stream_chat_jump_to_unread.dart

import 'package:flutter/material.dart';
import 'package:stream_chat_flutter/scrollable_positioned_list/scrollable_positioned_list.dart';
import 'package:stream_chat_flutter/stream_chat_flutter.dart';

Future<void> main() async {
  final client = StreamChatClient(
    'YOUR-KEY',
    logLevel: Level.INFO,
  );

  await client.connectUser(YOUR-USER);

  runApp(
    MyApp(
      client: client,
    ),
  );
}


class MyApp extends StatelessWidget {
  const MyApp({
    Key? key,
    required this.client,
  }) : super(key: key);

  final StreamChatClient client;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      builder: (context, child) => StreamChat(
        client: client,
        child: child,
      ),
      home: const ChannelListPage(),
    );
  }
}

class ChannelListPage extends StatelessWidget {
  const ChannelListPage({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ChannelsBloc(
        child: ChannelListView(
          filter: Filter.in_(
            'members',
            [StreamChat.of(context).currentUser!.id],
          ),
          sort: const [SortOption('last_message_at')],
          limit: 20,
          channelWidget: const ChannelPage(),
        ),
      ),
    );
  }
}

class ChannelPage extends StatelessWidget {
  const ChannelPage({
    Key? key,
    this.jumpToLastUnread = 0,
  }) : super(key: key);

  final int jumpToLastUnread;

  @override
  Widget build(BuildContext context) {
    final channel = StreamChannel.of(context).channel;

    return Scaffold(
      appBar: const ChannelHeader(),
      body: Column(
        children: <Widget>[
          Expanded(
            child: MyMessageList(
              channel: channel,
            ),
          ),
          const MessageInput(),
        ],
      ),
    );
  }
}

class MyMessageList extends StatefulWidget {
  const MyMessageList({
    Key? key,
    required this.channel,
  }) : super(key: key);

  final Channel channel;

  @override
  State<MyMessageList> createState() => _MyMessageListState();
}

class _MyMessageListState extends State<MyMessageList> {
  final controller = ItemScrollController();

  @override
  Widget build(BuildContext context) => Stack(
        children: [
          MessageListView(
            showScrollToBottom: false,
            scrollController: controller,
          ),
          Align(
            alignment: Alignment.bottomRight,
            child: Padding(
              padding: const EdgeInsets.all(16),
              child: ScrollToUnreadButton(
                channel: widget.channel,
                scrollController: controller,
              ),
            ),
          )
        ],
      );
}

class ScrollToUnreadButton extends StatelessWidget {
  const ScrollToUnreadButton({
    Key? key,
    required this.channel,
    required this.scrollController,
  }) : super(key: key);

  final Channel channel;

  final ItemScrollController scrollController;

  Future<void> _scrollTo(
    BuildContext context, {
    required int index,
    required double alignment,
  }) async {
    if (!channel.state!.isUpToDate) {
      await StreamChannel.of(context).loadChannelAtMessage(
        null,
        before: index + 25,
      );

      WidgetsBinding.instance?.addPostFrameCallback((_) {
        scrollController.jumpTo(index: index);
      });
    } else {
      scrollController.scrollTo(
        index: index,
        duration: const Duration(seconds: 1),
        curve: Curves.easeInOut,
      );
    }
  }

  @override
  Widget build(BuildContext context) => BetterStreamBuilder<int>(
        stream: channel.state?.unreadCountStream,
        initialData: channel.state?.unreadCount,
        builder: (context, count) {
          if (count == 0 || count == 0) {
            return const SizedBox.shrink();
          }
          return FloatingActionButton(
            onPressed: () {
              _scrollTo(context, index: count, alignment: 0);
            },
            child: const Icon(Icons.arrow_downward),
          );
        },
      );
}