[core] Ask pattern POC

packages/core/test/ask.test.ts

@@ -0,0 +1,109 @@
+import {
+  Actor,
+  ActorRef,
+  AnyActorLogic,
+  createActor,
+  enqueueActions,
+  EventFromLogic,
+  fromTransition,
+  setup,
+  Snapshot
+} from '../src';
+
+/** Extracts askable events (events with respondTo) from actor logic. */
+type AskableEvents<TLogic extends AnyActorLogic> = Extract<
+  EventFromLogic<TLogic>,
+  { respondTo: ActorRef<any, any, any> }
+>;
+
+/** Extracts the response event type from an askable event. */
+type ResponseOf<TAskEvent> = TAskEvent extends {
+  respondTo: ActorRef<any, infer TResponse, any>;
+}
+  ? TResponse
+  : never;
+
+/**
+ * Type-safe ask pattern for XState actors. Sends an event to an actor and waits
+ * for a response.
+ */
+function ask<TLogic extends AnyActorLogic>(
+  actor: Actor<TLogic>,
+  event: Omit<AskableEvents<TLogic>, 'respondTo'>
+): {
+  map: <TResult>(
+    responseMapper: (response: ResponseOf<AskableEvents<TLogic>>) => TResult
+  ) => Promise<TResult>;
+} {
+  type TResponse = ResponseOf<AskableEvents<TLogic>>;
+
+  return {
+    map: <TResult>(responseMapper: (response: TResponse) => TResult) => {
+      type ResolveEvent = { type: 'resolve'; value: TResult };
+
+      const tempActor = createActor(
+        fromTransition<undefined, TResponse, any, undefined, ResolveEvent>(
+          (_, e, actorScope) => {
+            actorScope.emit({ type: 'resolve', value: responseMapper(e) });
+            return undefined;
+          },
+          undefined
+        )
+      ).start();
+
+      const promise = new Promise<TResult>((res) => {
+        tempActor.on('resolve', (ev) => {
+          res(ev.value);
+        });
+      });
+
+      actor.send({
+        ...event,
+        respondTo: tempActor
+      } as AskableEvents<TLogic>);
+
+      return promise;
+    }
+  };
+}
+
+describe('ask pattern', () => {
+  it('should work', async () => {
+    // Define response event type
+    type AnswerEvent = { type: 'answer'; answer: number };
+
+    const machine = setup({
+      types: {
+        events: {} as {
+          type: 'ask';
+          respondTo: ActorRef<Snapshot<unknown>, AnswerEvent>;
+        }
+      }
+    }).createMachine({
+      initial: 'idle',
+      states: {
+        idle: {
+          on: {
+            ask: {
+              actions: enqueueActions(({ event, enqueue }) => {
+                enqueue.sendTo(event.respondTo, { type: 'answer', answer: 42 });
+              })
+            }
+          }
+        }
+      }
+    });
+
+    const actor = createActor(machine).start();
+
+    const result = await ask(actor, { type: 'ask' }).map(
+      (response) => response.answer
+    );
+
+    result satisfies number;
+    // @ts-expect-error
+    result satisfies string;
+
+    expect(result).toEqual(42);
+  });
+});