import { AbortablePromise } from './AbortablePromise';
/**
 * The protocol provided to the {@link AsyncQueue} consumer, so it can acknowledge that the value was processed and
 * should be removed from the queue.
 *
 * @template T The value taken from the queue.
 */
export type ValueAck<T> = [value: T, ack: (isTaken: boolean) => void];
/**
 * Asynchronous queue decouples value producers and value consumers.
 *
 * @template T The value stored in the queue.
 */
export declare class AsyncQueue<T = any> {
    /**
     * The elements stored in the queue.
     */
    private _elements;
    /**
     * The promise that resolves after the most recent take was acknowledged.
     */
    private _promise;
    /**
     * Resolves a pending acknowledgement promise, so the consumer can obtain the value from the queue. `undefined` if
     * there's no pending consumer.
     */
    private _resolveTake?;
    /**
     * Returns the number of values stored in this queue.
     */
    get size(): number;
    /**
     * Appends a new value to the end of the queue.
     *
     * @param value The value to append.
     */
    append(value: T): 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(): AbortablePromise<T>;
    /**
     * 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(): AbortablePromise<ValueAck<T>>;
    /**
     * Iterates over elements that are available in the queue.
     */
    [Symbol.iterator](): IterableIterator<T>;
}