event-emitter.ts

type BaseEventMap = { [event: string]: [] }
type EventListener<EventMap extends BaseEventMap, Event extends keyof EventMap>
    = (...args: EventMap[Event]) => any

type EventListenerItem<EventMap extends BaseEventMap, Event extends keyof EventMap> = {
    once: boolean
    listener: EventListener<EventMap, Event>
}

type Events<EventMap extends BaseEventMap> = {
    [Event in keyof EventMap]?: EventListenerItem<EventMap, Event>[]
}

export class EventEmitter<EventMap extends BaseEventMap> {
    private _events: Events<EventMap> = {}

    emit<Event extends keyof EventMap>(event: Event, ...args: EventMap[Event]): boolean {
        const listeners = this._events[event]
        if (listeners && listeners.length > 0) {
            listeners.forEach(({ listener }) => listener(...args))
            this._events[event] = listeners.filter(({ once }) => !once)
            return true
        } else {
            return false
        }
    }

    on<Event extends keyof EventMap>(event: Event, listener: (...args: EventMap[Event]) => any): this {
        const listeners = this._events[event]
        const item = { listener, once: false }
        if (listeners) {
            listeners.push(item)
        } else {
            this._events[event] = [item]
        }
        return this
    }

    once<Event extends keyof EventMap>(event: Event, listener: (...args: EventMap[Event]) => any): this {
        const listeners = this._events[event]
        const item = { listener, once: true }
        if (listeners) {
            listeners.push(item)
        } else {
            this._events[event] = [item]
        }
        return this
    }

    off<Event extends keyof EventMap>(event?: Event, listener?: (...args: EventMap[Event]) => any): this {
        if (!event) {
            this._events = {}
        } else if (!listener) {
            delete this._events[event]
        } else {
            const listeners = this._events[event]
            if (listeners) {
                this._events[event] = listeners.filter(item => item.listener !== listener)
            }
        }
        return this
    }
}