Dio().fetch() hangs in isolate

[mikes222]

Package

dio

Version

5.8.0+1

Operating-System

Windows

Adapter

Default Dio

Output of flutter doctor -v

[√] Flutter (Channel stable, 3.29.3, on Microsoft Windows [Version 10.0.22631.5189], locale de-AT) [361ms]
    • Flutter version 3.29.3 on channel stable at C:\develop\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision ea121f8859 (3 weeks ago), 2025-04-11 19:10:07 +0000
    • Engine revision cf56914b32
    • Dart version 3.7.2
    • DevTools version 2.42.3

[√] Windows Version (11 Pro 64-bit, 23H2, 2009) [2,7s]

[!] Android toolchain - develop for Android devices (Android SDK version 35.0.0) [283ms]
    • Android SDK at C:\Users\micro\AppData\Local\Android\sdk
    X cmdline-tools component is missing
      Run `path/to/sdkmanager --install "cmdline-tools;latest"`
      See https://developer.android.com/studio/command-line for more details.
    X Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/to/windows-android-setup for more details.

[√] Chrome - develop for the web [143ms]
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.8.2) [142ms]
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.8.34322.80
    • Windows 10 SDK version 10.0.22621.0

[√] Android Studio (version 2024.3) [13ms]
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 21.0.6+-13355223-b631.42)

[√] Connected device (4 available) [214ms]
    • sdk gphone64 x86 64 (mobile) • emulator-5554 • android-x64    • Android 15 (API 35) (emulator)
    • Windows (desktop)            • windows       • windows-x64    • Microsoft Windows [Version 10.0.22631.5189]
    • Chrome (web)                 • chrome        • web-javascript • Google Chrome 135.0.7049.115
    • Edge (web)                   • edge          • web-javascript • Microsoft Edge 136.0.3240.50

[√] Network resources [656ms]
    • All expected network resources are available.

! Doctor found issues in 1 category.
P

Dart Version

Dart SDK version: 3.7.2 (stable) (Tue Mar 11 04:27:50 2025 -0700) on "windows_x64"

Steps to Reproduce

I have created a method to fetch data from http(s) server. When calling this method in the main thread everything works. Unfortunately if I call the very same method in an isolate the last line in the following code snippet Response response = await responseFuture; hangs indefinitely. I am unable to debug what exactly causes this problem and therefore would like to get some hints how to find the root cause.

RequestOptions requestOptions = RequestOptions(
  path: source,
  method: "GET",
  connectTimeout: const Duration(seconds: 35),
  receiveTimeout: const Duration(seconds: 40),
  sendTimeout: const Duration(seconds: 62),
  //receiveDataWhenStatusError: true,
  followRedirects: true,
  responseType: ResponseType.stream,
);

final Dio _httpClient = Dio();

print("vor response");
Future<Response> responseFuture = _httpClient.fetch(requestOptions);
Response response = await responseFuture;

Expected Result

Not hanging in an isolate created with Isolate.spawn

Actual Result

hanging when awaiting the result in the isolate

[AlexV525]

Please try to provide a minimal reproducible example using Isolate.spawn.

[mikes222]

1. Please create a flutter project and insert the code below in a separate dart file (e.g. dio_test.dart).
2. add dio to pubspec.yaml
3. In your initState() method add the following line: DioTest.inIsolate = false;
4. In your onPressed() method add either

    DioTest dioTest = DioTest();
    dioTest.perform();

or

IsolateDioTest dioTest = IsolateDioTest();
dioTest.perform();

---

import 'dart:async';
import 'dart:isolate';
import 'dart:ui';

import 'package:dio/dio.dart';

/// This is the wrapper method to perform a computation in an isolate.
@pragma('vm:entry-point')
class IsolateDioTest {
  Future<void> perform() async {
    return await FlutterIsolateInstance.isolateCompute(performStatic);
  }

  /// The entry point for the isolate.
  @pragma('vm:entry-point')
  static Future<void> performStatic() async {
    DioTest dioTest = DioTest();
    dioTest.perform();
  }
}

//////////////////////////////////////////////////////////////////////////////

typedef EntryPoint<V> = Future<V> Function();

//////////////////////////////////////////////////////////////////////////////

/// One instance for an isolate. Do not instantiate directly. Use [FlutterIsolateInstance.isolateCompute] instead.
class FlutterIsolateInstance<V> {
  SendPort? _sendPort;

  Isolate? _isolate;

  // complete() will be called if the isolate is ready to receive commands.
  final Completer _isolateCompleter = Completer();

  // complete() will be called if the isolate returns a result.
  Completer<V>? _isolateResultCompleter;

  FlutterIsolateInstance._();

  void dispose() {
    _isolate?.kill();
    _isolate = null;
    _sendPort = null;
  }

  /// Performs a single computation in an isolate and disposes the isolate afterwards.
  static Future<V> isolateCompute<V>(EntryPoint<V> entryPoint) async {
    FlutterIsolateInstance instance = FlutterIsolateInstance<V>._();
    await instance.start();
    V result = await instance.compute(entryPoint);
    instance.dispose();
    return result;
  }

  /// Starts a new isolate. Do not call this directly. Use [isolateCompute] instead. This runs in the main isolate.
  Future<void> start() async {
    ReceivePort receivePort = ReceivePort();

    _isolate = await Isolate.spawn<SendPort>(isolateEntryPoint, receivePort.sendPort);
    // let the listener run in background of the main isolate
    unawaited(_listenToIsolate(receivePort));
    return _isolateCompleter.future;
  }

  /// listen to the results of an isolate. This method runs in the main isolate.
  Future<void> _listenToIsolate(ReceivePort receivePort) async {
    await for (var data in receivePort) {
      //tileCache.addTileBitmap(job.tile, tileBitmap);
      //print("received from isolate: ${data.toString()}");
      if (data is SendPort) {
        // Receive the SendPort from the Isolate
        _sendPort = data;
        _isolateCompleter.complete();
      } else {
        _isolateResultCompleter!.complete(data);
      }
    }
  }

  /// Starts a single computation in an isolate. Do not call this method
  /// directly. Use [isolateCompute] instead.
  /// This method runs in the main isolate.
  Future<V> compute(EntryPoint<V> entryPoint) {
    final entryPointHandle = PluginUtilities.getCallbackHandle(entryPoint)!.toRawHandle();
    _sendPort!.send(entryPointHandle);
    _isolateResultCompleter = Completer();
    return _isolateResultCompleter!.future;
  }

  //////////////////////////////////////////////////////////////////////////////

  /// The first entry point called in the isolate. It establishes the communication with the main isolate, instantiates the isolate's class if necessary and
  /// waits for computational commands.
  @pragma('vm:entry-point')
  static Future<void> isolateEntryPoint(SendPort sendPort) async {
    // Open the ReceivePort to listen for incoming messages
    var receivePort = new ReceivePort();
    // Send message to main Isolate and inform it about this receiver
    sendPort.send(receivePort.sendPort);

    // Listen for messages from main isolate and returns the results.
    await for (var data in receivePort) {
      if (data is int) {
        try {
          final entryPointHandle = CallbackHandle.fromRawHandle(data);
          final entryPoint = PluginUtilities.getCallbackFromHandle(entryPointHandle);
          entryPoint!().then((result) {
            // return result to main isolate
            sendPort.send(result);
          });
        } catch (error, stacktrace) {
          print("Error: $error");
        }
      }
    }
    return;
  }
}

//////////////////////////////////////////////////////////////////////////////

class DioTest {
  // variable will be set to false in the main method
  static bool inIsolate = true;

  Future<void> perform() async {
    RequestOptions requestOptions = RequestOptions(path: "https://www.google.com", method: "GET");

    final Dio httpClient = Dio();

    Future<Response> responseFuture = httpClient.fetch(requestOptions);
    print("Request (in Isolate: $inIsolate): $requestOptions");
    Response response = await responseFuture;
    print("Response (in Isolate: $inIsolate): ${response.headers}");
  }
}

[mikes222]

I found the problem myself. In

/// The entry point for the isolate.
@pragma('vm:entry-point')
static Future<void> performStatic() async {
  DioTest dioTest = DioTest();
  dioTest.perform();
}

The last statement should rather be await dioTest.perform();

Because of this the method returned instantly and the isolate was killed in the dispose() method :-(

Therefore I will close this issue.

[AlexV525]

Cool, glad you've found the cause without much pain.