/**
 * A reference to an object that automatically disappears after some time.
 *
 * The object has three states:
 * - in the initial, _empty_ state, the reference points to no object and
 *   the reference count is zero
 * - in the _using_ state, the reference points to some object, and the reference
 *   count is positive; the object will not be released while in this state
 * - in the _expiring_ state, the reference points to some object, but the
 *   reference count is zero; after a timeout, if still in the expiring state,
 *   the object will be released and the reference will revert to the empty state
 */
export default class TimedReference<T> {
    private readonly _maxAge;
    private _cached;
    private _refCount;
    private _age;
    private _timeout;
    private _releasefn;
    /**
     * Construct a new timed reference
     *
     * @param maxAge the maximum time to be in the expiring state
     * @param releasefn a function called to release the underlying object; defaults
     *   to a no-op
     */
    constructor(maxAge: number, releasefn?: (x: T) => Promise<void>);
    /**
     * Acquire the object referenced by this reference.
     *
     * If `ref` is true, this function increases the reference count, so the
     * object will not be automatically released until {@link release} is called.
     * If `ref` is false, this function does not affect the reference count. If
     * the reference is currently expiring, the timeout is reset.
     *
     * @param ref
     * @param ifabsent how to compute the reference if absent; if unspecified, the
     *    object might be null if absent
     * @returns
     */
    acquire(ref: boolean, ifabsent: () => T | Promise<T>): Promise<T>;
    acquire(ref: boolean): Promise<T | null>;
    private _tryClear;
    /**
     * Release the reference.
     *
     * If the reference count becomes zero, the reference enters the expiring state
     * and will disappear after a timeout.
     */
    release(): void;
    /**
     * Release the reference now.
     *
     * This function decreases the reference count if it is positive. If the reference
     * count is or becomes zero, it releases the object immediately.
     */
    releaseNow(): void;
}