fix(scrolling): update virtual scroll viewport size on resize (#18058)

We currently cache the size of the virtual scroll viewport, but it may become inaccurate if it's a percentage of the page viewport and the page is resized. These changes add an extra call that'll update the virtual scroll viewport on resize.

Fixes #16802.

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

@@ -16,7 +16,15 @@ import {
   Directive,
   ViewContainerRef
 } from '@angular/core';
-import {async, ComponentFixture, fakeAsync, flush, inject, TestBed} from '@angular/core/testing';
+import {
+  async,
+  ComponentFixture,
+  fakeAsync,
+  flush,
+  inject,
+  TestBed,
+  tick,
+} from '@angular/core/testing';
 import {animationFrameScheduler, Subject} from 'rxjs';
 
 
@@ -74,6 +82,17 @@ describe('CdkVirtualScrollViewport', () => {
       expect(viewport.getViewportSize()).toBe(500);
     }));
 
+    it('should update the viewport size when the page viewport changes', fakeAsync(() => {
+      finishInit(fixture);
+      spyOn(viewport, 'checkViewportSize').and.callThrough();
+
+      dispatchFakeEvent(window, 'resize');
+      fixture.detectChanges();
+      tick(20); // The resize listener is debounced so we need to flush it.
+
+      expect(viewport.checkViewportSize).toHaveBeenCalled();
+    }));
+
     it('should get the rendered range', fakeAsync(() => {
       finishInit(fixture);
 

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

@@ -23,13 +23,20 @@ import {
   ViewChild,
   ViewEncapsulation,
 } from '@angular/core';
-import {animationFrameScheduler, asapScheduler, Observable, Subject, Observer} from 'rxjs';
+import {
+  animationFrameScheduler,
+  asapScheduler,
+  Observable,
+  Subject,
+  Observer,
+  Subscription,
+} from 'rxjs';
 import {auditTime, startWith, takeUntil} from 'rxjs/operators';
 import {ScrollDispatcher} from './scroll-dispatcher';
 import {CdkScrollable, ExtendedScrollToOptions} from './scrollable';
 import {CdkVirtualForOf} from './virtual-for-of';
 import {VIRTUAL_SCROLL_STRATEGY, VirtualScrollStrategy} from './virtual-scroll-strategy';
-
+import {ViewportRuler} from './viewport-ruler';
 
 /** Checks if the given ranges are equal. */
 function rangesEqual(r1: ListRange, r2: ListRange): boolean {
@@ -142,18 +149,33 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
   /** A list of functions to run after the next change detection cycle. */
   private _runAfterChangeDetection: Function[] = [];
 
+  /** Subscription to changes in the viewport size. */
+  private _viewportChanges = Subscription.EMPTY;
+
   constructor(public elementRef: ElementRef<HTMLElement>,
               private _changeDetectorRef: ChangeDetectorRef,
               ngZone: NgZone,
               @Optional() @Inject(VIRTUAL_SCROLL_STRATEGY)
                   private _scrollStrategy: VirtualScrollStrategy,
               @Optional() dir: Directionality,
-              scrollDispatcher: ScrollDispatcher) {
+              scrollDispatcher: ScrollDispatcher,
+              /**
+               * @deprecated `viewportRuler` parameter to become required.
+               * @breaking-change 11.0.0
+               */
+              @Optional() viewportRuler?: ViewportRuler) {
     super(elementRef, scrollDispatcher, ngZone, dir);
 
     if (!_scrollStrategy) {
       throw Error('Error: cdk-virtual-scroll-viewport requires the "itemSize" property to be set.');
     }
+
+    // @breaking-change 11.0.0 Remove null check for `viewportRuler`.
+    if (viewportRuler) {
+      this._viewportChanges = viewportRuler.change().subscribe(() => {
+        this.checkViewportSize();
+      });
+    }
   }
 
   ngOnInit() {
@@ -188,6 +210,7 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
     // Complete all subjects
     this._renderedRangeSubject.complete();
     this._detachedSubject.complete();
+    this._viewportChanges.unsubscribe();
 
     super.ngOnDestroy();
   }

tools/public_api_guard/cdk/scrolling.d.ts

@@ -105,7 +105,8 @@ export declare class CdkVirtualScrollViewport extends CdkScrollable implements O
     orientation: 'horizontal' | 'vertical';
     renderedRangeStream: Observable<ListRange>;
     scrolledIndexChange: Observable<number>;
-    constructor(elementRef: ElementRef<HTMLElement>, _changeDetectorRef: ChangeDetectorRef, ngZone: NgZone, _scrollStrategy: VirtualScrollStrategy, dir: Directionality, scrollDispatcher: ScrollDispatcher);
+    constructor(elementRef: ElementRef<HTMLElement>, _changeDetectorRef: ChangeDetectorRef, ngZone: NgZone, _scrollStrategy: VirtualScrollStrategy, dir: Directionality, scrollDispatcher: ScrollDispatcher,
+    viewportRuler?: ViewportRuler);
     attach(forOf: CdkVirtualForOf<any>): void;
     checkViewportSize(): void;
     detach(): void;