How to use concurrency with data passing without blocking the main isolate.

Please take a look at the following example:

import 'dart:async';
import 'dart:convert';
import 'dart:math';
import 'package:isolate/isolate.dart';

String str = _generateString();

Future<void> main(List<String> args) async {
  final IsolateRunner r = await IsolateRunner.spawn();
  final List<Operation> operationAndDuration = [];
  const int frameLengthIsMS = 1000 ~/ 120;
  final List<int> frames = [];
  final Timer timer =
      Timer.periodic(const Duration(milliseconds: frameLengthIsMS), (timer) => frames.add(timer.tick));
  Stopwatch s;

  Future<T> runAsyncOperation<T>(String description, Future<T> Function() operation) async {
    s = Stopwatch()..start();
    final value = await operation();
    operationAndDuration.add(Operation(description, s.elapsedMilliseconds ~/ frameLengthIsMS, timer.tick));
    return value;

  T runSyncOperation<T>(String description, T Function() operation) {
    s = Stopwatch()..start();
    final value = operation();
    operationAndDuration.add(Operation(description, s.elapsedMilliseconds ~/ frameLengthIsMS, timer.tick));
    return value;

  print("${str.length} bytes, please wait ~10 seconds.");

  final map = await runAsyncOperation("Isolate Decode", () =>, str));
  final encodedMap = await runAsyncOperation("Isolate Encode", () =>, map));

  await Future(() {});

  final map2 = runSyncOperation("Decode", () => decode(str));
  await Future(() {});

  final encodedMap2 = runSyncOperation("Encode", () => encode(map2));
  await Future(() {});

  assert(encodedMap == encodedMap2);


  print(" • ${_zip(operationAndDuration, findSkippedFrames(frames)).map((v) {
    assert(v.value.key ==;
    final dif = v.value.value - v.value.key;
    return [
      "Took          : ${v.key.frames}",
      "SkippedFrames : ${dif}     (Frame ${v.value.key} to ${v.value.value})",
    ].join("\n   - ");
  }).join("\n\n • ")}");

class Operation {
  final String description;
  final int frames;
  final int at;

  const Operation(this.description, this.frames,;

  String toString() => "Operation '$description' took '$frames' frames at '$at'";

List<MapEntry<A, B>> _zip<A, B>(List<A> a, List<B> b) {
  assert(a.length == b.length);
  return a.asMap() => MapEntry(entry.value, b[entry.key])).toList();

List<MapEntry<int, int>> findSkippedFrames(List<int> frames) {
  final List<MapEntry<int, int>> skippedFrames = [];
  frames.fold<int>(null, (previousValue, element) {
    if (previousValue == null) return element;
    if (previousValue + 1 != element) {
      skippedFrames.add(MapEntry(previousValue, element));
    return element;
  return skippedFrames;

Map<dynamic, dynamic> decode(String str) => json.decode(str) as Map<dynamic, dynamic>;

String encode(Map<dynamic, dynamic> str) => json.encode(str);

String _generateString() {
  final Random rnd = Random();

  final Map<dynamic, dynamic> map = <dynamic, dynamic>{};
  for (int i = 0; i < 500000; i++) {
    map[i.toString()] = [
      (int length) {
        final rand = Random();
        final codeUnits = List.generate(length, (index) {
          return rand.nextInt(33) + 89;

        return String.fromCharCodes(codeUnits);

  return json.encode(map);


13985982 bytes, please wait ~10 seconds.
 • Isolate Decode
   - Took          : 196
   - SkippedFrames : 152     (Frame 241 to 393)

 • Isolate Encode
   - Took          : 187
   - SkippedFrames : 6     (Frame 482 to 488)

 • Decode
   - Took          : 47
   - SkippedFrames : 47     (Frame 488 to 535)

 • Encode
   - Took          : 29
   - SkippedFrames : 30     (Frame 535 to 565)

I’d like to do expensive operations outside of the main isolate and receive a lot of data (e.g. large decoded json maps) back.

This example is supposed to show that using an isolate to decode a large json would result in 152 skipped frames for this particular run while doing the same work on the main isolate would only cost 47.

How do I decode large JSON strings where

  • all of the decoded data is needed in the main isolate (only passing back parts of it is not an option)
  • the main isolate is supposed to be able to run smoothly at 120FPS during the whole process.

Related issues: #29480 dart-lang/language#124

Author: Fantashit

1 thought on “How to use concurrency with data passing without blocking the main isolate.

  1. I would be curious to hear more about this requirement.

    I would like to be able to visualize vast amounts of data on a virtual canvas. without having to worry about blocking the UI and without adding unecessary complexity due to limitations by dart.

    Since I need that canvas to be scalable, all of the data should optimally be available on the first frame. If that is not possible then at least the UI should not skip any frames so that a progress indicator can be displayed.

    Bildschirmfoto 2020-02-09 um 20 28 51

    I will try a chunked approach, but I should have mentioned that encoding and decoding json is just a placeholder for many other problems. Working with audio/video/images, calculating technical indicators, any other form of data visualization with big data sets.

    Yes, there are be workarounds for most of those problems with which one might be able to get a smooth UI, but i should not have to worry about that, just to get a smooth UI.

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