Reflex Multiplayer

multiplayer.ts

// Under the MIT
import type { Producer, InferState, InferActions } from "@rbxts/reflex";
import { createProducer } from "@rbxts/reflex";
import { Dictionary as Object } from "@rbxts/sift";

// from reflex!
type ReplaceStateParameters<State, Actions> = {
	readonly [K in keyof Actions]: Actions[K] extends (state: any, ...args: infer Args) => any
		? (state: State, player: Player, ...args: Args) => State
		: never;
};

// we do NOT want to add setPlayerState, as that would set the player's state for all mutiplayer producers
type PlayerActions<State> = {
	readonly removePlayer: (state: State, player: Player) => State;
};

export const withMultiplayer = <
	P extends Producer,
	S extends InferState<P>,
	A extends ReplaceStateParameters<Map<Player, S>, InferActions<P>> & PlayerActions<Map<Player, S>>,
>(
	producer: P,
): Producer<Map<Player, S>, A> => {
	const actions = Object.map(
		producer.getActions() as Record<string, (state: S, ...args: unknown[]) => S>,
		(action) => {
			return (combinedState: Map<Player, S>, player: Player, ...args: unknown[]) => {
				const nextState = table.clone(combinedState);

				if (!nextState.has(player)) {
					nextState.set(player, producer.getState());
				}

				const producerState = nextState.get(player)!;
				nextState.set(player, action(producerState, ...args));

				return nextState;
			};
		},
	) as never;

	(actions as Writable<PlayerActions<Map<Player, S>>>).removePlayer = (state, player) => {
		const nextState = table.clone(state);
		nextState.delete(player);
		return nextState;
	};

	return createProducer(new Map(), actions);
};

Massive thanks to @littensy for typings help!

Please note - due to TS being strange you need to call this as combineProducers(DatabaseRoot).enhance(withMultiplayer) not withMultiplayer(combineProducers(DatabaseRoot))