From 5362aa77e360fdb6bf76ca9f98e9478ad3ba9663 Mon Sep 17 00:00:00 2001
From: Richie Bendall <richiebendall@gmail.com>
Date: Fri, 29 Jul 2022 19:52:41 +1200
Subject: [PATCH] Add `empty` event

Signed-off-by: Richie Bendall <richiebendall@gmail.com>
---
 package.json    |  1 +
 readme.md       |  8 +++++
 source/index.ts | 86 ++++++++++++++++++-------------------------------
 test/test.ts    | 33 +++++++++++++++++++
 4 files changed, 74 insertions(+), 54 deletions(-)

diff --git a/package.json b/package.json
index 52db7cc..10b9b85 100644
--- a/package.json
+++ b/package.json
@@ -57,6 +57,7 @@
 		"delay": "^5.0.0",
 		"in-range": "^3.0.0",
 		"nyc": "^15.1.0",
+		"p-defer": "^4.0.0",
 		"random-int": "^3.0.0",
 		"time-span": "^5.0.0",
 		"ts-node": "^10.4.0",
diff --git a/readme.md b/readme.md
index 578a6f8..66ceb29 100644
--- a/readme.md
+++ b/readme.md
@@ -308,10 +308,18 @@ queue.on('error', error => {
 queue.add(() => Promise.reject(new Error('error')));
 ```
 
+#### empty
+
+Emitted every time the queue becomes empty.
+
+Useful if you for example add additional items at a later time.
+
 #### idle
 
 Emitted every time the queue becomes empty and all promises have completed; `queue.size === 0 && queue.pending === 0`.
 
+The difference with `empty` is that `idle` guarantees that all work from the queue has finished. `empty` merely signals that the queue is empty, but it could mean that some promises haven't completed yet.
+
 ```js
 import delay from 'delay';
 import PQueue from 'p-queue';
diff --git a/source/index.ts b/source/index.ts
index 2965430..b91c61e 100644
--- a/source/index.ts
+++ b/source/index.ts
@@ -4,15 +4,10 @@ import {Queue, RunFunction} from './queue.js';
 import PriorityQueue from './priority-queue.js';
 import {QueueAddOptions, Options, TaskOptions} from './options.js';
 
-type ResolveFunction<T = void> = (value?: T | PromiseLike<T>) => void;
-
 type Task<TaskResultType> =
 	| ((options: TaskOptions) => PromiseLike<TaskResultType>)
 	| ((options: TaskOptions) => TaskResultType);
 
-// eslint-disable-next-line @typescript-eslint/no-empty-function
-const empty = (): void => {};
-
 const timeoutError = new TimeoutError();
 
 /**
@@ -20,10 +15,12 @@ The error thrown by `queue.add()` when a job is aborted before it is run. See `s
 */
 export class AbortError extends Error {}
 
+type EventName = 'active' | 'idle' | 'empty' | 'add' | 'next' | 'completed' | 'error';
+
 /**
 Promise queue with concurrency control.
 */
-export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsType> = PriorityQueue, EnqueueOptionsType extends QueueAddOptions = QueueAddOptions> extends EventEmitter<'active' | 'idle' | 'add' | 'next' | 'completed' | 'error'> {
+export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsType> = PriorityQueue, EnqueueOptionsType extends QueueAddOptions = QueueAddOptions> extends EventEmitter<EventName> {
 	readonly #carryoverConcurrencyCount: boolean;
 
 	readonly #isIntervalIgnored: boolean;
@@ -51,13 +48,14 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 
 	#isPaused: boolean;
 
-	#resolveEmpty: ResolveFunction = empty;
-
-	#resolveIdle: ResolveFunction = empty;
+	readonly #throwOnTimeout: boolean;
 
-	#timeout?: number;
+	/**
+	Per-operation timeout in milliseconds. Operations fulfill once `timeout` elapses if they haven't already.
 
-	readonly #throwOnTimeout: boolean;
+	Applies to each future operation.
+	*/
+	timeout?: number;
 
 	constructor(options?: Options<QueueType, EnqueueOptionsType>) {
 		super();
@@ -88,7 +86,7 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 		this.#queue = new options.queueClass!();
 		this.#queueClass = options.queueClass!;
 		this.concurrency = options.concurrency!;
-		this.#timeout = options.timeout;
+		this.timeout = options.timeout;
 		this.#throwOnTimeout = options.throwOnTimeout === true;
 		this.#isPaused = options.autoStart === false;
 	}
@@ -107,13 +105,10 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 		this.emit('next');
 	}
 
-	#resolvePromises(): void {
-		this.#resolveEmpty();
-		this.#resolveEmpty = empty;
+	#emitEvents(): void {
+		this.emit('empty');
 
 		if (this.#pendingCount === 0) {
-			this.#resolveIdle();
-			this.#resolveIdle = empty;
 			this.emit('idle');
 		}
 	}
@@ -124,7 +119,7 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 		this.#timeoutId = undefined;
 	}
 
-	#isIntervalPaused(): boolean {
+	get #isIntervalPaused(): boolean {
 		const now = Date.now();
 
 		if (this.#intervalId === undefined) {
@@ -161,13 +156,13 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 
 			this.#intervalId = undefined;
 
-			this.#resolvePromises();
+			this.#emitEvents();
 
 			return false;
 		}
 
 		if (!this.#isPaused) {
-			const canInitializeInterval = !this.#isIntervalPaused();
+			const canInitializeInterval = !this.#isIntervalPaused;
 			if (this.#doesIntervalAllowAnother && this.#doesConcurrentAllowAnother) {
 				const job = this.#queue.dequeue();
 				if (!job) {
@@ -251,9 +246,9 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 						return;
 					}
 
-					const operation = (this.#timeout === undefined && options.timeout === undefined) ? fn({signal: options.signal}) : pTimeout(
+					const operation = (this.timeout === undefined && options.timeout === undefined) ? fn({signal: options.signal}) : pTimeout(
 						Promise.resolve(fn({signal: options.signal})),
-						(options.timeout === undefined ? this.#timeout : options.timeout)!,
+						(options.timeout === undefined ? this.timeout : options.timeout)!,
 						() => {
 							if (options.throwOnTimeout === undefined ? this.#throwOnTimeout : options.throwOnTimeout) {
 								reject(timeoutError);
@@ -331,13 +326,7 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 			return;
 		}
 
-		return new Promise<void>(resolve => {
-			const existingResolve = this.#resolveEmpty;
-			this.#resolveEmpty = () => {
-				existingResolve();
-				resolve();
-			};
-		});
+		await this.#onEvent('empty');
 	}
 
 	/**
@@ -353,16 +342,7 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 			return;
 		}
 
-		return new Promise<void>(resolve => {
-			const listener = () => {
-				if (this.#queue.size < limit) {
-					this.removeListener('next', listener);
-					resolve();
-				}
-			};
-
-			this.on('next', listener);
-		});
+		await this.#onEvent('next', () => this.#queue.size < limit);
 	}
 
 	/**
@@ -376,12 +356,21 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 			return;
 		}
 
-		return new Promise<void>(resolve => {
-			const existingResolve = this.#resolveIdle;
-			this.#resolveIdle = () => {
-				existingResolve();
+		await this.#onEvent('idle');
+	}
+
+	async #onEvent(event: EventName, filter?: () => boolean): Promise<void> {
+		return new Promise(resolve => {
+			const listener = () => {
+				if (filter && !filter()) {
+					return;
+				}
+
+				this.off(event, listener);
 				resolve();
 			};
+
+			this.on(event, listener);
 		});
 	}
 
@@ -415,17 +404,6 @@ export default class PQueue<QueueType extends Queue<RunFunction, EnqueueOptionsT
 	get isPaused(): boolean {
 		return this.#isPaused;
 	}
-
-	get timeout(): number | undefined {
-		return this.#timeout;
-	}
-
-	/**
-	Set the timeout for future operations.
-	*/
-	set timeout(milliseconds: number | undefined) {
-		this.#timeout = milliseconds;
-	}
 }
 
 // TODO: Rename `DefaultAddOptions` to `QueueAddOptions` in next major version
diff --git a/test/test.ts b/test/test.ts
index 1aaa58d..c8ce4be 100644
--- a/test/test.ts
+++ b/test/test.ts
@@ -5,6 +5,7 @@ import delay from 'delay';
 import inRange from 'in-range';
 import timeSpan from 'time-span';
 import randomInt from 'random-int';
+import pDefer from 'p-defer';
 import PQueue, {AbortError} from '../source/index.js';
 
 const fixture = Symbol('fixture');
@@ -892,6 +893,38 @@ test('should emit idle event when idle', async t => {
 	t.is(timesCalled, 2);
 });
 
+test('should emit empty event when empty', async t => {
+	const queue = new PQueue({concurrency: 1});
+
+	let timesCalled = 0;
+	queue.on('empty', () => {
+		timesCalled++;
+	});
+
+	const {resolve: resolveJob1, promise: job1Promise} = pDefer();
+	const {resolve: resolveJob2, promise: job2Promise} = pDefer();
+
+	const job1 = queue.add(async () => job1Promise);
+	const job2 = queue.add(async () => job2Promise);
+	t.is(queue.size, 1);
+	t.is(queue.pending, 1);
+	t.is(timesCalled, 0);
+
+	resolveJob1();
+	await job1;
+
+	t.is(queue.size, 0);
+	t.is(queue.pending, 1);
+	t.is(timesCalled, 0);
+
+	resolveJob2();
+	await job2;
+
+	t.is(queue.size, 0);
+	t.is(queue.pending, 0);
+	t.is(timesCalled, 1);
+});
+
 test('should emit add event when adding task', async t => {
 	const queue = new PQueue({concurrency: 1});