export type Emitter<TEvents extends Record<string | symbol, unknown>> = {
on<K extends keyof TEvents>(
type: K,
handler: (event: TEvents[K]) => void,
): void;
off<K extends keyof TEvents>(
type: K,
handler: (event: TEvents[K]) => void,
): void;
emit<K extends keyof TEvents>(type: K, event: TEvents[K]): void;
};
export function createEmitter<
TEvents extends Record<string | symbol, unknown>,
>(): Emitter<TEvents> {
const listeners = new Map<keyof TEvents, Set<(event: unknown) => void>>();
function on<K extends keyof TEvents>(
type: K,
handler: (event: TEvents[K]) => void,
) {
let set = listeners.get(type);
if (!set) {
set = new Set();
listeners.set(type, set);
}
set.add(handler as (event: unknown) => void);
}
function off<K extends keyof TEvents>(
type: K,
handler: (event: TEvents[K]) => void,
) {
const set = listeners.get(type);
if (!set) return;
set.delete(handler as (event: unknown) => void);
if (set.size === 0) listeners.delete(type);
}
function emit<K extends keyof TEvents>(type: K, event: TEvents[K]) {
const set = listeners.get(type);
if (!set) return;
// Iterate a snapshot, but consult the live set so removals during dispatch
// can prevent later handlers from firing in this cycle.
[...set].forEach((fn) => {
if (set.has(fn)) {
fn(event);
}
});
}
return { on, off, emit };
}
