msglist: add scroll-to-bottom button to MessageList

lib/widgets/message_list.dart

@@ -1,3 +1,5 @@
+import 'dart:math';
+
 import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
@@ -58,7 +60,6 @@ class _MessageListPageState extends State<MessageListPage> {
 
               child: Expanded(
                 child: MessageList(narrow: widget.narrow))),
-
             ComposeBox(controllerKey: _composeBoxKey, narrow: widget.narrow),
           ]))));
   }
@@ -97,7 +98,6 @@ class MessageListAppBarTitle extends StatelessWidget {
   }
 }
 
-
 class MessageList extends StatefulWidget {
   const MessageList({super.key, required this.narrow});
 
@@ -109,6 +109,14 @@ class MessageList extends StatefulWidget {
 
 class _MessageListState extends State<MessageList> {
   MessageListView? model;
+  final ScrollController scrollController = ScrollController();
+  final ValueNotifier<bool> _scrollToBottomVisibleValue = ValueNotifier<bool>(false);
+
+  @override
+  void initState() {
+    super.initState();
+    scrollController.addListener(_scrollChanged);
+  }
 
   @override
   void didChangeDependencies() {
@@ -126,6 +134,8 @@ class _MessageListState extends State<MessageList> {
   @override
   void dispose() {
     model?.dispose();
+    scrollController.dispose();
+    _scrollToBottomVisibleValue.dispose();
     super.dispose();
   }
 
@@ -142,6 +152,23 @@ class _MessageListState extends State<MessageList> {
     });
   }
 
+  void _adjustButtonVisibility(ScrollMetrics scrollMetrics) {
+    if (scrollMetrics.extentBefore == 0) {
+      _scrollToBottomVisibleValue.value = false;
+    } else {
+      _scrollToBottomVisibleValue.value = true;
+    }
+  }
+
+  void _scrollChanged() {
+    _adjustButtonVisibility(scrollController.position);
+  }
+
+  bool _metricsChanged(ScrollMetricsNotification scrollMetricsNotification) {
+    _adjustButtonVisibility(scrollMetricsNotification.metrics);
+    return true;
+  }
+
   @override
   Widget build(BuildContext context) {
     assert(model != null);
@@ -161,7 +188,18 @@ class _MessageListState extends State<MessageList> {
           child: Center(
             child: ConstrainedBox(
               constraints: const BoxConstraints(maxWidth: 760),
-              child: _buildListView(context))))));
+              child: NotificationListener<ScrollMetricsNotification>(
+                onNotification: _metricsChanged,
+                child: Stack(
+                  children: <Widget>[
+                    _buildListView(context),
+                    Positioned(
+                      bottom: 0,
+                      right: 0,
+                      child: ScrollToBottomButton(
+                        scrollController: scrollController,
+                        visibleValue: _scrollToBottomVisibleValue)),
+                  ])))))));
   }
 
   Widget _buildListView(context) {
@@ -179,6 +217,7 @@ class _MessageListState extends State<MessageList> {
         _ => ScrollViewKeyboardDismissBehavior.manual,
       },
 
+      controller: scrollController,
       itemCount: length,
       // Setting reverse: true means the scroll starts at the bottom.
       // Flipping the indexes (in itemBuilder) means the start/bottom
@@ -194,6 +233,39 @@ class _MessageListState extends State<MessageList> {
   }
 }
 
+class ScrollToBottomButton extends StatelessWidget {
+  const ScrollToBottomButton({super.key, required this.scrollController, required this.visibleValue});
+
+  final ValueNotifier<bool> visibleValue;
+  final ScrollController scrollController;
+
+  Future<void> _navigateToBottom() async {
+    final distance = scrollController.position.pixels;
+    final durationMsAtSpeedLimit = (1000 * distance / 8000).ceil();
+    final durationMs = max(300, durationMsAtSpeedLimit);
+    scrollController.animateTo(
+      0,
+      duration: Duration(milliseconds: durationMs),
+      curve: Curves.ease);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return ValueListenableBuilder<bool>(
+      valueListenable: visibleValue,
+      builder: (BuildContext context, bool value, Widget? child) {
+        return (value && child != null) ? child : const SizedBox.shrink();
+      },
+      // TODO: fix hardcoded values for size and style here
+      child: IconButton(
+        tooltip: "Scroll to bottom",
+        icon: const Icon(Icons.expand_circle_down_rounded),
+        iconSize: 40,
+        color: const HSLColor.fromAHSL(0.5,240,0.96,0.68).toColor(),
+        onPressed: _navigateToBottom));
+  }
+}
+
 class MessageItem extends StatelessWidget {
   const MessageItem({
     super.key,

test/widgets/message_list_test.dart

@@ -0,0 +1,125 @@
+import 'package:checks/checks.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:zulip/api/model/model.dart';
+import 'package:zulip/api/route/messages.dart';
+import 'package:zulip/model/narrow.dart';
+import 'package:zulip/widgets/message_list.dart';
+import 'package:zulip/widgets/sticky_header.dart';
+import 'package:zulip/widgets/store.dart';
+
+import '../api/fake_api.dart';
+import '../example_data.dart' as eg;
+import '../model/binding.dart';
+
+Future<void> setupMessageListPage(WidgetTester tester, {
+  required Narrow narrow,
+}) async {
+  addTearDown(TestZulipBinding.instance.reset);
+  addTearDown(tester.view.resetPhysicalSize);
+
+  tester.view.physicalSize = const Size(600, 800);
+
+  await TestZulipBinding.instance.globalStore.add(eg.selfAccount, eg.initialSnapshot());
+  final store = await TestZulipBinding.instance.globalStore.perAccount(eg.selfAccount.id);
+  final connection = store.connection as FakeApiConnection;
+
+  // prepare message list data
+  final List<StreamMessage> messages = List.generate(10, (index) {
+    return eg.streamMessage(id: index);
+  });
+  connection.prepare(json: GetMessagesResult(
+    anchor: messages[0].id,
+    foundNewest: true,
+    foundOldest: true,
+    foundAnchor: true,
+    historyLimited: false,
+    messages: messages,
+  ).toJson());
+
+  await tester.pumpWidget(
+    MaterialApp(
+      home: GlobalStoreWidget(
+        child: PerAccountStoreWidget(
+          accountId: eg.selfAccount.id,
+          child: MessageListPage(narrow: narrow)))));
+
+  // global store, per-account store, and message list get loaded
+  await tester.pumpAndSettle();
+}
+
+void main() {
+  TestZulipBinding.ensureInitialized();
+
+  group('ScrollToBottomButton interactions', () {
+    ScrollController? findMessageListScrollController(WidgetTester tester) {
+      final stickyHeaderListView = tester.widget<StickyHeaderListView>(find.byType(StickyHeaderListView));
+      return stickyHeaderListView.controller;
+    }
+
+    bool isButtonVisible(WidgetTester tester) {
+      return tester.any(find.descendant(
+        of: find.byType(ScrollToBottomButton),
+        matching: find.byTooltip("Scroll to bottom")));
+    }
+
+    testWidgets('scrolling changes visibility', (WidgetTester tester) async {
+      final stream = eg.stream();
+      await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId));
+
+      final scrollController = findMessageListScrollController(tester)!;
+
+      // Initial state should be not visible, as the message list renders with latest message in view
+      check(isButtonVisible(tester)).equals(false);
+
+      scrollController.jumpTo(600);
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(true);
+
+      scrollController.jumpTo(0);
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(false);
+    });
+
+    testWidgets('dimension updates changes visibility', (WidgetTester tester) async {
+      final stream = eg.stream();
+      await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId));
+
+      final scrollController = findMessageListScrollController(tester)!;
+
+      // Initial state should be not visible, as the message list renders with latest message in view
+      check(isButtonVisible(tester)).equals(false);
+
+      scrollController.jumpTo(600);
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(true);
+
+      tester.view.physicalSize = const Size(2000, 40000);
+      await tester.pump();
+      // Dimension changes use NotificationListener<ScrollMetricsNotification
+      // which has a one frame lag. If that ever gets resolved this extra pump
+      // would ideally be removed
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(false);
+    });
+
+    testWidgets('button functionality', (WidgetTester tester) async {
+      final stream = eg.stream();
+      await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId));
+
+      final scrollController = findMessageListScrollController(tester)!;
+
+      // Initial state should be not visible, as the message list renders with latest message in view
+      check(isButtonVisible(tester)).equals(false);
+
+      scrollController.jumpTo(600);
+      await tester.pump();
+      check(isButtonVisible(tester)).equals(true);
+
+      await tester.tap(find.byType(ScrollToBottomButton));
+      await tester.pumpAndSettle();
+      check(isButtonVisible(tester)).equals(false);
+      check(scrollController.position.pixels).equals(0);
+    });
+  });
+}