---
title: "Web (experimental)"
description: Service Worker based background execution on the web
---

Experimental web support is provided by a new `workmanager_web` package. The
web cannot run a Dart isolate or the Flutter engine inside a Service Worker,
and there is **no web API for exact-time background scheduling** — so the
plugin contract ("execute Dart code in the background, even when the app is
closed") is approximated as honestly as browsers allow.

## How it works

- **Page open**: tasks run in a dedicated Web Worker executing the compiled,
  Flutter-free callback dispatcher (dart2js) — real parallel execution.
- **Page closed — Periodic Background Sync** (Chromium): once the PWA is
  installed and the user has engagement, Chrome wakes the Service Worker
  roughly every `max(frequency, 12h)` and the Service Worker runs the compiled
  Dart dispatcher itself (`importScripts`), recording the result in IndexedDB.
- **Page closed — Web Push**: a push message wakes the Service Worker and
  triggers the same execution path (server-initiated).
- **Page closed — fetch interception**: opportunistic best-effort wake.

Results are replayed into the app's log on the next page load.

## Messaging with the background worker

Beyond task results, the page and the worker can exchange free-form messages
while the page is open:

```dart
// Page side — send a message, listen for replies.
WorkmanagerWeb().workerMessages.listen((payload) {
  print('worker says: $payload');
});
WorkmanagerWeb().sendMessageToWorker({'op': 'watch', 'city': 'cardiff', 'threshold': 5.0});
```

```dart
// Dispatcher side (Flutter-free bundle) — receive and reply.
WorkmanagerExecution.instance.messageHandler = handleWorkerMessage;
// …inside the handler, send back:
WorkmanagerExecution.instance.sendToPage?.call(reply);
```

## Live demo

A self-contained demo (landing page + worker chat + persistent background
task queue + Service Worker notifications) is hosted at
<https://fluttercommunity.github.io/flutter_workmanager/> and lives in the
repo's `example/` folder. The demo's Guide tab walks through the "install the
PWA, close the tab, trigger periodic sync, reopen" flow step by step.

## Honest limitations

- **No exact scheduling** — there is no "run at 15:00" on the web.
- **Chromium only** for periodic sync; PWA install + engagement required;
  ~12-hour minimum interval.
- **Secure context** (HTTPS or `localhost`) required for Service Workers.
- The callback dispatcher must be **Flutter-free** to compile into the bundle.
- One-off tasks are best-effort when the page is closed.
- Experimental: API and layout may change.

## Quick start

Follow the [`workmanager_web` README](../../workmanager_web/README.md) — it
includes a runnable example, exact testing steps for Chrome (including how to
trigger periodic sync and push from DevTools) and the full API reference.