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Pub/Sub with handleInfo

Pub/Sub is a common pattern for decoupling processes by allowing messages to be sent to a topic by one process and received asynchronously by another. LiveViewJS (and Phoenix LiveView, for that matter) are built on top of Pub/Sub, and Pub/Sub is what enables building the real-time, multi-player features with such ease (along with the LiveView programming model). We will go into more detail on Pub/Sub in the Real-Time Multi-Player docs.

Example Pub/Sub LiveView

We're going to extend our counter example to use Pub/Sub, which will make it a real-time, multi-player counter. Here is the code with the Pub/Sub changes highlighted:

import { createLiveView, html, SingleProcessPubSub } from "liveviewjs";

// An in-memory count simulating state outside of the LiveView
let count = 0;
// Use a single process pub/sub implementation (for simplicity)
const pubSub = new SingleProcessPubSub();

* A basic counter that increments and decrements a number.
export const rtCounterLiveView = createLiveView<
{ count: number }, // Define LiveView Context / State
{ type: "increment" } | { type: "decrement" }, // Define LiveView Events
{ type: "counter"; count: number } // Define LiveView Info messages
mount: (socket) => {
// init state, set count to current count
socket.assign({ count });
// subscribe to counter events
handleEvent: (event, socket) => {
// handle increment and decrement events
const { count } = socket.context;
switch (event.type) {
case "increment":
// broadcast the new count
pubSub.broadcast("counter", { count: count + 1 });
case "decrement":
// broadcast the new count
pubSub.broadcast("counter", { count: count - 1 });
handleInfo: (info, socket) => {
// receive updates from pubsub and update the context
count = info.count;
socket.assign({ count });
render: async (context) => {
// render the view based on the state
const { count } = context;
return html`
<h1>Count is: ${count}</h1>
<button phx-click="decrement">-</button>
<button phx-click="increment">+</button>

We're using a SingleProcessPubSub implementation for simplicity. In a real application, you would use a RedisPubSub implementation in NodeJS or a BroadcastChannelPubSub implementation in for Deno. See the Pub/Sub docs for more details.

How it works

  • This works just like the counter.ts example except we're using Pub/Sub to broadcast the new count to all connected clients and subscribe to updates from other clients.
  • When a client clicks the increment or decrement button, we broadcast the new count to all connected clients using pubSub.broadcast.
  • The LiveViewJS framework automatically routes messages from pubSub.broadcast to the handleInfo function for any LiveView subscribed to the topic.
  • In this case, handleInfo receives the new count and updates the LiveView context which re-renders the view.

It's that easy!

In ~10 lines of code, we've built a real-time, multi-player counter! Sure, that isn't particularly useful, but it shows you how easy it is to create real-time, multi-player applications with very little code and effort.