Retrieve number of members who haven't read a message

try {
  // Call the markAsRead() method when the current user views unread messages in a group channel.
  await groupChannel.markAsRead();
} catch (e) {
  // Handle error.
}

// To listen to an update from all the other channel members' client apps, implement onReadStatusUpdated() with actions to do when notified.
class MyGroupChannelHandler with GroupChannelHandler {
  @override
  void onReadStatusUpdated(GroupChannel channel) {
    if (currentGroupChannel.channelUrl == channel.channelUrl) {
      messages.forEach((message) {
        final unreadCount = channel.getUnreadMembers(message);
        if (unreadCount <= 0) {
          // 0 means that all members have read this message.
        } else {
          // If the value isn't 0, 
          // some members haven't read this message.
        }
      });
    }
  }
}