@cruise-automation/rpc

Add RPC to WebWorkers with transferrable object support

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@cruise-automation/rpc
An RPC layer to make it easier to communicate between a WebWorker and the main thread. Has support for sending and responding with transferable objects to avoid structured cloning of large array buffers. It also propagates errors thrown in receivers back to the calling thread.

Example

// worker.js
import Rpc from '@cruise-automation/rpc'

const rpc = new Rpc(global);

rpc.receive('message', async ({ url }) => {
  const res = await fetch(url);
  if (!res.ok) {
    throw new Error('Bad response ' + res.status);
  }
  const json = await res.json()
  return { body: json }
});

// ui-thread.js
const worker = new WebWorker('./worker.js')
const rpc = new Rpc(worker);
rpc.send('message', { url }).then(({ body }) => {
  console.log('I got a response', body);
});

API

The Rpc constructor takes a MessagePort as its constructor argument. In a WebWorker you generally would use global and on the UI thread you would use the instance of the WebWorker as the MessagePort.

rpc.send<TResult>(topic: string, data: any, transferables: Transferable[]): Promise<TResult>

The send method takes a topic name and any data. This data is sent over the MessagePort and can be received on the other end with a registered rpc.receive() receiver on the same topic. You may also specify an optional array of transferable objects. This returns a promise which resolves with whatever the handler registered on rpc.receive returns.
const rpc = new Rpc(new WebWorker('/worker-script.js'))

rpc.send('fetch-and-parse', { url: '/lots-of-binary-data' }).then(({ result }) => {
  console.log(result);
});

rpc.receive<T, TOut>(topic: string, handler: (T) => TOut): void

The receive method registers a function to be called whenever a message is received on the specified topic. This function's return value can be waited on by a promise from the caller. To return an object with a list of transferable objects in the graph you can add the list with a special key to the response from your receiver.
// worker-script.js
rpc.receive('fetch-and-parse', async ({ url }) => {
  const res = await fetch(url);
  if (!res.ok) {
    throw new Error('Bad response ' + res.status);
  }
  const arrayBuffer = await res.arrayBuffer();
  const result = doLongRunningParseOperation(arrayBuffer);
  return {
    result,
    [Rpc.transferables]: [result]
  }
});

If the handler throws or rejects the error message will be sent through the MessagePort and calling thread's promise will reject.

Rpc.transferable

This is a static property on the Rpc class that contains the magic string you must use as a key when responding to a message in a receiver and attaching transferables to the response.