SubscriptionChannel

The SubscriptionChannel class is used to send messages from the backend to the frontend, using the same mechanism used by live queries.

example:

// Defined in code that is shared between the frontend and the backend
export const notif = new SubscriptionChannel<{ coolInfo: string }>("notif");

// Backend: Publishing a message
notif.publish({ coolInfo: "Message coming from the backend" });

// Frontend: Subscribing to messages
notif.subscribe((message) => {
    console.log(`Something exciting happened: ${message.coolInfo}`);
});

// Note: If you want to publish from the frontend, use a BackendMethod for that.

---

Use cases:

A user (Paul) wants to notify everyone (Eric & Simona)

// shared code between the frontend and the backend
export const notif = new SubscriptionChannel<{ from: UserInfo, msg: string }>("notif");

// Frontend, Paul can call a BackendMethod to send his message
NotificationContoller.send("Hello everyone!")

// Backend
export class NotificationContoller {
  @BackendMethod({ allowed: true })
  static async send(message: string) {
    notif.publish({ from: remult.user, msg: message });
  }
}

// Frontend: Eric & Simona can subscribe to messages
notif.subscribe((message) => {
  if (message.from === remult.user) return;
  console.log(`Message from ${message.from.name}: ${message.msg}`);
});

A complexe chart that needs to be updated on multiple changes

// shared code between the frontend and the backend
export const chart = new SubscriptionChannel<{ kind: "pie" | "bar" | "line", data: number[] }>("chart");

// Entity
@Entity<Task>("tasks", {
  saved: async (e) => {
    if (e.completed) chart.publish({ kind: "pie", data: [1, 2, 3] });
  }
})
export class Task { ... }

// Frontend: in the chart component, we can subscribe to messages
chart.subscribe((message) => {
  console.log(`Chart updated: ${message.kind}`, message.data);
});

constructor

Constructs a new SubscriptionChannel instance.

Arguments:

• channelKey - The key that identifies the channel.

channelKey

The key that identifies the channel.

publish

Publishes a message to the channel. This method should only be used on the backend.

Arguments:

• message - The message to be published.
• remult - An optional instance of Remult to use for publishing the message.

subscribe

Subscribes to messages from the channel. This method should only be used on the frontend.

returns:

A promise that resolves to a function that can be used to unsubscribe from the channel.

Arguments:

• next - A function that will be called with each message received.
• remult - An optional instance of Remult to use for the subscription.