Isolate Manager


  • An easy way to create multiple isolates for a function, also keep it active to send and receive data multiple times.

  • Supports Worker on Web (Worker is the real Isolate on Web). The plugin will use Future (and Stream) if Worker is unavailable on the working browser or is not configured.

  • Multiple computes are allowed because the plugin will queue the input data and sends it to free isolate later.

  • Support try-catch block.

  • If you don't need to control your own function, you can use isolates_helper - the simple version of this package that will allow you to compute with multiple functions.

Basic Usage (Use build-in function)

There are multiple ways to use this package, the only thing to notice is that the function has to be a static or top-level function to make it works.

Step 1: Create a top-level or static function

double add(dynamic value) => value[0] + value[1];

You have to add @pragma('vm:entry-point') anotation to all methods that you want to use for isolation since Flutter 3.3.0. Without this annotation, the dart compiler could strip out unused functions, inline them, shrink names, etc, and the native code would fail to call it.

Step 2: Create IsolateManager instance for that function

final isolateManager = IsolateManager.create(
  add, // Function that you want to compute
  concurrent: 4, // Number of concurrent isolates. Default is 1

Step 3 Optional: Initialize the instance, this step is not required because it's automatically called when you use .compute for the first time

await isolateManager.start();

You can also run this method when creating the instance:

final isolateManager = IsolateManager.create(
  add, // Function that you want to compute
  concurrent: 4, // Number of concurrent isolates. Default is 1

Step 4: Send and receive data

You can listen to the result as stream => print(result));

You can send even more times then concurrent because the plugin will queues the input data and sends it to free isolate later.

// add([10, 20])
final result = await isolateManager.compute([10, 20]);

You can use try-catch to catch the exception:

try {
  final result = await isolateManager.compute([10, 20]);
} on Exception catch (e1) {
} catch (e2) {

Build your widget with StreamBuilder

  builder: (context, snapshot) {
    if (!snapshot.hasData) {
      return const Center(
        child: CircularProgressIndicator(),
    return Text('Result of the `add` function: ${}');

Step 5: Restart the IsolateManager if you need it

await isolateManager.restart();

Step 6: Stop IsolateManager when it finishes work

await isolateManager.stop();

Advanced Usage (Use your own function)

You can control everything with this method when you want to create multiple isolates for a function. With this method, you can also do one-time stuff when the isolate is started or each-time stuff when you call compute or sendMessage.

Step 1: Create a function of this form

/// Create your own function here. This function will be called when your isolate started.
void isolateFunction(dynamic params) {
  // Initial the controller for child isolate
  final controller = IsolateManagerController<double>(
    onDispose: () {
      print('Dispose isolateFunction');

  // Get your initialParams.
  // Notice that this `initialParams` different from the `params` above.
  final initialParams = controller.initialParams;

  // Do your one-time stuff here, this area of code will be called only one-time when you `start`
  // this instance of `IsolateManager`

  // Listen to the message receiving from main isolate, this area of code will be called each time
  // you use `compute` or `sendMessage`.
  controller.onIsolateMessage.listen((message) {
    // Create a completer
    Completer completer = Completer();

    // Handle the result an exceptions
      (value) => controller.sendResult(value),
      onError: (err, stack) =>
          controller.sendResult(IsolateException(err, stack)),

    // Use try-catch to send the exception to the main app
    try {

      // Do your stuff here. 
      completer.complete(add(message[0], message[1]));

    } catch (err, stack) {
      // Send the exception to your main app
      controller.sendResult(IsolateException(err, stack));

Step 2: Create IsolateManager instance for your own function

final isolateManager = IsolateManager.createOwnIsolate(
    initialParams: 'This is initialParams',
    debugMode: true,

Step 3: Now you can use everything as above from this step

Worker Configuration

  • Step 1: Download isolate_manager/worker/worker.dart or copy the below code to the file named worker.dart:

    // ignore_for_file: avoid_web_libraries_in_flutter, depend_on_referenced_packages
    import 'dart:async';
    import 'dart:convert';
    import 'dart:html' as html;
    import 'dart:js' as js;
    import 'package:isolate_manager/isolate_manager.dart';
    import 'package:js/js.dart' as pjs;
    import 'package:js/js_util.dart' as js_util;
    external dynamic get globalScopeSelf;
    /// dart compile js worker.dart -o worker.js -O4
    /// In most cases you don't need to modify this function
    main() {
      callbackToStream('onmessage', (html.MessageEvent e) {
        return js_util.getProperty(e, 'data');
      }).listen((message) async {
        final Completer completer = Completer();
          (value) => jsSendMessage(value),
          onError: (err, stack) => jsSendMessage(IsolateException(err, stack).toJson()),
        try {
        } catch (err, stack) {
          jsSendMessage(IsolateException(err, stack).toJson());
    /// TODO: Modify your function here:
    ///  Do this if you need to throw an exception
    ///  You should only throw the `message` instead of a whole Object because it may
    ///  not show as expected when sending back to the main app.
    /// ``` dart
    ///  return throw 'This is an error that you need to catch in your main app';
    /// ```
    FutureOr<dynamic> worker(dynamic message) {
      // Best way to use this method is encoding the result to JSON
      // before sending to the main app, then you can decode it back to
      // the return type you want with `workerConverter`.
      return jsonEncode(message);
    /// Internal function
    Stream<T> callbackToStream<J, T>(String name, T Function(J jsValue) unwrapValue) {
      var controller = StreamController<T>.broadcast(sync: true);
      js_util.setProperty(js.context['self'], name, js.allowInterop((J event) {
    /// Internal function
    void jsSendMessage(dynamic m) {
      js.context.callMethod('postMessage', [m]);
  • Step 2: Modify the function FutureOr<dynamic> worker(dynamic message) in the script to serves your purposes. You can also use the top-level or static function that you have created above.

You should copy that function to separated file or copy to worker.dart file to prevent the dart compile js error because some other functions depend on flutter library.

  • Step 3: Run dart compile js worker.dart -o worker.js -O4 to compile dart to js (-O0 to -O4 is the obfuscated level of js).

  • Step 4: Copy worker.js to web folder (the same folder with index.html).

  • Step 5: Now you can add worker to workerName like below:

    final isolateManager = IsolateManager.create(
        workerName: 'worker', // Don't need to add the extension

    Now the plugin will handle all other action to make the real isolate works on Web.


  • If the worker.dart show errors for js package, you can add js to dev_dependencies:

  • The result that you get from the isolate (or Worker) is sometimes different from the result that you want to get from the return type in the main app, you can use converter and workerConverter parameters to convert the result received from the Isolate (converter) and Worker (workerConverter). Example:

    final isolateManager = IsolateManager.create(
      // Ex: 'map_result' if the name is 'map_result.js'
      workerName: 'worker',
      // Convert the data from worker to fix the issue related to the different data type between dart and js
      workerConverter: (result) {
        final Map<int, double> convert = {};
        // Convert Map<String, String> (received from Worker) to Map<int, double>
        final decodedMap = jsonDecode(result) as Map;
        decodedMap.forEach((key, value) => convert.addAll({int.parse(key): double.parse(value)}));
        return convert;

    Data flow: Main -> Isolate or Worker -> Converter -> Result


  • This plugin is an enhanced plugin for isolate_contactor: pub | git

  • If you encounter any problems or feel the library is missing a feature, feel free to open an issue. Pull requests are also welcome.

  • If you like my work or the free stuff on this channel and want to say thanks, or encourage me to do more, you can buy me a coffee. Thank you so much!

To-do list

  • Find the best way to prevent using dart compile js.