【flutter_chat_ui】chatGPT-APIからの戻り値を綺麗に表示できない

          // 'content'を取得して文字列を追加していく
          List<dynamic> choices = data["choices"];
          Map<String, dynamic> choice = choices[0];
          Map<String, dynamic> delta = choice["delta"];
          String content = delta["content"];

          returnMessageText += content;

          final returnMessage = types.TextMessage(
            author: _ai,
            createdAt: DateTime.now().millisecondsSinceEpoch,
            id: randomString(),
            text: returnMessageText,
          );
          setState(() {
            _messages[0] = returnMessage;
          });

import "dart:convert";
import 'package:flutter/material.dart';
import 'dart:math';

import "package:http/http.dart" as http;
import 'package:flutter_chat_types/flutter_chat_types.dart' as types;
import 'package:flutter_chat_ui/flutter_chat_ui.dart';
import 'package:flutter_dotenv/flutter_dotenv.dart';

/* 
 * チャット画面
 */
String randomString() {
  final random = Random.secure();
  final values = List<int>.generate(16, (i) => random.nextInt(255));
  return base64UrlEncode(values);
}

/*
 * チャットルーム画面を生成するクラス
 */
class ExecuteAIChatPage extends StatefulWidget {
  final bool isQuestion;

  const ExecuteAIChatPage({Key? key, required this.isQuestion})
      : super(key: key);

  @override
  State<ExecuteAIChatPage> createState() => ExecuteAIChatState();
}

/*
 * チャットルーム画面ウィジェットのクラス
 */
class ExecuteAIChatState extends State<ExecuteAIChatPage> {
  final List<types.Message> _messages = [];
  final _user = const types.User(id: '82091008-a484-4a89-ae75-a22bf8d6f3ac');
  final _ai = const types.User(id: 'otheruser');
  String returnMessageText = "";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Chat(
        user: _user,
        messages: _messages,
        onSendPressed: _handleSendPressed,
        l10n: const ChatL10nEn(
            emptyChatPlaceholder: 'メッセージがありません。\nAIに質問してみましょう',
            inputPlaceholder: 'AIに質問する'),
      ),
    );
  }

  void _handleSendPressed(types.PartialText message) async {
    final textMessage = types.TextMessage(
      author: _user,
      createdAt: DateTime.now().millisecondsSinceEpoch,
      id: randomString(),
      text: message.text,
    );
    setState(() {
      _messages.insert(0, textMessage);
    });

    final returnMessage = types.TextMessage(
      author: _ai,
      createdAt: DateTime.now().millisecondsSinceEpoch,
      id: randomString(),
      text: "",
    );
    setState(() {
      _messages.insert(0, returnMessage);
    });
    sendChatCompletionRequest(message.text);
  }

  /*
   * chatgpt-API アクセス
   */
  void sendChatCompletionRequest(String text) {
    final client = http.Client();

    var request = http.Request(
      'POST',
      Uri.parse('https://api.openai.com/v1/chat/completions'),
    );

    Map<String, String> header = {
      'accept': 'text/event-stream',
      'Authorization': 'Bearer ${dotenv.env['OPENAI_APIKEY']!}',
      'Content-Type': 'application/json'
    };

    header.forEach((key, value) {
      request.headers[key] = value;
    });

    Map<String, dynamic> body = {
      "model": "gpt-3.5-turbo",
      "messages": [
        {"role": "user", "content": text},
      ],
      "stream": true
    };

    request.body = jsonEncode(body);

    Future<http.StreamedResponse> response = client.send(request);

    response.asStream().listen((data) {
      // ByteStreamをStringに変換し、改行で分割して1行ずつ処理する
      data.stream
          .transform(const Utf8Decoder())
          .transform(const LineSplitter())
          .listen(
        (dataLine) {
          // dataLineが空の場合や、[DONE]が返ってきた場合は早期リターン
          if (dataLine.isEmpty || dataLine == 'data: [DONE]') {
            return;
          }

          // dataLineの中身は'data: 'で始まっているので、それを削除してからMapに変換
          final map = dataLine.replaceAll('data: ', '');
          Map<String, dynamic> data = json.decode(map);
          // finish_reasonがstopの場合は早期リターン
          if (data['choices'][0]['finish_reason'] == 'stop') {
            return;
          }

          // 'content'を取得して文字列を追加していく
          List<dynamic> choices = data["choices"];
          Map<String, dynamic> choice = choices[0];
          Map<String, dynamic> delta = choice["delta"];
          String content = delta["content"];

          returnMessageText += content;

          final returnMessage = types.TextMessage(
            author: _ai,
            createdAt: DateTime.now().millisecondsSinceEpoch,
            id: randomString(),
            text: returnMessageText,
          );
          setState(() {
            _messages[0] = returnMessage;
          });
        },
      );
    });
  }
}