Monday, 3 September, 2018 UTC
Summary
Service Workers are isolated from the main JavaScript thread. Being a special kind of Web Worker, they share the same limitations. How do you communicate back to the main thread? Browsers offer us the Channel Messaging API. This API allows two scripts to communicate by passing simple messages on a shared channel.
It’s also useful to communicate with iframes, but we’ll focus on the worker use case in this post.
Instantiate the Channel You can instantiate a channel using the following syntax:
const channel = new MessageChannel() On the
channel object, which is of type MessageChannel, you have access to the two ports, port1 and port2.The gist is that at each end of the channel listen on one port, and you send messages to the other port.
When you create a channel, you send your messages to
Sending Messages Through the Channel port2.You send a message through the channel using the
postMessage() method, which is made available on the window object if the script is running in the browser:const data = { color: 'green' } const channel = new MessageChannel() window.postMessage(data, [channel.port2]) On the receiving end, in a Service Worker for example, you add a listener on
self, a global that workers have to access themselves:self.addEventListener('message', event => { console.log('Incoming message') }) The
event object passed as parameter contains the data attached to this event in the data property:self.addEventListener('message', event => { console.log('Incoming message') console.log(event.data) }) To send a message back, in the event listener we have access to the other port by referencing
event.ports[0].On this object we can call
postMessage() and post back additional data:self.addEventListener('message', event => { console.log('Incoming message') console.log(event.data) const data = { shape: 'rectangle' } event.ports[0].postMessage(data) }) The sender has access to the channel object it created, so it can attach an
onmessage handler to the message.port1 object:channel.port1.onmessage = event => { console.log(event.data) } In
postMessage() you can send any basic JavaScript data type. This means booleans, strings, numbers, and objects. You can use arrays to pass multiple objects.