import { Observable } from '../Observable';
/**
 * Returns an Observable that emits all items emitted by the source Observable that are distinct by comparison from the previous item,
 * using a property accessed by using the key provided to check if the two items are distinct.
 * If a comparator function is provided, then it will be called for each item to test for whether or not that value should be emitted.
 * If a comparator function is not provided, an equality check is used by default.
 * @param {string} key string key for object property lookup on each item.
 * @param {function} [compare] optional comparison function called to test if an item is distinct from the previous item in the source.
 * @returns {Observable} an Observable that emits items from the source Observable with distinct values based on the key specified.
 */
export declare function distinctUntilKeyChanged<T>(key: string, compare?: (x: T, y: T) => boolean): Observable<T>;