virtual-scroll: switch throttleTime to sampleTime (#10179)

* virtual-scroll: switch throttleTime to sampleTime

* add comment

Author: mmalerba

src/cdk-experimental/scrolling/virtual-scroll-viewport.ts

@@ -22,8 +22,8 @@ import {
 } from '@angular/core';
 import {Observable} from 'rxjs/Observable';
 import {fromEvent} from 'rxjs/observable/fromEvent';
+import {sampleTime} from 'rxjs/operators/sampleTime';
 import {takeUntil} from 'rxjs/operators/takeUntil';
-import {throttleTime} from 'rxjs/operators/throttleTime';
 import {animationFrame} from 'rxjs/scheduler/animationFrame';
 import {Subject} from 'rxjs/Subject';
 import {CdkVirtualForOf} from './virtual-for-of';
@@ -177,7 +177,9 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
 
       this._ngZone.runOutsideAngular(() => {
         fromEvent(this.elementRef.nativeElement, 'scroll')
-            .pipe(throttleTime(0, animationFrame))
+            // Sample the scroll stream at every animation frame. This way if there are multiple
+            // scroll events in the same frame we only need to recheck our layout once
+            .pipe(sampleTime(0, animationFrame))
             .subscribe(() => this._scrollStrategy.onContentScrolled());
       });
     });

tools/package-tools/rollup-globals.ts

@@ -71,6 +71,8 @@ export const rollupGlobals = {
   'rxjs/observable/of': 'Rx.Observable',
   'rxjs/observable/throw': 'Rx.Observable',
 
+  'rxjs/scheduler/animationFrame': 'Rx.Scheduler',
+
   'rxjs/operators/auditTime': 'Rx.operators',
   'rxjs/operators/catchError': 'Rx.operators',
   'rxjs/operators/combineLatest': 'Rx.operators',
@@ -81,6 +83,7 @@ export const rollupGlobals = {
   'rxjs/operators/first': 'Rx.operators',
   'rxjs/operators/map': 'Rx.operators',
   'rxjs/operators/pairwise': 'Rx.operators',
+  'rxjs/operators/sampleTime': 'Rx.operators',
   'rxjs/operators/share': 'Rx.operators',
   'rxjs/operators/shareReplay': 'Rx.operators',
   'rxjs/operators/startWith': 'Rx.operators',