Comlink
Comlink makes WebWorkers enjoyable. Comlink is a tiny library (1.1kB), that removes the mental barrier of thinking about postMessage
and hides the fact that you are working with workers.
At a more abstract level it is an RPC implementation for postMessage
and ES6 Proxies.
$ npm install --save comlink
Browsers support & bundle size
Browsers without ES6 Proxy support can use the proxy-polyfill.
Size: ~2.5k, ~1.2k gzip’d, ~1.1k brotli’d
Introduction
On mobile phones, and especially on low-end mobile phones, it is important to keep the main thread as idle as possible so it can respond to user interactions quickly and provide a jank-free experience. The UI thread ought to be for UI work only. WebWorkers are a web API that allow you to run code in a separate thread. To communicate with another thread, WebWorkers offer the postMessage
API. You can send JavaScript objects as messages using myWorker.postMessage(someObject)
, triggering a message
event inside the worker.
Comlink turns this messaged-based API into a something more developer-friendly by providing an RPC implementation: Values from one thread can be used within the other thread (and vice versa) just like local values.
API
Comlink.wrap(endpoint)
and Comlink.expose(value, endpoint?)
Comlink’s goal is to make exposed values from one thread available in the other. expose
exposes value
on endpoint
, where endpoint
is a postMessage
-like interface.
wrap
wraps the other end of the message channel and returns a proxy. The proxy will have all properties and functions of the exposed value, but access and invocations are inherently asynchronous. This means that a function that returns a number will now return a promise for a number. As a rule of thumb: If you are using the proxy, put await
in front of it. Exceptions will be caught and re-thrown on the other side.
Comlink.transfer(value, transferables)
and Comlink.proxy(value)
By default, every function parameter, return value and object property value is copied, in the sense of structured cloning. Structured cloning can be thought of as deep copying, but has some limitations. See this table for details.
If you want a value to be transferred rather than copied — provided the value is or contains a Transferable
— you can wrap the value in a transfer()
call and provide a list of transferable values:
const data = new Uint8Array([1, 2, 3, 4, 5]);
await myProxy.someFunction(Comlink.transfer(data, [data.buffer]));
Lastly, you can use Comlink.proxy(value)
. When using this Comlink will neither copy nor transfer the value, but instead send a proxy. Both threads now work on the same value. This is useful for callbacks, for example, as functions are neither structured cloneable nor transferable.
myProxy.onready = Comlink.proxy(data => {
/* ... */
});
Transfer handlers and event listeners
It is common that you want to use Comlink to add an event listener, where the event source is on another thread:
button.addEventListener("click", myProxy.onClick.bind(myProxy));
While this won’t throw immediately, onClick
will never actually be called. This is because Event
is neither structured cloneable nor transferable. As a workaround, Comlink offers transfer handlers.
Each function parameter and return value is given to all registered transfer handlers. If one of the event handler signals that it can process the value by returning true
from canHandle()
, it is now responsible for serializing the value to structured cloneable data and for deserializing the value. A transfer handler has be set up on both sides of the message channel. Here’s an example transfer handler for events:
Comlink.transferHandlers.set("EVENT", {
canHandle: obj => obj instanceof Event,
serialize: ev => {
return [{
target: {
id: ev.target.id
classList: [...ev.target.classList]
}
}, []];
},
deserialize: obj => obj,
});
Note that this particular transfer handler won’t create an actual Event
, but just an object that has the event.target.id
and event.target.classList
property. Often, this enough. If not, the transfer handler can be easily augmented to provide all necessary data.
Comlink.releaseProxy
Every proxy created by Comlink has the [releaseProxy]
method.
Calling it will detach the proxy and the exposed object from the message channel, allowing both ends to be garbage collected.
const proxy = Comlink.wrap(port);
// ... use the proxy ...
proxy[Comlink.releaseProxy]();
Comlink.createEndpoint
Every proxy created by Comlink has the [createEndpoint]
method.
Calling it will return a new MessagePort
, that has been hooked up to the same object as the proxy that [createEndpoint]
has been called on.
const port = myProxy[Comlink.createEndpoint]();
const newProxy = Comlink.wrap(port);
Node
Comlink works with Node’s worker_threads
module. Take a look at the example in the docs
folder.
License Apache-2.0