Add util/async.ts

util/async.ts

@@ -0,0 +1,60 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+import { assert } from "../testing/asserts.ts";
+
+// TODO(ry) It'd be better to make Deferred a class that inherits from
+// Promise, rather than an interface. This is possible in ES2016, however
+// typescript produces broken code when targeting ES5 code.
+// See https://github.com/Microsoft/TypeScript/issues/15202
+// At the time of writing, the github issue is closed but the problem remains.
+export interface Deferred<T> extends Promise<T> {
+  resolve: (value?: T | PromiseLike<T>) => void;
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  reject: (reason?: any) => void;
+}
+
+/** Creates a Promise with the `reject` and `resolve` functions
+ * placed as methods on the promise object itself. It allows you to do:
+ *
+ *     const p = deferred<number>();
+ *     // ...
+ *     p.resolve(42);
+ */
+export function deferred<T>(): Deferred<T> {
+  let methods;
+  const promise = new Promise<T>(
+    (resolve, reject): void => {
+      methods = { resolve, reject };
+    }
+  );
+  return Object.assign(promise, methods) as Deferred<T>;
+}
+
+export class Latch<T> {
+  // TODO(ry) Can this be done without using Arrays?
+
+  // Array of `[resolve_function, value]` tuples.
+  private sendQueue: Array<[() => void, T]> = [];
+  // Array of `resolve_function` values.
+  private recvQueue: Array<(value: T) => void> = [];
+
+  send(value: T): Promise<void> {
+    const recvResolve = this.recvQueue.shift();
+    if (recvResolve) {
+      recvResolve(value);
+      return Promise.resolve();
+    } else {
+      return new Promise((resolve, _) => this.sendQueue.push([resolve, value]));
+    }
+  }
+
+  recv(): Promise<T> {
+    const s = this.sendQueue.shift();
+    if (s) {
+      const [sendResolve, value] = s;
+      sendResolve();
+      return Promise.resolve(value);
+    } else {
+      return new Promise((res, _) => this.recvQueue.push(res));
+    }
+  }
+}

util/async_test.ts

@@ -0,0 +1,27 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+import { test, runIfMain } from "../testing/mod.ts";
+import { assertEquals } from "../testing/asserts.ts";
+import { Latch, deferred } from "./async.ts";
+
+test(async function asyncDeferred(): Promise<void> {
+  const d = deferred<number>();
+  d.resolve(12);
+});
+
+async function send3(latch: Latch<number>): Promise<void> {
+  await latch.send(1);
+  await latch.send(2);
+  await latch.send(3);
+}
+
+test(async function asyncLatch(): Promise<void> {
+  const latch = new Latch<number>();
+  send3(latch);
+
+  assertEquals(1, await latch.recv());
+  assertEquals(2, await latch.recv());
+  assertEquals(3, await latch.recv());
+  let _lastPromise = latch.recv();
+});
+
+runIfMain(import.meta);

util/test.ts

@@ -1 +1,2 @@
+import "./async_test.ts";
 import "./deep_assign_test.ts";