type EventDeclaration = Record<string, any[]>;
type Handler<T extends EventDeclaration, K extends keyof T> = (...params: T[K]) => void;

/**
 * Basic event broker class. Provides `on`, `off`, and `emit` methods.
 */
export class EventEmitter<T extends EventDeclaration, E = keyof T> {
    constructor() {
        this.on = this.on.bind(this);
        this.off = this.off.bind(this);
    }
    protected handlers: any = {};
    protected getHandlers<E extends keyof T>(event: E) {
        return (this.handlers[event] || []) as Handler<T, E>[];
    }
    emit<E extends keyof T>(event: E, params: T[E]) {
        const handlers = this.getHandlers(event);
        for (let h of handlers) {
            h(...params);
        }
    }
    on<E extends keyof T>(event: E, f: Handler<T, E>)
    on<E extends keyof T>(events: E[], f: Handler<T, E>)
    // on<E extends keyof T>(events: E[], f: (event: E, ...args: T[E]) => void)
    on<E extends keyof T>(event, f) {
        if (!Array.isArray(event)) event = [event];
        for (let e of event) {
            this.handlers[e] ||= [];
            const handlers = this.getHandlers(e);
            if (!handlers.includes(f)) {
                handlers.push(f);
            }
        }
        return () => this.off(event, f);
    }
    off<E extends keyof T>(event: E, f: Handler<T, E>)
    off<E extends keyof T>(events: E[], f: Handler<T, E>)
    off<E extends keyof T>(event, f: Handler<T, E>) {
        if (!Array.isArray(event)) event = [event];
        for (let e of event) {
            const handlers = this.getHandlers(e);
            const idx = handlers.indexOf(f);
            if (idx >= 0) handlers.splice(idx, 1);
        }
    }
}