parallel-universe/AsyncQueue.js

The set of async flow control structures and promise utils.

'use strict';

var AbortablePromise = require('./AbortablePromise.js');

/**
 * Asynchronous queue decouples value producers and value consumers.
 *
 * @template T The value stored in the queue.
 */
class AsyncQueue {
    constructor() {
        /**
         * The elements stored in the queue.
         */
        this._elements = [];
        /**
         * The promise that resolves after the most recent take was acknowledged.
         */
        this._promise = Promise.resolve();
    }
    /**
     * Returns the number of values stored in this queue.
     */
    get size() {
        return this._elements.length;
    }
    /**
     * Appends a new value to the end of the queue.
     *
     * @param value The value to append.
     */
    append(value) {
        this._elements.push(value);
        if (this._resolveTake !== undefined) {
            this._resolveTake();
        }
        return this;
    }
    /**
     * Returns a promise that is fulfilled with a value when it is available.
     *
     * Values are taken in the same order they were appended. Taken values are removed from the queue.
     *
     * @returns The promise that is fulfilled with a value that was added to the queue. Aborting the returned  promise
     * after the value was taken is a no-op.
     */
    take() {
        return new AbortablePromise.AbortablePromise((resolve, reject, signal) => {
            this.takeAck()
                .withSignal(signal)
                .then(([value, ack]) => {
                ack(!signal.aborted);
                resolve(value);
            }, reject);
        });
    }
    /**
     * Returns a promise that is fulfilled with a value and an acknowledgement callback.
     *
     * The promise is fulfilled when a value is available. Consequent consumers are blocked until the acknowledgement
     * callback is invoked. Invoking acknowledgement callback multiple times is a no-op.
     *
     * **Note:** Be sure to always call an acknowledgement callback. Otherwise, consequent consumers would never be
     * fulfilled.
     *
     * @returns A tuple that contains a value available in the queue, and a callback that acknowledges that the value was
     * processed and should be removed from the queue. Aborting the returned promise after a consumer received an
     * acknowledgement callback is a no-op.
     */
    takeAck() {
        return new AbortablePromise.AbortablePromise((resolveTake, _rejectTake, signal) => {
            this._promise = this._promise.then(() => {
                if (signal.aborted) {
                    return;
                }
                return new Promise(resolveAck => {
                    let isAcked = false;
                    const ack = (isTaken) => {
                        if (isAcked) {
                            return;
                        }
                        if (isTaken) {
                            this._elements.shift();
                        }
                        resolveAck();
                        isAcked = true;
                    };
                    if (this._elements.length !== 0) {
                        resolveTake([this._elements[0], ack]);
                        return;
                    }
                    const abortTake = () => {
                        this._resolveTake = undefined;
                        resolveAck();
                    };
                    signal.addEventListener('abort', abortTake);
                    this._resolveTake = () => {
                        signal.removeEventListener('abort', abortTake);
                        this._resolveTake = undefined;
                        resolveTake([this._elements[0], ack]);
                    };
                });
            });
        });
    }
    /**
     * Iterates over elements that are available in the queue.
     */
    [Symbol.iterator]() {
        return this._elements[Symbol.iterator]();
    }
}

exports.AsyncQueue = AsyncQueue;