ -164,6 +191,7 @@ mixin _MessageSequence {
   /// Either the bottom slices of both [items] and [messages] are empty,
   /// or the first item in the bottom slice of [items] is a [MessageListMessageItem]
   /// for the first message in the bottom slice of [messages].
+  // TODO(#1453) update this in the context of outbox messages.
   int middleItem = 0;
 
   int _findMessageWithId(int messageId) {
@@ -179,9 +207,10 @@ mixin _MessageSequence {
     switch (item) {
       case MessageListRecipientHeaderItem(:var message):
       case MessageListDateSeparatorItem(:var message):
-        if (message.id == null)                  return 1;  // TODO(#1441): test
+        if (message.id == null)                  return 1;
         return message.id! <= messageId ? -1 : 1;
       case MessageListMessageItem(:var message): return message.id.compareTo(messageId);
+      case MessageListOutboxMessageItem():       return 1;
     }
   }
 
@@ -298,10 +327,46 @@ mixin _MessageSequence {
     _reprocessAll();
   }
 
+  /// Append [outboxMessage] to [outboxMessages], and update derived data
+  /// accordingly.
+  ///
+  /// The caller is responsible for ensuring this is an appropriate thing to do
+  /// given [narrow] and other concerns.
+  void _addOutboxMessage(OutboxMessage outboxMessage) {
+    assert(!outboxMessages.contains(outboxMessage));
+    outboxMessages.add(outboxMessage);
+    _processOutboxMessage(outboxMessages.length - 1);
+  }
+
+  /// Remove the [outboxMessage] from the view.
+  ///
+  /// Returns true if the outbox message was removed, false otherwise.
+  bool _removeOutboxMessage(OutboxMessage outboxMessage) {
+    if (!outboxMessages.remove(outboxMessage)) {
+      return false;
+    }
+    _reprocessOutboxMessages();
+    return true;
+  }
+
+  /// Remove all outbox messages that satisfy [test] from [outboxMessages].
+  ///
+  /// Returns true if any outbox messages were removed, false otherwise.
+  bool _removeOutboxMessagesWhere(bool Function(OutboxMessage) test) {
+    final count = outboxMessages.length;
+    outboxMessages.removeWhere(test);
+    if (outboxMessages.length == count) {
+      return false;
+    }
+    _reprocessOutboxMessages();
+    return true;
+  }
+
   /// Reset all [_MessageSequence] data, and cancel any active fetches.
   void _reset() {
     generation += 1;
     messages.clear();
+    outboxMessages.clear();
     middleMessage = 0;
     _fetched = false;
     _haveOldest = false;
@@ -374,6 +439,7 @@ mixin _MessageSequence {
   /// The previous messages in the list must already have been processed.
   /// This message must already have been parsed and reflected in [contents].
   void _processMessage(int index) {
+    assert(items.lastOrNull is! MessageListOutboxMessageItem);
     final prevMessage = index == 0 ? null : messages[index - 1];
     final message = messages[index];
     final content = contents[index];
@@ -385,13 +451,67 @@ mixin _MessageSequence {
         message, content, showSender: !canShareSender, isLastInBlock: true));
   }
 
-  /// Recompute [items] from scratch, based on [messages], [contents], and flags.
+  /// Append to [items] based on the index-th outbox message.
+  ///
+  /// All [messages] and previous messages in [outboxMessages] must already have
+  /// been processed.
+  void _processOutboxMessage(int index) {
+    final prevMessage = index == 0 ? messages.lastOrNull
+                                   : outboxMessages[index - 1];
+    final message = outboxMessages[index];
+
+    _addItemsForMessage(message,
+      isMiddleMessage: false,
+      prevMessage: prevMessage,
+      buildItem: (bool canShareSender) => MessageListOutboxMessageItem(
+        message, showSender: !canShareSender, isLastInBlock: true));
+  }
+
+  /// Remove items associated with [outboxMessages] from [items].
+  ///
+  /// This is designed to be idempotent; repeated calls will not change the
+  /// content of [items].
+  ///
+  /// This is efficient due to the expected small size of [outboxMessages].
+  void _removeOutboxMessageItems() {
+    // This loop relies on the assumption that all items that follow
+    // the last [MessageListMessageItem] are derived from outbox messages.
+    // If there is no [MessageListMessageItem] at all,
+    // this will end up removing end markers.
+    while (items.isNotEmpty && items.last is! MessageListMessageItem) {
+      items.removeLast();
+      assert(items.length >= middleItem);
+    }
+    assert(items.none((e) => e is MessageListOutboxMessageItem));
+
+    if (items.isNotEmpty) {
+      final lastItem = items.last as MessageListMessageItem;
+      lastItem.isLastInBlock = true;
+    }
+  }
+
+  /// Recompute the portion of [items] derived from outbox messages,
+  /// based on [outboxMessages] and [messages].
+  ///
+  /// All [messages] should have been processed when this is called.
+  void _reprocessOutboxMessages() {
+    _removeOutboxMessageItems();
+    for (var i = 0; i < outboxMessages.length; i++) {
+      _processOutboxMessage(i);
+    }
+  }
+
+  /// Recompute [items] from scratch, based on [messages], [contents],
+  /// [outboxMessages] and flags.
   void _reprocessAll() {
     items.clear();
     for (var i = 0; i < messages.length; i++) {
       _processMessage(i);
     }
     if (middleMessage == messages.length) middleItem = items.length;
+    for (var i = 0; i < outboxMessages.length; i++) {
+      _processOutboxMessage(i);
+    }
   }
 }
 
@@ -435,7 +555,9 @@ class MessageListView with ChangeNotifier, _MessageSequence {
 
   factory MessageListView.init(
       {required PerAccountStore store, required Narrow narrow}) {
-    final view = MessageListView._(store: store, narrow: narrow);
+    final view = MessageListView._(store: store, narrow: narrow)
+      .._syncOutboxMessages()
+      .._reprocessOutboxMessages();
     store.registerMessageList(view);
     return view;
   }
@@ -538,12 +660,14 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     store.recentSenders.handleMessages(result.messages); // TODO(#824)
 
     // We'll make the bottom slice start at the last visible message, if any.
+    _removeOutboxMessageItems();
     for (final message in result.messages) {
       if (!_messageVisible(message)) continue;
       middleMessage = messages.length;
       _addMessage(message);
       // Now [middleMessage] is the last message (the one just added).
     }
+    _reprocessOutboxMessages();
     _fetched = true;
     _haveOldest = result.foundOldest;
     notifyListeners();
@@ -647,9 +771,42 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     }
   }
 
+  bool _shouldAddOutboxMessage(OutboxMessage outboxMessage, {
+    bool wasUnmuted = false,
+  }) {
+    return !outboxMessage.hidden
+      && narrow.containsMessage(outboxMessage)
+      && (_messageVisible(outboxMessage) || wasUnmuted);
+  }
+
+  /// Copy outbox messages from the store, keeping the ones belong to the view.
+  ///
+  /// This does not recompute [items].  The caller is expected to call
+  /// [_reprocessOutboxMessages] later to keep [items] up-to-date.
+  ///
+  /// This assumes that [outboxMessages] is empty.
+  void _syncOutboxMessages() {
+    assert(outboxMessages.isEmpty);
+    for (final outboxMessage in store.outboxMessages.values) {
+      if (_shouldAddOutboxMessage(outboxMessage)) {
+        outboxMessages.add(outboxMessage);
+      }
+    }
+  }
+
   /// Add [outboxMessage] if it belongs to the view.
   void addOutboxMessage(OutboxMessage outboxMessage) {
-    // TODO(#1441) implement this
+    assert(outboxMessages.none(
+      (message) => message.localMessageId == outboxMessage.localMessageId));
+    if (_shouldAddOutboxMessage(outboxMessage)) {
+      _addOutboxMessage(outboxMessage);
+      if (fetched) {
+        // Only need to notify listeners when [fetched] is true, because
+        // otherwise the message list just shows a loading indicator with
+        // no other items.
+        notifyListeners();
+      }
+    }
   }
 
   /// Remove the [outboxMessage] from the view.
@@ -658,7 +815,9 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   ///
   /// This should only be called from [MessageStore.takeOutboxMessage].
   void removeOutboxMessage(OutboxMessage outboxMessage) {
-    // TODO(#1441) implement this
+    if (_removeOutboxMessage(outboxMessage)) {
+      notifyListeners();
+    }
   }
 
   void handleUserTopicEvent(UserTopicEvent event) {
@@ -667,10 +826,17 @@ class MessageListView with ChangeNotifier, _MessageSequence {
         return;
 
       case VisibilityEffect.muted:
-        if (_removeMessagesWhere((message) =>
-            (message is StreamMessage
-             && message.streamId == event.streamId
-             && message.topic == event.topicName))) {
+        bool removed = _removeOutboxMessagesWhere((message) =>
+          message is StreamOutboxMessage
+          && message.conversation.streamId == event.streamId
+          && message.conversation.topic == event.topicName);
+
+        removed |= _removeMessagesWhere((message) =>
+          message is StreamMessage
+          && message.streamId == event.streamId
+          && message.topic == event.topicName);
+
+        if (removed) {
           notifyListeners();
         }
 
@@ -683,6 +849,18 @@ class MessageListView with ChangeNotifier, _MessageSequence {
           notifyListeners();
           fetchInitial();
         }
+
+        outboxMessages.clear();
+        for (final outboxMessage in store.outboxMessages.values) {
+          if (_shouldAddOutboxMessage(
+            outboxMessage,
+            wasUnmuted: outboxMessage is StreamOutboxMessage
+              && outboxMessage.conversation.streamId == event.streamId
+              && outboxMessage.conversation.topic == event.topicName,
+          )) {
+            outboxMessages.add(outboxMessage);
+          }
+        }
     }
   }
 
@@ -696,14 +874,34 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   void handleMessageEvent(MessageEvent event) {
     final message = event.message;
     if (!narrow.containsMessage(message) || !_messageVisible(message)) {
+      assert(event.localMessageId == null || outboxMessages.none((message) =>
+        message.localMessageId == int.parse(event.localMessageId!, radix: 10)));
       return;
     }
     if (!_fetched) {
       // TODO mitigate this fetch/event race: save message to add to list later
       return;
     }
+    if (outboxMessages.isEmpty) {
+      assert(items.none((item) => item is MessageListOutboxMessageItem));
+      _addMessage(message);
+      notifyListeners();
+      return;
+    }
+
+    // We always remove all outbox message items
+    // to ensure that message items come before them.
+    _removeOutboxMessageItems();
     // TODO insert in middle instead, when appropriate
     _addMessage(message);
+    if (event.localMessageId != null) {
+      final localMessageId = int.parse(event.localMessageId!);
+      // [outboxMessages] is epxected to be short, so removing the corresponding
+      // outbox message and reprocessing them all in linear time is efficient.
+      outboxMessages.removeWhere(
+        (message) => message.localMessageId == localMessageId);
+    }
+    _reprocessOutboxMessages();
     notifyListeners();
   }
 
@@ -735,6 +933,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     // TODO in cases where we do have data to do better, do better.
     _reset();
     notifyListeners();
+    _syncOutboxMessages();
     fetchInitial();
   }
 
@@ -750,6 +949,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       case PropagateMode.changeLater:
         narrow = newNarrow;
         _reset();
+        _syncOutboxMessages();
         fetchInitial();
       case PropagateMode.changeOne:
     }
@@ -824,7 +1024,11 @@ class MessageListView with ChangeNotifier, _MessageSequence {
 
   /// Notify listeners if the given outbox message is present in this view.
   void notifyListenersIfOutboxMessagePresent(int localMessageId) {
-    // TODO(#1441) implement this
+    final isAnyPresent =
+      outboxMessages.any((message) => message.localMessageId == localMessageId);
+    if (isAnyPresent) {
+      notifyListeners();
+    }
   }
 
   /// Called when the app is reassembled during debugging, e.g. for hot reload.