import { Observable } from 'rxjs';
import { finalize } from 'rxjs/operators';

/**
 * Returns an Observable that mirrors the source Observable, but will call a specified function when
 * the source terminates on complete, error or unsubscribe.
 *
 * <span class="informal">Ensure a given function will be called when a stream ends, no matter why it ended.</span>
 *
 * `finally` method accepts as a single parameter a function. This function does not accept any parameters and
 * should not return anything. It will be called whenever source Observable completes, errors or is unsubscribed,
 * which makes it good candidate to perform any necessary clean up or side effects when Observable terminates,
 * no matter how or why it terminated.
 *
 * Observable returned by `finally` will simply mirror source Observable - each time it is subscribed, source
 * Observable will be subscribed underneath.
 *
 * Note that behavior of `finally` will be repeated per every subscription, so if resulting Observable has
 * many subscribers, function passed to `finally` might be potentially called multiple times.
 *
 * Remember also that `finally` differs quite a lot from passing complete or error handler to {@link subscribe}. It will
 * return an Observable which can be further chained, while `subscribe` returns Subscription, basically ending Observable
 * chain. Function passed to `finally` will be called also when consumer of resulting Observable unsubscribes from it,
 * while handlers passed to `subscribe` will not (even complete handler). But most importantly, `finally` does not start
 * an execution of source Observable, like `subscribe` does, allowing you to set up all necessary hooks before
 * passing Observable further, even without specific knowledge how or when it will be used.
 *
 *
 * @example <caption>Call finally after complete notification</caption>
 * Rx.Observable.of(1, 2, 3)
 * .finally(() => console.log('I was finalized!'))
 * .map(x => x * 2) // `finally` returns an Observable, so we still can chain operators.
 * .subscribe(
 *   val => console.log(val),
 *   err => {},
 *   () => console.log('I completed!')
 * );
 *
 * // Logs:
 * // 1
 * // 2
 * // 3
 * // "I completed!"
 * // "I was finalized!"
 *
 *
 *
 * @example <caption>Call finally after consumer unsubscribes</caption>
 * const o = Rx.Observable.interval(1000)
 * .finally(() => console.log('Timer stopped'));
 *
 * const subscription = o.subscribe(
 *   val => console.log(val),
 *   err => {},
 *   () => console.log('Complete!') // Will not be called, since complete handler
 * );                               // does not react to unsubscription, just to
 *                                  // complete notification sent by the Observable itself.
 *
 * setTimeout(() => subscription.unsubscribe(), 2500);
 *
 * // Logs:
 * // 0 after 1s
 * // 1 after 2s
 * // "Timer stopped" after 2.5s
 *
 * @see {@link using}
 *
 * @param {function} callback Function to be called when source terminates (completes, errors or is unsubscribed).
 * @return {Observable} An Observable that mirrors the source, but will call the specified function on termination.
 * @method finally
 * @name finally
 * @owner Observable
 */
export function _finally<T>(this: Observable<T>, callback: () => void): Observable<T> {
  return finalize(callback)(this) as Observable<T>;
}