vessel/src/classes/EventBus/EventEmitter.ts

import Listener from "./Listener"
import type {
	EventTemplateT,
	TemplateEventT,
	TemplateListenerArgsT,
	TemplateListenerContextT,
	TemplateListenerT,
} from "./Listener"

export default class EventEmitter<
	Template extends EventTemplateT = EventTemplateT,
> {
	protected events: EventsT<Template> = {}

	public on = this.addListener

	public off = this.removeListener

	public id = null

	/**
	 * Returns an array listing the events for which the emitter has registered listeners.
	 */
	public eventNames(): TemplateEventT<Template>[] {
		const events = this.events

		return Object.keys(events).filter(
			(event) => events[event] !== undefined,
		)
	}

	/**
	 * Returns the array of listeners for the specified `event`.
	 *
	 * @param event
	 */
	public rawListeners<Event extends TemplateEventT<Template>>(
		event: Event,
	): Listener<Template, Event, this>[] {
		const listeners = this.events[event]

		if (listeners === undefined) return []

		if (isSingleListener(listeners)) return [listeners]

		return listeners
	}

	/**
	 * Returns a copy of the array of listeners for the specified `event`.
	 *
	 * @param event
	 */
	public listeners<Event extends TemplateEventT<Template>>(
		event: Event,
	): TemplateListenerT<Template, Event, this>[] {
		const listeners = this.rawListeners(event)
		const length = listeners.length

		if (length === 0) return []

		const result = new Array(length)

		for (let index = 0; index < length; index++)
			result[index] = listeners[index].fn

		return result
	}

	/**
	 * Returns the number of listeners listening to the specified `event`.
	 *
	 * @param event
	 */
	public listenerCount<Event extends TemplateEventT<Template>>(
		event: Event,
	): number {
		return this.rawListeners(event).length
	}

	/**
	 * Synchronously calls each of the listeners registered for the specified `event`,
	 * in the order they were registered, passing the supplied arguments to each.
	 *
	 * @param event
	 * @param args
	 * @returns - `true` if the `event` had listeners, `false` otherwise.
	 */
	public emit<Event extends TemplateEventT<Template>>(
		event: Event,
		...args: TemplateListenerArgsT<Template, Event>
	): boolean {
		const events = this.events
		const listeners = events[event]

		if (listeners === undefined) {
			if (event === "error") throw args[0]

			return false
		}

		// console.debug(
		// 	`[eventbus][${this.id}] emitted event (${event.toString()})`,
		// 	{
		// 		args: args,
		// 		listeners,
		// 	},
		// )

		if (isSingleListener(listeners)) {
			const { fn, context, once } = listeners

			if (once) events[event] = undefined

			fn.apply(context, args)

			return true
		}

		let length = listeners.length

		for (let index = 0; index < length; index++) {
			const { fn, context, once } = listeners[index]

			if (once) {
				listeners.splice(index--, 1)

				length--
			}

			fn.apply(context, args)
		}

		if (length === 0) events[event] = undefined
		else if (length === 1) events[event] = listeners[0]

		return true
	}

	/**
	 * Adds the `listener` function to the end of the listeners array for the specified `event`.
	 * No checks are made to see if the listener has already been added.
	 * Multiple calls passing the same combination of `event` and `listener` will result in the listener being added,
	 * and called, multiple times.
	 *
	 * @param event
	 * @param listener
	 * @param context - default: `this` (EventEmitter instance)
	 */
	public addListener<
		Event extends TemplateEventT<Template>,
		Context = TemplateListenerContextT<Template, Event, this>,
	>(
		event: Event,
		listener: TemplateListenerT<Template, Event, Context>,
		context?: Context,
	): this {
		return this._addListener(event, listener, context, true, false)
	}

	/**
	 * Adds a one-time `listener` function for the specified `event` to the end of the `listeners` array.
	 * The next time `event` is triggered, this listener is removed, and then invoked.
	 *
	 * @param event
	 * @param listener
	 * @param context - default: `this` (EventEmitter instance)
	 */
	public once<
		Event extends TemplateEventT<Template>,
		Context = TemplateListenerContextT<Template, Event, this>,
	>(
		event: Event,
		listener: TemplateListenerT<Template, Event, Context>,
		context?: Context,
	): this {
		return this._addListener(event, listener, context, true, true)
	}

	/**
	 * Adds the `listener` function to the beginning of the listeners array for the specified `event`.
	 * No checks are made to see if the listener has already been added.
	 * Multiple calls passing the same combination of `event` and `listener` will result in the listener being added,
	 * and called, multiple times.
	 *
	 * @param event
	 * @param listener
	 * @param context - default: `this` (EventEmitter instance)
	 */
	public prependListener<
		Event extends TemplateEventT<Template>,
		Context = TemplateListenerContextT<Template, Event, this>,
	>(
		event: Event,
		listener: TemplateListenerT<Template, Event, Context>,
		context?: Context,
	): this {
		return this._addListener(event, listener, context, false, false)
	}

	/**
	 * Adds a one-time `listener` function for the specified `event` to the beginning of the `listeners` array.
	 * The next time `event` is triggered, this listener is removed, and then invoked.
	 *
	 * @param event
	 * @param listener
	 * @param context - default: `this` (EventEmitter instance)
	 */
	public prependOnceListener<
		Event extends TemplateEventT<Template>,
		Context = TemplateListenerContextT<Template, Event, this>,
	>(
		event: Event,
		listener: TemplateListenerT<Template, Event, Context>,
		context?: Context,
	): this {
		return this._addListener(event, listener, context, false, true)
	}

	/**
	 * Removes all `listeners`, or those of the specified `event`.
	 *
	 * @param event
	 */
	public removeAllListeners<Event extends TemplateEventT<Template>>(
		event?: Event,
	): this {
		if (event === undefined) {
			this.events = {}

			return this
		}

		if (this.events[event] === undefined) return this

		this.events[event] = undefined

		return this
	}

	/**
	 * Removes the specified `listener` from the `listeners` array for the specified `event`.
	 *
	 * @param event
	 * @param listener
	 */
	public removeListener<Event extends TemplateEventT<Template>>(
		event: Event,
		listener: TemplateListenerT<Template, Event, this>,
	): this {
		const listeners = this.events[event]

		if (listeners === undefined) return this

		if (isSingleListener(listeners)) {
			if (listeners.fn === listener) this.events[event] = undefined

			return this
		}

		for (let index = listeners.length - 1; index >= 0; index--) {
			if (listeners[index].fn === listener) listeners.splice(index, 1)
		}

		if (listeners.length === 0) this.events[event] = undefined
		else if (listeners.length === 1) this.events[event] = listeners[0]

		return this
	}

	protected _addListener<
		Event extends TemplateEventT<Template>,
		Context = TemplateListenerContextT<Template, Event, this>,
	>(
		event: Event,
		fn: TemplateListenerT<Template, Event, Context>,
		context: Context = this as never,
		append: boolean,
		once: boolean,
	): this {
		const events = this.events
		const listeners = events[event]
		const listener = new Listener<Template, Event, Context>(
			fn,
			context,
			once,
		)

		if (listeners === undefined) {
			events[event] = listener

			return this
		}

		if (isSingleListener(listeners)) {
			events[event] = append
				? [listeners, listener]
				: [listener, listeners]

			return this
		}

		if (append) {
			listeners.push(listener)
		} else {
			listeners.unshift(listener)
		}

		return this
	}
}

function isSingleListener<Type>(value: Type | Type[]): value is Type {
	// eslint-disable-next-line @typescript-eslint/no-explicit-any
	return (value as any).length === undefined
}

export type EventsT<Template extends EventTemplateT> = {
	// eslint-disable-next-line @typescript-eslint/no-explicit-any
	[Event in TemplateEventT<Template>]?:
		| Listener<Template, Event, any>
		| Listener<Template, Event, any>[]
}