/**
 * @packageDocumentation
 * @module Utility
 */
import { Observable, Observer } from 'rxjs';
/**
 * Returns an Observable that emits values from an EventSource subscribing to the the passed
 * `eventName` stream.  Takes an optional Observer (e.g. Subject) to emit when the stream opens
 *
 * @category Streams
 *
 * @see {@link https://codesandbox.io/s/rxjs-ninja-eventsource-example-w7v4j|RxJS Event Source Example}
 *
 * @typeParam T The type of the value in the message `data` property
 *
 * @param source The event source to subscribe to
 * @param eventName The name of the event to listen to, by default this is `message`
 * @param openObserver Optional observer that is emitted when the event source is opened
 * @param signal Optional signal to end the event source
 *
 * @example
 * Subscribe to an EventSource, listen for it opening and provide a stop signal
 * ```ts
 * // The event source emits a time every 1 minute
 * const eventSource = new EventSource('/event-stream');
 * const stopSource = new AbortController();
 * const isOpen$ = new Subject<Event>();
 *
 * function endSource() {
 *   stopSource.abort();
 * }
 *
 * fromEventSource<string>(eventSource, 'message', isOpen$, stopSource.signal).pipe(
 *  tap(value => {
 *    const parsed = JSON.parse(value);
 *    outputSpan.innerHTML = `The time is ${parsed.message}`
 *  }),
 *  finalize(() => {
 *    outputSpan.innerHTML = `EventSource closed`
 *  })
 * ).subscribe();
 * ```
 * Output: `'The time is 12:01', 'The time is 12:02', ....`
 *
 * @returns Observable that emits the `data` value from an EventSource message
 */
export declare function fromEventSource<T extends unknown>(source: EventSource, eventName?: string, openObserver?: Observer<Event>, signal?: AbortSignal): Observable<T>;