Necati Özmen
In this post, we implement realtime broadcast and subscription of pixels updates in our refine based Pixels app. We do this with the liveProvider prop on <Refine /> and Supabase's Realtime servers. Applying a PubSub feature allows us to receive instant updates in one part of our app for database changes triggered from another part or by a different client.
Here's a quick rundown of the features we'll work on:
- Allow multiple users to draw pixels on a canvas.
- All contributors can see realtime updates on the canvas.
This is Day 4 in the series titled refineWeek. refineWeek is a quickfire tutorial guide that aims to help developers learn the ins-and-outs of refine's powerful capabilities and get going with refine within a week.
refineWeek series
- Day 1 - Pilot & refine architecture
- Day 2 - Setting Up the Client App
Overview
On Day Three, we implemented CRUD operations using Supabase dataProvider methods and the resources prop, which leveraged RESTful routes in the routerProvider object under the hood.
Today, we are going to explore the liveProvider prop as we implement realtime collaboration on a canvas so that pixels drawn on a canvas by one user is instantly seen by anyone else viewing it from another client.
There are two parts to our endeavor in this post, one in the Supabase backend and one in our refine app:
- Spin up Realtime servers from Supabase dashboard for the resource we want to publish changes for and subscribe to.
- Registering the
liveProviderprop inside<Refine />component and enabling subscription withliveMode: auto.
refine has already added the implementation code for Supabase Realtime provider in the liveProvider object it created for us during project initialization. We will mostly play the laymen role here as we tour around the underlying principles and the magic at work behind the scenes.
Let's start by first making sure Realtime servers are set up for the pixels table in the Supabase backend.
Setting Up Supabase Realtime
In order to make drawing collaborative between our users, we have to run Supabase Realtime servers and create channels for our pixels resource.
We can do this from the Supabase dashboard for the database we created. If you haven't already, please go ahead and select Enable Realtime for the pixels table from its editor:
Behind the scenes, Supabase spins up globally distributed Realtime servers that facilitate low latency communication between our app and Supabase database tables. Supabase's Realtime feature spares a channel for each resource to be broadcasted. Any change in the resource is published in the channel, and clients that subscribe to the channel receive updates as soon as the change is made.
We are using Supabase Realtime's PostgreSQL Change Data Capture feature which will now allow our app to publish mutation events to the pixels channel and also listen to those changes from a subscriber component.
This means mutation hooks such as useCreate() can now publish events to the pixels channel and consumer hooks like useList() are able to get instant updates for any change to pixels.
<Refine />'s liveProvider Prop
Now, it's time to move our attention back to the <Refine /> component in our app.
We already have the liveProvider prop passed in with the liveProvider() function from @pankod/refine-supabase:
// App.tsx
<Refine
...
liveProvider={liveProvider(supabaseClient)}
/>
And that's it! The channel for pixels resource that was specified above in Supabase will broadcast all mutations on the pixels table. And any subscriber will be able to receive real time updates about the changes.
Now, let's try opening the app in two browsers, one with Google account and one with GitHub. Navigate to a canvas page, the same one in both and try adding some pixels from each. We'll see that pixels created in one are displayed in the other in real time:
This is obnoxious, because we don't know how this is happening. And very pleasant because create refine-app already generated the code that handles the PubSub logic for Supabase PostregSQL CDC. Let's have a look to see what's happening in the Supabase liveProvider object.
refine's Supabase liveProvider Object
refine's liveProvider object has the following signature:
const liveProvider = {
subscribe: ({ channel, params: { ids }, types, callback }) => any,
unsubscribe: (subscription) => void,
publish?: (event) => void,
};
In @pankod/refine-supabase version 3.35.0, at the time of publishing this article, the liveProvider consists of only the subscribe and unsubscribe methods. Its implementation looks like this:
// Supabase data provider
const liveProvider = (supabaseClient: SupabaseClient): LiveProvider => {
return {
subscribe: ({
channel,
types,
params,
callback,object
}): RealtimeSubscription => {
const resource = channel.replace("resources/", "");
const listener = (payload: SupabaseRealtimePayload<any>) => {
if (
types.includes("*") ||
types.includes(liveTypes[payload.eventType])
) {
if (
liveTypes[payload.eventType] !== "created" &&
params?.ids !== undefined &&
payload.new?.id !== undefined
) {
if (
params.ids
.map(String)
.includes(payload.new.id.toString())
) {
callback({
channel,
type: liveTypes[payload.eventType],
date: new Date(payload.commit_timestamp),
payload: payload.new,
});
}
} else {
callback({
channel,
type: liveTypes[payload.eventType],
date: new Date(payload.commit_timestamp),
payload: payload.new,
});
}
}
};
const client = supabaseClient
.from(resource)
.on(supabaseTypes[types[0]], listener);
types
.slice(1)
.map((item) => client.on(supabaseTypes[item], listener));
return client.subscribe();
},
unsubscribe: async (subscription: RealtimeSubscription) => {
supabaseClient.removeSubscription(subscription);
},
};
};
Both methods are concerned with subscription to the changes. That's because the publishing the event is done by mutation methods. In our case, it is done from the useCreate() hook we invoke to create a pixel.
Let's have a look.
Broadcasting
If we look inside our <CanvasShow /> component that is rendered at /canvases/:id, we have a <CanvasItem /> component which basically displays all pixels that belong to the canvas item being visited. <CanvasItem /> is also the component from where a pixel is created, so let's look at that now:
// src/pages/components/canvas/item.tsx
import React from "react";
import { Typography } from "@pankod/refine-antd";
import { Pixel, Canvas } from "types";
import { DEFAULT_SCALE, PIXEL_SIZE } from "utility/constants";
const { Text } = Typography;
type CanvasItemProps = {
canvas: Canvas;
pixels: Pixel[];
scale?: number;
border?: boolean;
active?: boolean;
onPixelClick?: (x: number, y: number) => void;
};
export const CanvasItem: React.FC<CanvasItemProps> = ({
canvas: { id, name, width, height },
pixels,
scale = DEFAULT_SCALE,
border = true,
active = true,
onPixelClick,
}) => {
return (
<div>
{Array.from({ length: height }).map((_, i) => (
<div key={`row-${i}`} style={{ display: "flex" }}>
{Array.from({ length: width }).map((_, j) => (
<div key={`row-${i}-col-${j}`}>
<div
onClick={() => {
if (onPixelClick && active) {
onPixelClick(j, i);
}
}}
style={{
cursor: active ? "pointer" : undefined,
width: PIXEL_SIZE * scale,
height: PIXEL_SIZE * scale,
border: border
? "0.5px solid rgba(0,0,0,0.05)"
: undefined,
background:
pixels.find(
(el) => el.x === j && el.y === i,
)?.color ?? "transparent",
}}
/>
</div>
))}
</div>
))}
{!active && <Text className="canvas-name-text">{name ?? id}</Text>}
</div>
);
};
The most relevant thing to look in the component above is the onPixelClick click event handler. It is a prop received by the <CanvasItem /> from the <CanvasShow /> component. It's original value is the onSubmit() function defined inside the <CanvasShow /> component.
We'd like to focus on this onSubmit event handler, because it is what facilitates the creation of a pixel:
// src/pages/canvases/show.tsx
const { mutate } = useCreate();
const onSubmit = (x: number, y: number) => {
if (!identity) {
return push("/login");
}
if (typeof x === "number" && typeof y === "number" && canvas?.id) {
mutate({
resource: "pixels",
values: {
x,
y,
color,
canvas_id: canvas?.id,
user_id: identity.id,
},
successNotification: false,
});
}
};
As we can see, with the call to mutate method of the useCreate() hook, a new entry is being added to the pixels table upon every click on the canvas.
And since we enabled realtime for the pixels table, each successful create action broadcasts the change to the pixels channel for subscribers to pick.
refine usePublish() Hook
The exact way it happens looks like this:
-
useCreate()hook is called from the consumer component. -
useCreate(), behind the scenes, uses the liveusePublish()hook to publish thecreatedevent to thepixelschannel on Supabase Realtime servers.
The published event for pixels create action produces an object with the following signature:
{
channel: `resources/pixels`,
type: "created",
payload: {
ids: ["id-of-created-pixel"]
},
date: new Date(),
}
Feel free to go through the live hooks docs for details about how live publishing is supported by useCreate() and other mutation hooks.
Subscription
The changes to the pixels table can be subscribed by consumer components with the useList() hook. We are showing the pixels inside the <CanvasShow /> component itself, but they are fetched inside the <DisplayCanvas /> render-props component.
The useList() hook inside <DisplayCanvas /> looks like this:
// src/components/canvas/display.tsx
const { data } = useList<Pixel>
{
resource: "pixels",
liveMode: "auto",
config: {
filters: [
{
field: "canvas_id",
operator: "eq",
value: id,
},
],
sort: [
{
field: "created_at",
order: "desc",
},
],
hasPagination: false,
},
metaData: {
select: "*, users(id, full_name, avatar_url)",
},
};
Among the loads of arguments and options passed to the useList() hook, we have used the liveMode: auto property which allows us to subscribe to the Realtime channel for the pixels resource.
With liveProvider disabled in the <Refine /> component, useList() acts as a normal dataProvider hook.
With liveProvider activated, under the hood, useList() banks on refine's useResourceSubscription() live hook to communicate with the pixels channel.
refine useResourceSubscription() Hook
The actual subscription is done by the liveProvider.subscribe() method.
The subscribe() method is called from inside the useResourceSubscription() hook in order to subsribe to the pixels channel.
If you want to dive into the details, please feel free to do so in the liveProvider docs here.
Summary
In today's part, we implemented real time collaboration for drawing pixels on canvas items using <Refine /> component's liveProvider prop and its supporting hooks for publishing and subscribing to Supabase's Realtime servers.
The subscribe() method on Supabase's liveProvider object allows us to subscribe to a channel created for a resource. Subscription for the resource is triggered by the useResourceSubscription() hook called from data hooks that support live subscription - the useList() hook in our example.
Broadcasting, in turn, is initiated by the usePublish() hook called from a supported mutation hook for our resource - the useCreate() hook in this case.
We implemented real time collaboration very effortlessly due to the out-of-box solutions provided by refine's @pankod/refine-supabase package.
With this now, we have enabled multiple users to draw on a canvas at the same time and receive updates instantly.
