ClientException: Ошибка XMLHttpRequest. Flutter WEB

Gotcha! You're running into a ClientException: XMLHttpRequest error in your Flutter Web app when trying to communicate with your Django backend. This happens especially when the backend goes down, and even after you catch the error, your app stops making any more API calls once the server is back up. Let’s break down why this might be happening and how you can fix it.

Why This Happens

1. HTTP Client State Issues: Your HTTP client might be getting stuck in an error state after the first failed request, preventing any further requests.

2. State Management Problems: Your app's state might be updating in a way that stops further API calls after an error occurs.

3. Browser Caching or Network State: The browser might be caching the failed requests or getting into a bad network state that blocks future requests.

4. CORS Issues: After restarting your server, CORS settings might not be correctly configured, blocking your requests.

How to Fix It

1. Proper Exception Handling

Make sure you're handling exceptions properly without messing up your HTTP client or app state. Here’s a basic example:

import 'package:http/http.dart' as http;

Future<void> fetchData() async {
  try {
    final response = await http.get(Uri.parse('https://your-api-endpoint'));
    if (response.statusCode == 200) {
      // Handle the data
    } else {
      // Handle server errors
    }
  } catch (e) {
    if (e is http.ClientException) {
      // Handle client exceptions, maybe show a message
      print('ClientException: $e');
    } else {
      // Handle other exceptions
      print('Other Exception: $e');
    }
  }
}

2. Implement Retry Logic

Add a retry mechanism to try reconnecting after a failure. This helps your app recover once the server is back up.

import 'package:http/http.dart' as http;
import 'dart:async';

Future<void> fetchDataWithRetry({int retries = 3, Duration delay = const Duration(seconds: 2)}) async {
  for (int attempt = 0; attempt < retries; attempt++) {
    try {
      final response = await http.get(Uri.parse('https://your-api-endpoint'));
      if (response.statusCode == 200) {
        // Handle the data
        return;
      } else {
        throw Exception('Server error: ${response.statusCode}');
      }
    } catch (e) {
      if (attempt == retries - 1) {
        print('Failed after $retries attempts: $e');
      } else {
        await Future.delayed(delay);
      }
    }
  }
}

3. Reset the HTTP Client

If you're using a custom HTTP client, reset it after a failure to clear any bad state.

import 'package:http/http.dart' as http;

http.Client client = http.Client();

Future<void> fetchData() async {
  try {
    final response = await client.get(Uri.parse('https://your-api-endpoint'));
    if (response.statusCode == 200) {
      // Handle the data
    } else {
      // Handle server errors
    }
  } catch (e) {
    if (e is http.ClientException) {
      print('ClientException: $e');
      client.close();
      client = http.Client();
    } else {
      print('Other Exception: $e');
    }
  }
}

4. Check Your CORS Settings

Make sure your Django backend has the right CORS settings. Using django-cors-headers is a good way to manage this.

# settings.py

INSTALLED_APPS = [
    ...
    'corsheaders',
    ...
]

MIDDLEWARE = [
    'corsheaders.middleware.CorsMiddleware',
    ...
]

CORS_ALLOWED_ORIGINS = [
    'https://your-flutter-web-app-domain',
]

Restart your Django server after making these changes and verify the CORS headers using your browser’s developer tools.

5. Inspect Network and Cache in the Browser

Use the browser’s developer tools to:

• Disable Caching: Prevent the browser from using cached responses that might be causing issues.
• Clear Cache: Sometimes clearing the browser cache can resolve weird issues.

6. Use a Separate HTTP Client for Each Request

This can help avoid state issues with a shared client.

import 'package:http/http.dart' as http;

Future<void> fetchData() async {
  final client = http.Client();
  try {
    final response = await client.get(Uri.parse('https://your-api-endpoint'));
    if (response.statusCode == 200) {
      // Handle the data
    } else {
      // Handle server errors
    }
  } catch (e) {
    print('Exception: $e');
  } finally {
    client.close();
  }
}

7. Global Error Handler

Set up a global error handler to catch any uncaught exceptions and keep your app running smoothly.

void main() {
  runZonedGuarded(() {
    runApp(MyApp());
  }, (error, stackTrace) {
    print('Uncaught error: $error');
  });
}

8. Use Advanced HTTP Libraries Like Dio

Libraries like dio offer more features for handling requests and errors, making it easier to manage retries and interceptors.

import 'package:dio/dio.dart';

final Dio dio = Dio();

Future<void> fetchData() async {
  try {
    final response = await dio.get('https://your-api-endpoint');
    // Handle the data
  } on DioError catch (e) {
    if (e.type == DioErrorType.other) {
      print('ClientException: ${e.message}');
    } else {
      print('DioError: ${e.response?.statusCode}');
    }
  }
}

9. Ensure Server is Up and Running Properly

After restarting your server, double-check that it's running correctly by sending requests directly through the browser or tools like curl or Postman.

10. Check Browser Console Logs

Look at the browser’s console logs for any additional errors or warnings that could give you more clues about what’s going wrong.

Wrapping It Up

The ClientException: XMLHttpRequest error in Flutter Web usually points to some hiccup on the client side when trying to reach your backend. To keep your app making API calls smoothly even after encountering errors:

1. Handle exceptions properly so they don’t mess up your app’s state.
2. Implement retry logic to reconnect when the server comes back.
3. Reset your HTTP client if needed.
4. Double-check your CORS settings on the Django backend.
5. Inspect network and cache issues in the browser.
6. Use separate HTTP clients for each request to avoid shared state problems.
7. Set up a global error handler to catch unexpected issues.
8. Consider using advanced HTTP libraries like dio for better error and request management.