Posting from Dart to a URL with a redirect: Since Dart's stock http package (and even the dio package) chose to handle redirects differently than basically every other language+package out there and disregard redirect responses returned to a POST request you have to do a little extra work. Worse than anything is every solution I have found was #…

post_request_with_redirect_result.dart

import 'dart:convert';
import 'dart:io';

import 'package:http/http.dart';

String url =
    "https://script.google.com/macros/s/YOUR_DEPLOYMENT_HASH_HERE/exec";

final data = {
  'foo': 456,
  'bar': 'that',
  'biz': 23,
};

void main(List<String> args) {
  postRequestWithRedirect(url, jsonEncode(data)).then((r) {
    print('code: ' + r.statusCode.toString());
    print('body: ' + r.responseBody.toString());
  });
}

class postRequestWithRedirectResult {
  num? statusCode;
  String? responseBody;
}


Future<postRequestWithRedirectResult> postRequestWithRedirect(
  String url,
  String body,
) async {
  var httpclient = HttpClient();
  HttpClientRequest request;
  HttpClientResponse response;
  postRequestWithRedirectResult result = postRequestWithRedirectResult();
  try {
    request = await httpclient.postUrl(Uri.parse(url));
    request.headers.contentLength = body.length;
    request.write(body);
    response = await request.close();
    final location = response.headers.value(HttpHeaders.locationHeader);
    if (response.statusCode == 302 && location != null) {
      request = await httpclient.getUrl(Uri.parse(location));
      response = await request.close();
      result.statusCode = response.statusCode;
      result.responseBody = await response.transform(utf8.decoder).join();
    } else {
      result.statusCode = response.statusCode;
      result.responseBody = await response.transform(utf8.decoder).join();
    }
  } finally {
    httpclient.close(force: true);
    return result;
  }
}