Forwarding Subscriptions

FastEvent supports forwarding subscriptions, which allows publishing and subscribing to be forwarded to another FastEvent instance.

Forwarding Subscriptions

Forwarding subscriptions refers to forwarding subscriptions to another FastEvent instance.

To implement this feature, you need to use the Hook API onAddListener, which is called when the on/once/anAny subscription methods are called.

This HOOK can prevent subscription by returning false, or it can directly subscribe to other FastEvent instances. For subscribers, they are unaware of whether the subscription has been forwarded.

To implement subscription forwarding, you generally need to establish rules for forwarding subscriptions, such as: when the event name starts with @/, forward the subscription to another FastEvent instance.

const otherEmitter = new FastEvent()
const emitter = new FastEvent({
    onAddListener: (type, listener, options) => {
        // Subscription forwarding rule: when the event name starts with `@/`, forward the subscription to another `FastEvent` instance
        if (type.startsWith('@/')) {   
            return otherEmitter.on(type.substring(2), listener, options)
        }
    } 
})
const events: any[] = []
// Subscribe to the data event
emitter.on("@/data", ({ payload }) => {
    events.push(payload)
}) 
// Publish the data event
otherEmitter.emit("data", 1)

otherEmitter.on returns a FastEventSubscriber object, which can be used to cancel the subscription using the off method. In onAddListener, you can directly return it to the subscriber for cancellation.

Forwarding Publishing

You can also directly subscribe to events from another FastEvent instance in the current FastEvent instance.

To implement this feature, you need to use the Hook API onBeforeExecuteListener, which is called when emit is executed.

        const otherEmitter = new FastEvent()
        const emitter = new FastEvent({ 
            onBeforeExecuteListener: (message, args) => {
                // Event forwarding rule: when the event name starts with `@/`, publish to another `FastEvent` instance
                if (message.type.startsWith('@/')) {
                    // Extract the event name starting from `@/`
                    message.type = message.type.substring(2)
                    return otherEmitter.emit(message, args) 
                }
            }
        })
        const events: any[] = []
        otherEmitter.on("data", ({ payload }) => {
            events.push(payload)
        }) 
        const subscriber = emitter.emit("@/data", 1) 
        // You can cancel the subscription just like in the current instance
        subscriber.off()