70 lines
2.1 KiB
TypeScript
70 lines
2.1 KiB
TypeScript
// Copyright 2020 Signal Messenger, LLC
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// SPDX-License-Identifier: AGPL-3.0-only
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/**
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* This class tries to enforce a state machine that looks something like this:
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*
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* .--------------------. called .-----------. called .---------------------.
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* | | --------> | | -------> | |
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* | Nothing is running | | 1 running | | 1 running, 1 queued |
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* | | <-------- | | <------- | |
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* '--------------------' done '-----------' done '---------------------'
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* | ^
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* '-----------'
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* called
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*
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* Most notably, if something is queued and the function is called again, we discard the
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* previously queued task completely.
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*/
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export class LatestQueue {
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private isRunning: boolean;
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private queuedTask?: () => Promise<void>;
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private onceEmptyCallbacks: Array<() => unknown>;
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constructor() {
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this.isRunning = false;
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this.onceEmptyCallbacks = [];
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}
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/**
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* Does one of the following:
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*
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* 1. Runs the task immediately.
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* 2. Enqueues the task, destroying any previously-enqueued task. In other words, 0 or 1
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* tasks will be enqueued at a time.
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*/
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add(task: () => Promise<void>): void {
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if (this.isRunning) {
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this.queuedTask = task;
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} else {
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this.isRunning = true;
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task().finally(() => {
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this.isRunning = false;
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const { queuedTask } = this;
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if (queuedTask) {
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this.queuedTask = undefined;
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this.add(queuedTask);
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} else {
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try {
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this.onceEmptyCallbacks.forEach(callback => {
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callback();
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});
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} finally {
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this.onceEmptyCallbacks = [];
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}
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}
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});
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}
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}
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/**
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* Adds a callback to be called the first time the queue goes from "running" to "empty".
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*/
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onceEmpty(callback: () => unknown): void {
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this.onceEmptyCallbacks.push(callback);
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}
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}
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