List flift/Dart/Firestore-在动画列表中按天分组文档

如何按天在用户之间分组消息？到目前为止，它只是添加到列表中并按天排序，但我似乎无法在过去几天发送的消息之间添加间隔。列表显示在

AnimatedList（）中

代码：

List<Message> _messages = [];

... 

_initMessages() { // Being called in the initState
  APIs().chats.messages(
    chatID: widget.chat.chatID,
    onEmpty: () {
      if (mounted)
        setState(() {
          this._isLoading = false;
        });
    },
    onAdded: (m) {
      if (mounted)
        setState(() {
          if (m.type == 'Image') {
            m.message = NetworkImage(m.message);
          }
          this._messages.add(m);
          this._messages.sort((a, b) => a.createdAt.compareTo(b.createdAt));
          if (mounted) this._listKey.currentState.insertItem(this._messages.length, duration: Duration(milliseconds: 500));
          this._isLoading = false;
        });


    },
    onModified: (m) {
      int i = this._messages.indexWhere((message) => message.messageID == m.messageID);
      if (mounted)
        setState(() {
          this._messages[i] = m;
          this._messages.sort((a, b) => a.createdAt.compareTo(b.createdAt));
          this._isLoading = false;
        });
    },
    onRemoved: (m) {
      int i = this._messages.indexWhere((message) => message.messageID == m.messageID);
      if (mounted)
        setState(() {
          this._messages.removeAt(i);
          this._messages.sort((a, b) => a.createdAt.compareTo(b.createdAt));
          this._isLoading = false;
        });
    },
    onFailure: (e) {
      print(e);
    });
}

这是一个分为两部分的问题，将文档按天分组，并尝试使其在
动画列表中工作。日期为Firestore时间戳类型

[VERBOSE-2:ui_dart_state.cc(148)] Unhandled Exception: 'package:flutter/src/widgets/animated_list.dart': Failed assertion: line 279 pos 12: 'itemIndex >= 0 && itemIndex <= _itemsCount': is not true.