msglist: Add fetchNewer method to model

This completes the model layer of #82 and #80: the message list can
start at an arbitrary anchor, including a numeric message-ID anchor
or AnchorCode.firstUnread, and can fetch more history from there
in both directions.

Still to do is to work that into the widgets layer.  This change is
therefore NFC as to the live app: nothing calls this method yet.

Author: gnprice

lib/model/message_list.dart

@@ -643,6 +643,42 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       });
   }
 
+  /// Fetch the next batch of newer messages, if applicable.
+  ///
+  /// If there are no newer messages to fetch (i.e. if [haveNewest]),
+  /// or if this message list is already busy fetching more messages
+  /// (i.e. if [busyFetchingMore], which includes backoff from failed requests),
+  /// then this method does nothing and immediately returns.
+  /// That makes this method suitable to call frequently, e.g. every frame,
+  /// whenever it looks likely to be useful to have more messages.
+  Future<void> fetchNewer() async {
+    if (haveNewest) return;
+    if (busyFetchingMore) return;
+    assert(fetched);
+    assert(messages.isNotEmpty);
+    await _fetchMore(
+      anchor: NumericAnchor(messages.last.id),
+      numBefore: 0,
+      numAfter: kMessageListFetchBatchSize,
+      processResult: (result) {
+        if (result.messages.isNotEmpty
+            && result.messages.first.id == messages.last.id) {
+          // TODO(server-6): includeAnchor should make this impossible
+          result.messages.removeAt(0);
+        }
+
+        store.reconcileMessages(result.messages);
+        store.recentSenders.handleMessages(result.messages); // TODO(#824)
+
+        for (final message in result.messages) {
+          if (_messageVisible(message)) {
+            _addMessage(message);
+          }
+        }
+        _haveNewest = result.foundNewest;
+      });
+  }
+
   Future<void> _fetchMore({
     required Anchor anchor,
     required int numBefore,

test/example_data.dart

@@ -628,6 +628,24 @@ GetMessagesResult olderGetMessagesResult({
   );
 }
 
+/// A GetMessagesResult the server might return when we request newer messages.
+GetMessagesResult newerGetMessagesResult({
+  required int anchor,
+  bool foundAnchor = false, // the value if the server understood includeAnchor false
+  required bool foundNewest,
+  bool historyLimited = false,
+  required List<Message> messages,
+}) {
+  return GetMessagesResult(
+    anchor: anchor,
+    foundAnchor: foundAnchor,
+    foundOldest: false,
+    foundNewest: foundNewest,
+    historyLimited: historyLimited,
+    messages: messages,
+  );
+}
+
 PollWidgetData pollWidgetData({
   required String question,
   required List<String> options,

test/model/message_list_test.dart

@@ -28,6 +28,7 @@ import 'test_store.dart';
 const newestResult = eg.newestGetMessagesResult;
 const nearResult = eg.nearGetMessagesResult;
 const olderResult = eg.olderGetMessagesResult;
+const newerResult = eg.newerGetMessagesResult;
 
 void main() {
   // Arrange for errors caught within the Flutter framework to be printed
@@ -291,8 +292,8 @@ void main() {
     });
   });
 
-  group('fetchOlder', () {
-    test('smoke', () async {
+  group('fetching more', () {
+    test('fetchOlder smoke', () async {
       const narrow = CombinedFeedNarrow();
       await prepare(narrow: narrow);
       await prepareMessages(foundOldest: false,
@@ -321,14 +322,43 @@ void main() {
       );
     });
 
-    test('nop when already fetching', () async {
-      await prepare();
-      await prepareMessages(foundOldest: false,
+    test('fetchNewer smoke', () async {
+      const narrow = CombinedFeedNarrow();
+      await prepare(narrow: narrow, anchor: NumericAnchor(1000));
+      await prepareMessages(foundOldest: true, foundNewest: false,
         messages: List.generate(100, (i) => eg.streamMessage(id: 1000 + i)));
 
+      connection.prepare(json: newerResult(
+        anchor: 1099, foundNewest: false,
+        messages: List.generate(100, (i) => eg.streamMessage(id: 1100 + i)),
+      ).toJson());
+      final fetchFuture = model.fetchNewer();
+      checkNotifiedOnce();
+      check(model).busyFetchingMore.isTrue();
+
+      await fetchFuture;
+      checkNotifiedOnce();
+      check(model)
+        ..busyFetchingMore.isFalse()
+        ..messages.length.equals(200);
+      checkLastRequest(
+        narrow: narrow.apiEncode(),
+        anchor: '1099',
+        includeAnchor: false,
+        numBefore: 0,
+        numAfter: kMessageListFetchBatchSize,
+        allowEmptyTopicName: true,
+      );
+    });
+
+    test('nop when already fetching older', () async {
+      await prepare(anchor: NumericAnchor(1000));
+      await prepareMessages(foundOldest: false, foundNewest: false,
+        messages: List.generate(201, (i) => eg.streamMessage(id: 900 + i)));
+
       connection.prepare(json: olderResult(
-        anchor: 1000, foundOldest: false,
-        messages: List.generate(100, (i) => eg.streamMessage(id: 900 + i)),
+        anchor: 900, foundOldest: false,
+        messages: List.generate(100, (i) => eg.streamMessage(id: 800 + i)),
       ).toJson());
       final fetchFuture = model.fetchOlder();
       checkNotifiedOnce();
@@ -338,39 +368,90 @@ void main() {
       final fetchFuture2 = model.fetchOlder();
       checkNotNotified();
       check(model).busyFetchingMore.isTrue();
+      final fetchFuture3 = model.fetchNewer();
+      checkNotNotified();
+      check(model)..busyFetchingMore.isTrue()..messages.length.equals(201);
 
       await fetchFuture;
       await fetchFuture2;
+      await fetchFuture3;
       // We must not have made another request, because we didn't
       // prepare another response and didn't get an exception.
       checkNotifiedOnce();
-      check(model)
-        ..busyFetchingMore.isFalse()
-        ..messages.length.equals(200);
+      check(model)..busyFetchingMore.isFalse()..messages.length.equals(301);
     });
 
-    test('nop when already haveOldest true', () async {
-      await prepare();
-      await prepareMessages(foundOldest: true, messages:
-        List.generate(30, (i) => eg.streamMessage()));
+    test('nop when already fetching newer', () async {
+      await prepare(anchor: NumericAnchor(1000));
+      await prepareMessages(foundOldest: false, foundNewest: false,
+        messages: List.generate(201, (i) => eg.streamMessage(id: 900 + i)));
+
+      connection.prepare(json: newerResult(
+        anchor: 1100, foundNewest: false,
+        messages: List.generate(100, (i) => eg.streamMessage(id: 1101 + i)),
+      ).toJson());
+      final fetchFuture = model.fetchNewer();
+      checkNotifiedOnce();
+      check(model).busyFetchingMore.isTrue();
+
+      // Don't prepare another response.
+      final fetchFuture2 = model.fetchOlder();
+      checkNotNotified();
+      check(model).busyFetchingMore.isTrue();
+      final fetchFuture3 = model.fetchNewer();
+      checkNotNotified();
+      check(model)..busyFetchingMore.isTrue()..messages.length.equals(201);
+
+      await fetchFuture;
+      await fetchFuture2;
+      await fetchFuture3;
+      // We must not have made another request, because we didn't
+      // prepare another response and didn't get an exception.
+      checkNotifiedOnce();
+      check(model)..busyFetchingMore.isFalse()..messages.length.equals(301);
+    });
+
+    test('fetchOlder nop when already haveOldest true', () async {
+      await prepare(anchor: NumericAnchor(1000));
+      await prepareMessages(foundOldest: true, foundNewest: false, messages:
+        List.generate(151, (i) => eg.streamMessage(id: 950 + i)));
       check(model)
         ..haveOldest.isTrue()
-        ..messages.length.equals(30);
+        ..messages.length.equals(151);
 
       await model.fetchOlder();
       // We must not have made a request, because we didn't
       // prepare a response and didn't get an exception.
       checkNotNotified();
       check(model)
         ..haveOldest.isTrue()
-        ..messages.length.equals(30);
+        ..messages.length.equals(151);
+    });
+
+    test('fetchNewer nop when already haveNewest true', () async {
+      await prepare(anchor: NumericAnchor(1000));
+      await prepareMessages(foundOldest: false, foundNewest: true, messages:
+        List.generate(151, (i) => eg.streamMessage(id: 950 + i)));
+      check(model)
+        ..haveNewest.isTrue()
+        ..messages.length.equals(151);
+
+      await model.fetchNewer();
+      // We must not have made a request, because we didn't
+      // prepare a response and didn't get an exception.
+      checkNotNotified();
+      check(model)
+        ..haveNewest.isTrue()
+        ..messages.length.equals(151);
     });
 
     test('nop during backoff', () => awaitFakeAsync((async) async {
       final olderMessages = List.generate(5, (i) => eg.streamMessage());
       final initialMessages = List.generate(5, (i) => eg.streamMessage());
-      await prepare();
-      await prepareMessages(foundOldest: false, messages: initialMessages);
+      final newerMessages = List.generate(5, (i) => eg.streamMessage());
+      await prepare(anchor: NumericAnchor(initialMessages[2].id));
+      await prepareMessages(foundOldest: false, foundNewest: false,
+        messages: initialMessages);
       check(connection.takeRequests()).single;
 
       connection.prepare(apiException: eg.apiBadRequest());
@@ -385,20 +466,31 @@ void main() {
       check(model).busyFetchingMore.isTrue();
       check(connection.lastRequest).isNull();
 
+      await model.fetchNewer();
+      checkNotNotified();
+      check(model).busyFetchingMore.isTrue();
+      check(connection.lastRequest).isNull();
+
       // Wait long enough that a first backoff is sure to finish.
       async.elapse(const Duration(seconds: 1));
       check(model).busyFetchingMore.isFalse();
       checkNotifiedOnce();
       check(connection.lastRequest).isNull();
 
-      connection.prepare(json: olderResult(
-        anchor: 1000, foundOldest: false, messages: olderMessages).toJson());
+      connection.prepare(json: olderResult(anchor: initialMessages.first.id,
+        foundOldest: false, messages: olderMessages).toJson());
       await model.fetchOlder();
       checkNotified(count: 2);
       check(connection.takeRequests()).single;
+
+      connection.prepare(json: newerResult(anchor: initialMessages.last.id,
+        foundNewest: false, messages: newerMessages).toJson());
+      await model.fetchNewer();
+      checkNotified(count: 2);
+      check(connection.takeRequests()).single;
     }));
 
-    test('handles servers not understanding includeAnchor', () async {
+    test('fetchOlder handles servers not understanding includeAnchor', () async {
       await prepare();
       await prepareMessages(foundOldest: false,
         messages: List.generate(100, (i) => eg.streamMessage(id: 1000 + i)));
@@ -415,8 +507,25 @@ void main() {
         ..messages.length.equals(200);
     });
 
+    test('fetchNewer handles servers not understanding includeAnchor', () async {
+      await prepare(anchor: NumericAnchor(1000));
+      await prepareMessages(foundOldest: true, foundNewest: false,
+        messages: List.generate(101, (i) => eg.streamMessage(id: 1000 + i)));
+
+      // The old behavior is to include the anchor message regardless of includeAnchor.
+      connection.prepare(json: newerResult(
+        anchor: 1100, foundNewest: false, foundAnchor: true,
+        messages: List.generate(101, (i) => eg.streamMessage(id: 1100 + i)),
+      ).toJson());
+      await model.fetchNewer();
+      checkNotified(count: 2);
+      check(model)
+        ..busyFetchingMore.isFalse()
+        ..messages.length.equals(201);
+    });
+
     // TODO(#824): move this test
-    test('recent senders track all the messages', () async {
+    test('fetchOlder recent senders track all the messages', () async {
       await prepare();
       final initialMessages = List.generate(10, (i) => eg.streamMessage(id: 100 + i));
       await prepareMessages(foundOldest: false, messages: initialMessages);
@@ -434,6 +543,27 @@ void main() {
       recent_senders_test.checkMatchesMessages(store.recentSenders,
         [...initialMessages, ...oldMessages]);
     });
+
+    // TODO(#824): move this test
+    test('TODO fetchNewer recent senders track all the messages', () async {
+      await prepare(anchor: NumericAnchor(100));
+      final initialMessages = List.generate(10, (i) => eg.streamMessage(id: 100 + i));
+      await prepareMessages(foundOldest: true, foundNewest: false,
+        messages: initialMessages);
+
+      final newMessages = List.generate(10, (i) => eg.streamMessage(id: 110 + i))
+        // Not subscribed to the stream with id 10.
+        ..add(eg.streamMessage(id: 120, stream: eg.stream(streamId: 10)));
+      connection.prepare(json: newerResult(
+        anchor: 100, foundNewest: false,
+        messages: newMessages,
+      ).toJson());
+      await model.fetchNewer();
+
+      check(model).messages.length.equals(20);
+      recent_senders_test.checkMatchesMessages(store.recentSenders,
+        [...initialMessages, ...newMessages]);
+    });
   });
 
   group('MessageEvent', () {