lib/src/support/bus/LocalBus.ts

import {AwareOfContainerLifecycle, Inject, Injectable, StaticInstantiable} from '../../di'
import {BusSubscriber, Event, EventBus, EventHandler, EventHandlerReturn, EventHandlerSubscription} from './types'
import {Awaitable, Collection, ifDebugging, Pipeline, uuid4} from '../../util'
import {Logging} from '../../service/Logging'
import {Bus, BusInternalSubscription} from './Bus'
import {getEventName} from './getEventName'
import {CanonicalItemClass} from '../CanonicalReceiver'

/**
 * Non-connectable event bus implementation. Can forward events to the main Bus instance.
 */
@Injectable()
export class LocalBus<TEvent extends Event = Event> extends CanonicalItemClass implements EventBus<TEvent>, AwareOfContainerLifecycle {
    awareOfContainerLifecycle = true as const

    @Inject()
    protected readonly logging!: Logging

    @Inject()
    protected readonly bus!: Bus

    public readonly uuid = uuid4()

    /** Local listeners subscribed to events on this bus. */
    protected subscribers: Collection<BusSubscriber<TEvent>> = new Collection()

    protected subscriptions: Collection<BusInternalSubscription> = new Collection()

    /** True if the bus has been initialized. */
    private isUp = false

    /**
     * Push an event onto the bus.
     * @param event
     */
    async push(event: TEvent): Promise<void> {
        if ( event.originBusUuid === this.uuid ) {
            return
        }

        if ( !event.originBusUuid ) {
            event.originBusUuid = this.uuid
        }

        if ( await this.callSubscribers(event) ) {
            // One of the subscribers halted propagation of the event
            return
        }

        if ( await this.shouldBroadcast(event) ) {
            await this.bus.push(event)
        }
    }

    /** Returns true if the given event should be pushed to connected event busses. */
    protected async shouldBroadcast(event: TEvent): Promise<boolean> {
        if ( typeof event.shouldBroadcast === 'function' ) {
            return event.shouldBroadcast()
        }

        return Boolean(event.shouldBroadcast)
    }

    /**
     * Call all local listeners for the given event. Returns true if the propagation
     * of the event should be halted.
     * @param event
     * @protected
     */
    protected async callSubscribers(event: TEvent): Promise<boolean> {
        return this.subscribers
            .filter(sub => event instanceof sub.eventKey)
            .pluck('handler')
            .toAsync()
            .some(handler => handler(event))
    }

    /** Register a pipeline as an event handler. */
    pipe<T extends TEvent>(eventKey: StaticInstantiable<T>, line: Pipeline<T, EventHandlerReturn>): Awaitable<EventHandlerSubscription> {
        return this.subscribe(eventKey, event => line.apply(event))
    }

    /**
     * Subscribe to an event on the bus.
     * @param eventKey
     * @param handler
     */
    async subscribe<T extends TEvent>(eventKey: StaticInstantiable<T>, handler: EventHandler<T>): Promise<EventHandlerSubscription> {
        const uuid = uuid4()

        this.subscribers.push({
            eventName: getEventName(eventKey),
            handler,
            eventKey,
            uuid,
        } as unknown as BusSubscriber<TEvent>)

        return {
            unsubscribe: async () => {
                this.subscribers = this.subscribers.where('uuid', '!=', uuid)

                await this.subscriptions
                    .where('subscriberUuid', '=', uuid)
                    .tap(trashed => this.subscriptions.diffInPlace(trashed))
                    .pluck('subscription')
                    .awaitMapCall('unsubscribe')
            },
        }
    }

    async up(): Promise<void> {
        if ( this.isUp ) {
            this.logging.warn('Attempted to boot more than once. Skipping.')
            ifDebugging('extollo.bus', () => {
                throw new Error('Attempted to boot more than once. Skipping.')
            })
            return
        }

        this.isUp = true
    }

    isConnected(): boolean {
        return this.isUp
    }

    /** Clean up this event bus. */
    async down(): Promise<void> {
        if ( !this.isUp ) {
            this.logging.warn('Attempted to shut down but was never properly booted. Skipping.')
            return
        }

        await this.subscriptions
            .pluck('subscription')
            .awaitMapCall('unsubscribe')

        this.isUp = false
    }

    onContainerRelease(): Awaitable<void> {
        if ( this.isUp ) {
            this.down()
        }
    }
}