Fix async worker return type (#7249)

common/api/core-bentley.api.md

@@ -1298,6 +1298,9 @@ export class LRUMap<K, V> extends LRUCache<K, V> {
 // @public
 export type MarkRequired<T, K extends keyof T> = Pick<Required<T>, K> & Omit<T, K>;
 
+// @public
+export type MaybePromise<T> = T | Promise<T>;
+
 // @public
 export type Mutable<T> = {
     -readonly [K in keyof T]: T[K];

common/api/core-frontend.api.md

@@ -184,6 +184,7 @@ import { MassPropertiesRequestProps } from '@itwin/core-common';
 import { MassPropertiesResponseProps } from '@itwin/core-common';
 import { Matrix3d } from '@itwin/core-geometry';
 import { Matrix4d } from '@itwin/core-geometry';
+import { MaybePromise } from '@itwin/core-bentley';
 import { MeshEdges } from '@itwin/core-common';
 import { MeshPolyline } from '@itwin/core-common';
 import { MeshPolylineList } from '@itwin/core-common';
@@ -15710,10 +15711,10 @@ export type WorkerProxy<T> = WorkerInterface<T> & {
 };
 
 // @beta
-export type WorkerReturnType<T extends (...args: any) => any> = ReturnType<T> | {
-    result: ReturnType<T>;
+export type WorkerReturnType<T extends (...args: any) => any> = MaybePromise<ReturnType<T> | {
+    result: Awaited<ReturnType<T>>;
     transfer: Transferable[];
-};
+}>;
 
 // @beta
 export interface WorkerTextureParams {

common/api/summary/core-bentley.exports.csv

@@ -86,6 +86,7 @@ public;class;LRUCache
 public;class;LRUDictionary
 public;class;LRUMap
 public;type;MarkRequired
+public;type;MaybePromise
 public;type;Mutable
 public;class;MutableCompressedId64Set
 public;type;NonFunctionPropertiesOf

common/changes/@itwin/core-bentley/pmc-fix-async-worker-return-type_2024-10-09-13-27.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@itwin/core-bentley",
+      "comment": "Add MaybePromise utility type.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@itwin/core-bentley"
+}

common/changes/@itwin/core-frontend/pmc-fix-async-worker-return-type_2024-10-09-13-27.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@itwin/core-frontend",
+      "comment": "Fix WorkerReturnType for async implementations.",
+      "type": "none"
+    }
+  ],
+  "packageName": "@itwin/core-frontend"
+}

core/bentley/src/UtilityTypes.ts

@@ -98,6 +98,11 @@ export type PickAsyncMethods<T> = { [P in keyof T]: T[P] extends AsyncFunction ?
  */
 export type AsyncMethodsOf<T> = { [P in keyof T]: T[P] extends AsyncFunction ? P : never }[keyof T];
 
+/** A type that is either `T` or `Promise<T>`.
+ * @public
+ */
+export type MaybePromise<T> = T | Promise<T>;
+
 /** Extracts the type to which the Promise returned by an async function resolves.
  * @public
  */

core/frontend/src/common/WorkerProxy.ts

@@ -5,7 +5,7 @@
 /** @packageDocumentation
  * @module Utils
  */
-import { assert } from "@itwin/core-bentley";
+import { assert, MaybePromise } from "@itwin/core-bentley";
 
 /** Holds callbacks for a Promise produced for a particular call to postMessage. */
 interface Task {
@@ -48,11 +48,11 @@ export type WorkerInterface<T> = {
 };
 
 /** Augments each method of `T` with the ability to specify values to be transferred from the worker thread to the main thread.
- * Each return type `R` is replaced with `R | { result: R; transfer: Transferable[]; }`.
+ * Each return type `R` is replaced with `R | { result: R; transfer: Transferable[]; }`, or a promise that resolves to such a type.
  * @see [[WorkerImplementation]].
  * @beta
  */
-export type WorkerReturnType<T extends (...args: any) => any> = ReturnType<T> | { result: ReturnType<T>, transfer: Transferable[] };
+export type WorkerReturnType<T extends (...args: any) => any> = MaybePromise<ReturnType<T> | { result: Awaited<ReturnType<T>>, transfer: Transferable[] }>;
 
 /** Given an interface T that defines the operations provided by a worker, produce an interface to which the implementation of those operations must conform.
  * The return type of each function is enhanced to permit supplying a list of values to be transferred from the worker to the main thread.

core/frontend/src/test/worker/test-worker.ts

@@ -100,7 +100,7 @@ registerWorker<TestWorker>({
 
   someVeryLongRunningAsyncOperation: async () => {
     await waitNTicks(10);
-    return ++globalTickCounter;
+    return { result: ++globalTickCounter, transfer: [] };
   },
   someLongRunningAsyncOperation: async () => {
     await waitNTicks(5);