msglist: Fetch older messages on scrolling up

Fixes: #78

lib/model/message_list.dart

@@ -27,6 +27,15 @@ class MessageListMessageItem extends MessageListItem {
   MessageListMessageItem(this.message, this.content);
 }
 
+/// Indicates the app is loading more messages at the top or bottom.
+class MessageListLoadingItem extends MessageListItem {
+  final MessageListDirection direction;
+
+  const MessageListLoadingItem(this.direction);
+}
+
+enum MessageListDirection { older, newer }
+
 /// Indicates we've reached the oldest message in the narrow.
 class MessageListHistoryStartItem extends MessageListItem {
   const MessageListHistoryStartItem();
@@ -55,6 +64,10 @@ mixin _MessageSequence {
   bool get haveOldest => _haveOldest;
   bool _haveOldest = false;
 
+  /// Whether we are currently fetching the next batch of older messages.
+  bool get fetchingOlder => _fetchingOlder;
+  bool _fetchingOlder = false;
+
   /// The parsed message contents, as a list parallel to [messages].
   ///
   /// The i'th element is the result of parsing the i'th element of [messages].
@@ -70,7 +83,7 @@ mixin _MessageSequence {
   /// before, between, or after the messages.
   ///
   /// This information is completely derived from [messages] and
-  /// the flag [haveOldest].
+  /// the flags [haveOldest] and [fetchingOlder].
   /// It exists as an optimization, to memoize that computation.
   final QueueList<MessageListItem> items = QueueList();
 
@@ -86,6 +99,11 @@ 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 MessageListDirection.newer:       return 1;
+        }
       case MessageListMessageItem(:var message): return message.id.compareTo(messageId);
     }
   }
@@ -115,6 +133,19 @@ mixin _MessageSequence {
     _processMessage(messages.length - 1);
   }
 
+  void _insertAllMessages(int index, Iterable<Message> toInsert) {
+    // TODO parse/process messages in smaller batches, to not drop frames.
+    //   On a Pixel 5, a batch of 100 messages takes ~15-20ms in _insertAllMessages.
+    //   (Before that, ~2-5ms in jsonDecode and 0ms in fromJson,
+    //   so skip worrying about those steps.)
+    assert(contents.length == messages.length);
+    messages.insertAll(index, toInsert);
+    contents.insertAll(index, toInsert.map(
+      (message) => parseContent(message.content)));
+    assert(contents.length == messages.length);
+    _reprocessAll();
+  }
+
   /// Redo all computations from scratch, based on [messages].
   void _recompute() {
     assert(contents.length == messages.length);
@@ -137,12 +168,22 @@ mixin _MessageSequence {
 
   /// Update [items] to include markers at start and end as appropriate.
   void _updateEndMarkers() {
-    switch ((items.firstOrNull, haveOldest)) {
-      case (MessageListHistoryStartItem(), true): break;
-      case (MessageListHistoryStartItem(), _   ): items.removeFirst();
-
-      case (_,                             true): items.addFirst(const MessageListHistoryStartItem());
-      case (_,                                _): break;
+    assert(!(haveOldest && fetchingOlder));
+    final startMarker = switch ((fetchingOlder, 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;
     }
   }
 
@@ -164,13 +205,12 @@ mixin _MessageSequence {
 /// Lifecycle:
 ///  * Create with [init].
 ///  * Add listeners with [addListener].
-///  * Fetch messages with [fetch].  When the fetch completes, this object
+///  * Fetch messages with [fetchInitial].  When the fetch completes, this object
 ///    will notify its listeners (as it will any other time the data changes.)
+///  * Fetch more messages as needed with [fetchOlder].
 ///  * On reassemble, call [reassemble].
 ///  * When the object will no longer be used, call [dispose] to free
 ///    resources on the [PerAccountStore].
-///
-/// TODO support fetching another batch
 class MessageListView with ChangeNotifier, _MessageSequence {
   MessageListView._({required this.store, required this.narrow});
 
@@ -190,10 +230,11 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   final PerAccountStore store;
   final Narrow narrow;
 
-  Future<void> fetch() async {
+  /// Fetch messages, starting from scratch.
+  Future<void> fetchInitial() async {
     // TODO(#80): fetch from anchor firstUnread, instead of newest
     // TODO(#82): fetch from a given message ID as anchor
-    assert(!fetched && !haveOldest);
+    assert(!fetched && !haveOldest && !fetchingOlder);
     assert(messages.isEmpty && contents.isEmpty);
     // TODO schedule all this in another isolate
     final result = await getMessages(store.connection,
@@ -211,6 +252,39 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     notifyListeners();
   }
 
+  /// Fetch the next batch of older messages, if applicable.
+  Future<void> fetchOlder() async {
+    if (haveOldest) return;
+    if (fetchingOlder) return;
+    assert(fetched);
+    assert(messages.isNotEmpty);
+    _fetchingOlder = true;
+    _updateEndMarkers();
+    notifyListeners();
+    try {
+      final result = await getMessages(store.connection,
+        narrow: narrow.apiEncode(),
+        anchor: NumericAnchor(messages[0].id),
+        includeAnchor: false,
+        numBefore: kMessageListFetchBatchSize,
+        numAfter: 0,
+      );
+
+      if (result.messages.isNotEmpty
+          && result.messages.last.id == messages[0].id) {
+        // TODO(server-6): includeAnchor should make this impossible
+        result.messages.removeLast();
+      }
+
+      _insertAllMessages(0, result.messages);
+      _haveOldest = result.foundOldest;
+    } finally {
+      _fetchingOlder = false;
+      _updateEndMarkers();
+      notifyListeners();
+    }
+  }
+
   /// Add [message] to this view, if it belongs here.
   ///
   /// Called in particular when we get a [MessageEvent].

lib/widgets/message_list.dart

@@ -123,6 +123,19 @@ class MessageListAppBarTitle extends StatelessWidget {
   }
 }
 
+/// The approximate height of a short message in the message list.
+const _kShortMessageHeight = 80;
+
+/// The point at which we fetch more history, in pixels from the start or end.
+///
+/// When the user scrolls to within this distance of the start (or end) of the
+/// history we currently have, we make a request to fetch the next batch of
+/// older (or newer) messages.
+//
+// When the user reaches this point, they're at least halfway through the
+// previous batch.
+const kFetchMessagesBufferPixels = (kMessageListFetchBatchSize / 2) * _kShortMessageHeight;
+
 class MessageList extends StatefulWidget {
   const MessageList({super.key, required this.narrow});
 
@@ -160,7 +173,7 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
   void _initModel(PerAccountStore store) {
     model = MessageListView.init(store: store, narrow: widget.narrow);
     model!.addListener(_modelChanged);
-    model!.fetch();
+    model!.fetchInitial();
   }
 
   void _modelChanged() {
@@ -176,6 +189,17 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
     } else {
       _scrollToBottomVisibleValue.value = true;
     }
+
+    final extentRemainingAboveViewport = scrollMetrics.maxScrollExtent - scrollMetrics.pixels;
+    if (extentRemainingAboveViewport < kFetchMessagesBufferPixels) {
+      // TODO: This ends up firing a second time shortly after we fetch a batch.
+      //   The result is that each time we decide to fetch a batch, we end up
+      //   fetching two batches in quick succession.  This is basically harmless
+      //   but makes things a bit more complicated to reason about.
+      //   The cause seems to be that this gets called again with maxScrollExtent
+      //   still not yet updated to account for the newly-added messages.
+      model?.fetchOlder();
+    }
   }
 
   void _scrollChanged() {
@@ -251,6 +275,11 @@ class _MessageListState extends State<MessageList> with PerAccountStoreAwareStat
               child: Padding(
                 padding: EdgeInsets.symmetric(vertical: 16.0),
                 child: Text("No earlier messages."))), // TODO use an icon
+          MessageListLoadingItem() =>
+            const Center(
+              child: Padding(
+                padding: EdgeInsets.symmetric(vertical: 16.0),
+                child: CircularProgressIndicator())), // TODO perhaps a different indicator
           MessageListMessageItem(:var message, :var content) =>
             MessageItem(
               trailing: i == 0 ? const SizedBox(height: 8) : const SizedBox(height: 11),