概要

FlutterからGASにPOSTし、帰ってきたレスポンスJsonを扱いたい
POSTした際、GASのdoPost(e)は正常に受信できる。
GASの仕様上、ワンタイムURLにリダイレクトされる。

→Moved TemporarilyというHTMLのデータが返ってきてしまう。

-------[Flutter]Postコード---------

   HttpClient client = new HttpClient();
    HttpClientRequest request = await client.postUrl(Uri.parse('GASのURL'));
    request.followRedirects = true; //リダイレクトを許可する
    request.headers.contentType = ContentType("application", "json", charset: "utf-8");
    request.write(body);
    final HttpClientResponse response = await request.close();
    client.close();
    final String responseBody = await response.transform(utf8.decoder).join();
    debugPrint('$responseBody');
    final dynamic responseJson = jsonDecode(responseBody);
    debugPrint('$responseJson');

出力結果(responseBody)

flutter: <HTML>
flutter: <HEAD>
flutter: <TITLE>Moved Temporarily</TITLE>
flutter: </HEAD>
flutter: <BODY BGCOLOR="#FFFFFF" TEXT="#000000">
flutter: <H1>Moved Temporarily</H1>
flutter: The document has moved <A HREF="https://script.googleusercontent.com/macros/echo?user_content_key=リダイレクト先URLの為、割愛">here</A>.
flutter: </BODY>
flutter: </HTML>
flutter:

出力結果(responseJson)

[VERBOSE-2:ui_dart_state.cc(177)] Unhandled Exception: FormatException: Unexpected character (at character 1)
<HTML>
^

#0      _ChunkedJsonParser.fail (dart:convert-patch/convert_patch.dart:1404:5)
#1      _ChunkedJsonParser.parseNumber (dart:convert-patch/convert_patch.dart:1271:9)
#2      _ChunkedJsonParser.parse (dart:convert-patch/convert_patch.dart:936:22)
#3      _parseJson (dart:convert-patch/convert_patch.dart:40:10)
#4      JsonDecoder.convert (dart:convert/json.dart:505:36)
#5      JsonCodec.decode (dart:convert/json.dart:156:41)
#6      jsonDecode (dart:convert/json.dart:96:10)
#7      StartMenu._request (package:flutter_app/main.dart:62:34)
<asynchronous suspension>
#8      StartMenu.build._submission (package:flutter_app/main.dart:187:9)
#9      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:993:19)
#10     _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:1111:38)
#11     GestureRecognizer.invokeCallback (package:flutter/s<…>