msglist [nfc]: Handle start markers within widget code, not model

The view-model already exposes flags that express all the same
information.  By leaving these markers out of the list of items,
we can save ourselves a bunch of stateful updates.

Author: gnprice

lib/model/message_list.dart

@@ -63,21 +63,6 @@ class MessageListMessageItem extends MessageListMessageBaseItem {
   });
 }
 
-/// Indicates the app is loading more messages at the top.
-// TODO(#80): or loading at the bottom, by adding a [MessageListDirection.newer]
-class MessageListLoadingItem extends MessageListItem {
-  final MessageListDirection direction;
-
-  const MessageListLoadingItem(this.direction);
-}
-
-enum MessageListDirection { older }
-
-/// Indicates we've reached the oldest message in the narrow.
-class MessageListHistoryStartItem extends MessageListItem {
-  const MessageListHistoryStartItem();
-}
-
 /// The sequence of messages in a message list, and how to display them.
 ///
 /// This comprises much of the guts of [MessageListView].
@@ -161,11 +146,6 @@ mixin _MessageSequence {
 
   static int _compareItemToMessageId(MessageListItem item, int messageId) {
     switch (item) {
-      case MessageListHistoryStartItem():        return -1;
-      case MessageListLoadingItem():
-        switch (item.direction) {
-          case MessageListDirection.older:       return -1;
-        }
       case MessageListRecipientHeaderItem(:var message):
       case MessageListDateSeparatorItem(:var message):
         if (message.id == null)                  return 1;  // TODO(#1441): test
@@ -328,37 +308,12 @@ mixin _MessageSequence {
       showSender: !canShareSender, isLastInBlock: true));
   }
 
-  /// Update [items] to include markers at start and end as appropriate.
-  void _updateEndMarkers() {
-    assert(fetched);
-    assert(!(fetchingOlder && fetchOlderCoolingDown));
-    final effectiveFetchingOlder = fetchingOlder || fetchOlderCoolingDown;
-    assert(!(effectiveFetchingOlder && haveOldest));
-    final startMarker = switch ((effectiveFetchingOlder, haveOldest)) {
-      (true, _) => const MessageListLoadingItem(MessageListDirection.older),
-      (_, true) => const MessageListHistoryStartItem(),
-      (_,    _) => null,
-    };
-    final hasStartMarker = switch (items.firstOrNull) {
-      MessageListLoadingItem()      => true,
-      MessageListHistoryStartItem() => true,
-      _                             => false,
-    };
-    switch ((startMarker != null, hasStartMarker)) {
-      case (true, true): items[0] = startMarker!;
-      case (true, _   ): items.addFirst(startMarker!);
-      case (_,    true): items.removeFirst();
-      case (_,    _   ): break;
-    }
-  }
-
   /// Recompute [items] from scratch, based on [messages], [contents], and flags.
   void _reprocessAll() {
     items.clear();
     for (var i = 0; i < messages.length; i++) {
       _processMessage(i);
     }
-    _updateEndMarkers();
   }
 }
 
@@ -508,7 +463,6 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     }
     _fetched = true;
     _haveOldest = result.foundOldest;
-    _updateEndMarkers();
     notifyListeners();
   }
 
@@ -555,7 +509,6 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       || (narrow as TopicNarrow).with_ == null);
     assert(messages.isNotEmpty);
     _fetchingOlder = true;
-    _updateEndMarkers();
     notifyListeners();
     final generation = this.generation;
     bool hasFetchError = false;
@@ -601,13 +554,11 @@ class MessageListView with ChangeNotifier, _MessageSequence {
             .wait().then((_) {
               if (this.generation != generation) return;
               _fetchOlderCoolingDown = false;
-              _updateEndMarkers();
               notifyListeners();
             }));
         } else {
           _fetchOlderCooldownBackoffMachine = null;
         }
-        _updateEndMarkers();
         notifyListeners();
       }
     }

lib/widgets/message_list.dart

@@ -606,8 +606,10 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
           if (childIndex < 0) return null;
           return childIndex;
         },
-        childCount: topItems,
+        childCount: topItems + 1,
         (context, childIndex) {
+          if (childIndex == topItems) return _buildStartCap();
+
           final itemIndex = totalItems - 1 - (childIndex + bottomItems);
           final data = model.items[itemIndex];
           final item = _buildItem(data);
@@ -688,12 +690,21 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
       ]);
   }
 
+  Widget _buildStartCap() {
+    // These assertions are invariants of [MessageListView].
+    assert(!(model.fetchingOlder && model.fetchOlderCoolingDown));
+    final effectiveFetchingOlder =
+      model.fetchingOlder || model.fetchOlderCoolingDown;
+    assert(!(model.haveOldest && effectiveFetchingOlder));
+    return switch ((effectiveFetchingOlder, model.haveOldest)) {
+      (true, _) => const _MessageListLoadingMore(),
+      (_, true) => const _MessageListHistoryStart(),
+      (_,    _) => const SizedBox.shrink(),
+    };
+  }
+
   Widget _buildItem(MessageListItem data) {
     switch (data) {
-      case MessageListHistoryStartItem():
-        return const _MessageListHistoryStart();
-      case MessageListLoadingItem():
-        return const _MessageListLoadingMore();
       case MessageListRecipientHeaderItem():
         final header = RecipientHeader(message: data.message, narrow: widget.narrow);
         return StickyHeaderItem(allowOverflow: true,

test/model/message_list_test.dart

@@ -1791,7 +1791,6 @@ void main() {
     // We check showSender has the right values in [checkInvariants],
     // but to make this test explicit:
     check(model.items).deepEquals(<void Function(Subject<Object?>)>[
-      (it) => it.isA<MessageListHistoryStartItem>(),
       (it) => it.isA<MessageListRecipientHeaderItem>(),
       (it) => it.isA<MessageListMessageItem>().showSender.isTrue(),
       (it) => it.isA<MessageListMessageItem>().showSender.isFalse(),
@@ -1964,12 +1963,6 @@ void checkInvariants(MessageListView model) {
   }
 
   int i = 0;
-  if (model.haveOldest) {
-    check(model.items[i++]).isA<MessageListHistoryStartItem>();
-  }
-  if (model.fetchingOlder || model.fetchOlderCoolingDown) {
-    check(model.items[i++]).isA<MessageListLoadingItem>();
-  }
   for (int j = 0; j < model.messages.length; j++) {
     bool forcedShowSender = false;
     if (j == 0
@@ -1991,9 +1984,7 @@ void checkInvariants(MessageListView model) {
         i == model.items.length || switch (model.items[i]) {
           MessageListMessageItem()
           || MessageListDateSeparatorItem() => false,
-          MessageListRecipientHeaderItem()
-          || MessageListHistoryStartItem()
-          || MessageListLoadingItem()       => true,
+          MessageListRecipientHeaderItem()  => true,
         });
   }
   check(model.items).length.equals(i);