Fix #171 (#172)

* feat: address #171

* fix: fix type

* docs: update docs

README.md

@@ -236,7 +236,8 @@ using `send` will reject.
 
 - `subscribe`: You can register a callback that will be invoked
 on every state transition between the `onEntry` and `onExit` hooks.
-The callback returns the `subscriptionId` and receives `context` and `currentStateId`.
+The callback returns the `subscriptionId` and receives `context`,
+`event`, `sharedData`, and `currentStateId`.
 
 - `unsubscribe`: Remove the subscription with the given `subscriptionId`.
 

docs/api.md

@@ -66,7 +66,7 @@ using `send` will reject.
 
 - `subscribe`: You can register a callback that will be invoked
 on every state transition between the `onEntry` and `onExit` hooks.
-The callback returns the `subscriptionId` and receives `context` and `currentStateId`.
+The callback returns the `subscriptionId` and receives `context`, `event`, `sharedData`, and `currentStateId`.
 
 - `unsubscribe`: Remove the subscription with the given `subscriptionId`.
 

src/fsm.ts

@@ -40,7 +40,10 @@ export class StateMachine<
 	#initial: State<TContext, TEvent, TSharedData>;
 	#states: Map<StateIdentifier, State<TContext, TEvent, TSharedData>>;
 	#sharedData: TSharedData;
-	#subscriptions: Map<SubscriptionIdentifier, SubscriptionCallback>;
+	#subscriptions: Map<
+		SubscriptionIdentifier,
+		SubscriptionCallback<TContext, TEvent, TSharedData>
+	>;
 
 	private constructor(
 		states: Array<State<TContext, TEvent, TSharedData>>,
@@ -158,6 +161,7 @@ export class StateMachine<
 				context: this.context,
 				currentStateId: this.#current.id,
 				sharedData: this.#sharedData,
+				event,
 			});
 		}
 
@@ -212,7 +216,9 @@ export class StateMachine<
 		await this.enter(destination, event);
 	}
 
-	public subscribe(callback: SubscriptionCallback): SubscriptionIdentifier {
+	public subscribe(
+		callback: SubscriptionCallback<TContext, TEvent, TSharedData>,
+	): SubscriptionIdentifier {
 		const id = crypto.randomUUID();
 		this.#subscriptions.set(id, callback);
 		return id;

src/state.ts

@@ -3,15 +3,21 @@ export type SubscriptionIdentifier = string;
 
 export type SubscriptionCallbackInput<
 	TContext = unknown,
+	TEvent = unknown,
 	TSharedData = unknown,
 > = {
 	context: TContext;
 	currentStateId: StateIdentifier;
 	sharedData: TSharedData;
+	event?: TEvent;
 };
 
-export type SubscriptionCallback<TContext = unknown> = (
-	callback: SubscriptionCallbackInput<TContext>,
+export type SubscriptionCallback<
+	TContext = unknown,
+	TEvent = unknown,
+	TSharedData = unknown,
+> = (
+	callback: SubscriptionCallbackInput<TContext, TEvent, TSharedData>,
 ) => void;
 
 export type HookInput<

test/subscribe.spec.js

@@ -0,0 +1,59 @@
+import assert from "node:assert";
+import test from "node:test";
+import { StateMachine } from "../dist/index.js";
+
+test("#171 - event is available on 'subscribe' callback", async () => {
+	const subscriptionEvent = [];
+
+	const states = [
+		{
+			id: "OFF",
+			initial: true,
+			transitionTo: () => "ON",
+		},
+		{
+			id: "ON",
+			transitionTo: () => "OFF",
+		},
+	];
+
+	const context = {};
+	const sharedData = {};
+	const machine = StateMachine.from(states, {
+		context,
+		sharedData,
+	});
+
+	machine.subscribe((event) => {
+		subscriptionEvent.push(event);
+	});
+
+	// Start the state machine
+	await machine.start();
+
+	const event1 = { name: "foo", value: 1 };
+	await machine.send(event1);
+	const event2 = { name: "foo", value: 2 };
+	await machine.send(event2);
+
+	assert.deepStrictEqual(subscriptionEvent, [
+		{
+			context,
+			currentStateId: "OFF",
+			event: undefined,
+			sharedData,
+		},
+		{
+			context,
+			currentStateId: "ON",
+			event: event1,
+			sharedData,
+		},
+		{
+			context,
+			currentStateId: "OFF",
+			event: event2,
+			sharedData,
+		},
+	]);
+});