Ubuntu – Flutter app is working in debug mode, however build/web version is unable to communicate with the server because of CORS policy

I’m trying to learn Webdev and as part of my first project, I have built a simple chat-bot-app. The app consists of a Python-Flask backend (running on localhost/5000) and a Flutter app front end. The Flutter app takes our input, sends it to the Flask server, and will print us the response.

When I run the app in debug mode locally, it is working fine as expected. However, when I build a web version of the app using

flutter build web

and then try to run the app from /build/web/ by creating a Python server

python -m http.server 8081

There is a problem, the web app is launching in the browser: however, now it can not send and receive messages from the server running at localhost/5000. I’m getting an error:

flutter.js:368 Exception while loading service worker: Error: Service Worker API unavailable.
The current context is NOT secure.
Read more: https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts
    at FlutterServiceWorkerLoader.loadServiceWorker (flutter.js:130:11)
    at FlutterLoader.loadEntrypoint (flutter.js:366:33)
    at (index):46:23
(anonymous) @ flutter.js:368
0.0.0.0/:1 Access to XMLHttpRequest at 'http://127.0.0.1:5000/api/chat' from 
origin 'http://0.0.0.0:8081' has been blocked by CORS policy:
The request client is not a secure context and the resource is in more-private address space `local`.
main.dart.js:6506 Uncaught Error
    at Object.alx (main.dart.js:6573:24)
    at TE.$1 (main.dart.js:73160:53)
    at abl.adR (main.dart.js:36153:34)
    at abl.DP (main.dart.js:36155:21)
    at a95.$0 (main.dart.js:35896:11)
    at Object.r4 (main.dart.js:5825:40)
    at az.mx (main.dart.js:35827:3)
    at Object.ayw (main.dart.js:5908:8)
    at a4N.$1 (main.dart.js:35938:9)
    at a8N.$1 (main.dart.js:39265:21)
127.0.0.1:5000/api/chat:1     Failed to load resource: net::ERR_FAILED

It basically says I can not communicate with ‘http://127.0.0.1:5000/api/chat’ from origin ‘http://0.0.0.0:8081’ due to CORS security policy. When I try to run the same web app, by disabling the browser security, I’m no longer getting this error and the web app is able to communicate with the server.

chromium-browser --disable-web-security

How can I fix this without sacrificing security? I wish to deploy the app online after fixing this.

Why is this happening?
Here is the link to the Github repo which has the full code:
https://github.com/sri-gpt/simple_chat_app

I’m using:

• Ubuntu version 22.04 LTS,
• Flutter 3.13.0-0.2.pre
• Dart 3.1.0
• DevTools 2.25.0
• Flask 2.3.2
• Flask-Cors 4.0.0
• Python 3.9

Sample code:
Python Flask server

from flask import Flask, request,  jsonify
from flask_cors import CORS


app = Flask(__name__)

CORS(app)

@app.route('/')
def home():
    return 'ServerDEployed'

@app.route('/about')
def about():
    return 'About'

@app.route('/api/chat', methods=['POST'])
def chat():
    data = request.get_json()
    input_text = data.get('input_text')
    return jsonify({"response_mal": str("response_recived:")+str(input_text)})

if __name__ == '__main__':
    app.run(debug=True)

Here is the main.dart file of the Flutter app:

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

void main() {
  runApp(const ChatApp());
}

class ChatApp extends StatelessWidget {
  const ChatApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Chat_app',
      theme: ThemeData(
        primarySwatch: Colors.indigo, // Change primary color
        fontFamily: 'Montserrat', // Add custom font (Montserrat can be replaced with your preferred font)
      ),
      home: const MyHomePage(title: 'Chat_app'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final _messages = <Map<String, dynamic>>[];
  final _controller = TextEditingController();
  final double fontText = 24.0; // <-- This is your pseudo variable for font size

  Future<void> _sendText() async {
    if (_controller.text.isEmpty) {
      return;
    }

    setState(() {
      _messages.add({"text": _controller.text, "type": "outgoing"});
    });

    final url = Uri.parse('http://127.0.0.1:5000/api/chat');
    final response = await http.post(
      url,
      headers: <String, String>{
        'Content-Type': 'application/json; charset=UTF-8',
      },
      body: jsonEncode(<String, String>{
        'input_text': _controller.text,
      }),
    );

    if (response.statusCode == 200) {
      final Map<String, dynamic> data = jsonDecode(response.body);

      setState(() {
        _messages.add({"text": data["response_mal"], "type": "incoming"});
        _controller.text = "";
      });
    } else {
      throw Exception('Failed to send message');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title, style: TextStyle(fontSize: fontText)), // <-- Apply font size here.
      ),
      body: Column(
        children: <Widget>[
          Expanded(
            child: ListView.builder(
              padding: const EdgeInsets.all(8),
              itemCount: _messages.length,
              itemBuilder: (_, int index) {
                final message = _messages[index];
                final isOutgoing = message["type"] == "outgoing";

                return Container(
                  margin: const EdgeInsets.symmetric(vertical: 5),
                  padding: const EdgeInsets.all(10),
                  alignment:
                      isOutgoing ? Alignment.centerRight : Alignment.centerLeft,
                  child: Text(
                    message["text"],
                    style: TextStyle(fontSize: fontText), // <-- Apply font size here.
                  ),
                  decoration: BoxDecoration(
                    color: isOutgoing ? Colors.orange[100] : Colors.blue[100],
                    borderRadius: BorderRadius.circular(5),
                    boxShadow: [
                      BoxShadow(
                        color: Colors.black12,
                        blurRadius: 5,
                        offset: Offset(0, 2),
                      ),
                    ],
                  ),
                );
              },
            ),
          ),
          Row(
            children: <Widget>[
              Expanded(
                child: Padding(
                  padding: EdgeInsets.all(8.0),
                  child: TextField(
                    controller: _controller,
                    onSubmitted: (_) => _sendText(),
                    decoration: InputDecoration(
                      border: OutlineInputBorder(),
                      labelText: 'Enter your message',
                    ),
                    style: TextStyle(fontSize: fontText), // <-- Apply font size here.
                  ),
                ),
              ),
              Padding(
                padding: EdgeInsets.all(8.0),
                child: ElevatedButton( // Change the send button to ElevatedButton
                  onPressed: _sendText,
                  child: Icon(Icons.send),
                ),
              ),
            ],
          ),
        ],
      ),
    );
  }
}