fix(delayWhen): Emit source value if duration selector completes synchronously (#3664)

* fix(delayWhen): Emit source value if duration selector completes synchronously

This fixes an issue where delayWhen would not re-emit a source emission if the duration selector
completed synchronously.

fixes #3663

* docs(delayWhen): Deprecate completion of notifier triggering source emission

This deprecates the behavior that the completion of the notifier observable will cause the source
emission to be emitted on the output observable.

Author: Airblader

spec/operators/delayWhen-spec.ts

@@ -1,4 +1,4 @@
-import { of } from 'rxjs';
+import { of, EMPTY } from 'rxjs';
 import { delayWhen } from 'rxjs/operators';
 import { TestScheduler } from 'rxjs/testing';
 import { hot, cold, expectObservable, expectSubscriptions } from '../helpers/marble-testing';
@@ -106,6 +106,28 @@ describe('delayWhen operator', () => {
     expectObservable(result).toBe(expected);
     expectSubscriptions(e1.subscriptions).toBe(subs);
     expectSubscriptions(selector.subscriptions).toBe(selectorSubs);
+    });
+
+  it('should emit if the selector completes synchronously', () => {
+    const e1 =        hot('a--|');
+    const expected =      'a--|';
+    const subs =          '^  !';
+
+    const result = e1.pipe(delayWhen((x: any) => EMPTY));
+
+    expectObservable(result).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(subs);
+  });
+
+  it('should emit if the source completes synchronously and the selector completes synchronously', () => {
+    const e1 =        hot('(a|)');
+    const expected =      '(a|)';
+    const subs =          '(^!)';
+
+    const result = e1.pipe(delayWhen((x: any) => EMPTY));
+
+    expectObservable(result).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(subs);
   });
 
   it('should not emit if selector never emits', () => {

src/internal/operators/delayWhen.ts

@@ -7,6 +7,12 @@ import { InnerSubscriber } from '../InnerSubscriber';
 import { subscribeToResult } from '../util/subscribeToResult';
 import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
 
+/* tslint:disable:max-line-length */
+/** @deprecated In future versions, empty notifiers will no longer re-emit the source value on the output observable. */
+export function delayWhen<T>(delayDurationSelector: (value: T) => Observable<never>, subscriptionDelay?: Observable<any>): MonoTypeOperatorFunction<T>;
+export function delayWhen<T>(delayDurationSelector: (value: T) => Observable<any>, subscriptionDelay?: Observable<any>): MonoTypeOperatorFunction<T>;
+/* tslint:disable:max-line-length */
+
 /**
  * Delays the emission of items from the source Observable by a given time span
  * determined by the emissions of another Observable.
@@ -22,6 +28,8 @@ import { MonoTypeOperatorFunction, TeardownLogic } from '../types';
  * argument, and should return an Observable, called the "duration" Observable.
  * The source value is emitted on the output Observable only when the duration
  * Observable emits a value or completes.
+ * The completion of the notifier triggering the emission of the source value
+ * is deprecated behavior and will be removed in future versions.
  *
  * Optionally, `delayWhen` takes a second argument, `subscriptionDelay`, which
  * is an Observable. When `subscriptionDelay` emits its first value or
@@ -79,7 +87,6 @@ class DelayWhenOperator<T> implements Operator<T, T> {
 class DelayWhenSubscriber<T, R> extends OuterSubscriber<T, R> {
   private completed: boolean = false;
   private delayNotifierSubscriptions: Array<Subscription> = [];
-  private values: Array<T> = [];
 
   constructor(destination: Subscriber<T>,
               private delayDurationSelector: (value: T) => Observable<any>) {
@@ -126,15 +133,11 @@ class DelayWhenSubscriber<T, R> extends OuterSubscriber<T, R> {
     subscription.unsubscribe();
 
     const subscriptionIdx = this.delayNotifierSubscriptions.indexOf(subscription);
-    let value: T = null;
-
     if (subscriptionIdx !== -1) {
-      value = this.values[subscriptionIdx];
       this.delayNotifierSubscriptions.splice(subscriptionIdx, 1);
-      this.values.splice(subscriptionIdx, 1);
     }
 
-    return value;
+    return subscription.outerValue;
   }
 
   private tryDelay(delayNotifier: Observable<any>, value: T): void {
@@ -144,8 +147,6 @@ class DelayWhenSubscriber<T, R> extends OuterSubscriber<T, R> {
       this.add(notifierSubscription);
       this.delayNotifierSubscriptions.push(notifierSubscription);
     }
-
-    this.values.push(value);
   }
 
   private tryComplete(): void {