Add event to subscription event

[allevo]

Use case

I would like to implement event sourcing.

const machine = StateMachine.from(states, {
	context: { events: [] },
});
machine.subscribe(({ context, event }) => {
	context.events.push(event)
});

subscription callback is invoked when a new state is reached. The subscription callback is invoked with context, sharedData, and currentStateId, available on the machine instance. The only element not available is the event. This limits the capability because I cannot update the context using event.

Proposed enhancement

From type perspective:

export type SubscriptionCallbackInput<
	TContext = unknown,
	TEvent = unknown, // added
	TSharedData = unknown,
> = {
	context: TContext;
	currentStateId: StateIdentifier;
	sharedData: TSharedData;
	event?: TEvent // added
};

I can provide a PR to address this.

[marco-ippolito]

soonds like a good idea, pr welcome