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Native events (NativeEventEmitter)

Workers can subscribe to native device events with the familiar NativeEventEmitter API, so a background worker can react to native notifications — geolocation, bluetooth, custom module events — without involving the JS thread.

Subscribing inside a worker​

const worker = new Worker({
  inline: `
    const emitter = new NativeEventEmitter(); // or new NativeEventEmitter(someModule)
    const sub = emitter.addListener('LocationChanged', (payload) => {
      // this fires INSIDE the worker
      self.postMessage({ location: payload });
    });
    // sub.remove(); // when done
  `,
});

worker.onmessage = (e) => console.log('worker saw event:', e.data);

The API mirrors React Native's: addListener(type, cb) returns a subscription with .remove(); removeAllListeners(type) and listenerCount(type) are available too.

How delivery works​

There are two paths, and the difference is which runtime's module emitted the event — not what language it was written in:

  • Events from the worker's own modules — C++, Java or Objective-C — land directly on that worker. They are never dispatched on the host runtime and never copied through it. This is what makes a worker's network response independent of the RN JS thread; see isolation rules.
  • Events from the host app — the app's own DeviceEventEmitter.emit(...), or a module owned by the host — originate on the host runtime. The library forwards those to any worker that has registered a listener, structured-cloning the payload.

You don't have to think about which path an event takes; subscribing is the same.

This changed in 1.0.0-alpha.3

Before that release, a worker module's Java/ObjC events were raised on the host runtime and forwarded back — so a worker's own events depended on the RN JS thread. That is fixed on both platforms.

Example: forwarding an app event to a worker​

import { DeviceEventEmitter } from 'react-native';

const worker = new Worker({
  inline: `
    new NativeEventEmitter().addListener('sync-requested', (p) => {
      self.postMessage({ started: p.id });
      // ...do the sync work here, off the JS thread...
    });
  `,
});

// somewhere in your app, on the JS thread:
DeviceEventEmitter.emit('sync-requested', { id: 42 });

Notes​

  • A worker only receives forwarded host events after it has added at least one listener (it opts in on first addListener). Workers with no listeners cost nothing.
  • Event payloads must be structured-cloneable to reach a worker; non-cloneable payloads are delivered to the host only.