fix(sample): don't signal on complete

BREAKING CHANGE: the sample operator's notifier observable must emit a next notification to effect a sample. Complete notifications no longer effect a sample.

spec/operators/sample-spec.ts

@@ -56,12 +56,12 @@ describe('sample operator', () => {
     expectSubscriptions(e2.subscriptions).toBe(e2subs);
   });
 
-  it('should sample when the notifier completes', () => {
+  it('should not sample when the notifier completes', () => {
     const e1 =   hot('----a-^------b----------|');
     const e1subs =         '^                 !';
     const e2 =   hot(      '-----x-----|');
     const e2subs =         '^          !';
-    const expected =       '-----------b------|';
+    const expected =       '------------------|';
 
     expectObservable(e1.pipe(sample(e2))).toBe(expected);
     expectSubscriptions(e1.subscriptions).toBe(e1subs);

src/internal/operators/sample.ts

@@ -1,8 +1,8 @@
 /** @prettier */
 import { Observable } from '../Observable';
-
 import { MonoTypeOperatorFunction } from '../types';
 import { operate } from '../util/lift';
+import { noop } from '../util/noop';
 import { OperatorSubscriber } from './OperatorSubscriber';
 
 /**
@@ -14,7 +14,7 @@ import { OperatorSubscriber } from './OperatorSubscriber';
  *
  * ![](sample.png)
  *
- * Whenever the `notifier` Observable emits a value or completes, `sample`
+ * Whenever the `notifier` Observable emits a value, `sample`
  * looks at the source Observable and emits whichever value it has most recently
  * emitted since the previous sampling, unless the source has not emitted
  * anything since the previous sampling. The `notifier` is subscribed to as soon
@@ -41,7 +41,7 @@ import { OperatorSubscriber } from './OperatorSubscriber';
  * source Observable.
  * @return {Observable<T>} An Observable that emits the results of sampling the
  * values emitted by the source Observable whenever the notifier Observable
- * emits value or completes.
+ * emits value.
  */
 export function sample<T>(notifier: Observable<any>): MonoTypeOperatorFunction<T> {
   return operate((source, subscriber) => {
@@ -61,6 +61,6 @@ export function sample<T>(notifier: Observable<any>): MonoTypeOperatorFunction<T
         subscriber.next(value);
       }
     };
-    notifier.subscribe(new OperatorSubscriber(subscriber, emit, undefined, emit));
+    notifier.subscribe(new OperatorSubscriber(subscriber, emit, undefined, noop));
   });
 }