refactor: add virtual scroll repeater interface

Author: MichaelJamesParsons

src/cdk/scrolling/public-api.ts

@@ -14,3 +14,4 @@ export * from './viewport-ruler';
 export * from './virtual-for-of';
 export * from './virtual-scroll-strategy';
 export * from './virtual-scroll-viewport';
+export * from './virtual-scroll-repeater';

src/cdk/scrolling/virtual-for-of.ts

@@ -33,6 +33,7 @@ import {
 import {Observable, Subject, of as observableOf, isObservable} from 'rxjs';
 import {pairwise, shareReplay, startWith, switchMap, takeUntil} from 'rxjs/operators';
 import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
+import {CdkVirtualScrollRepeater} from './virtual-scroll-repeater';
 
 
 /** The context for an item rendered by `CdkVirtualForOf` */
@@ -74,7 +75,8 @@ function getSize(orientation: 'horizontal' | 'vertical', node: Node): number {
 @Directive({
   selector: '[cdkVirtualFor][cdkVirtualForOf]',
 })
-export class CdkVirtualForOf<T> implements CollectionViewer, DoCheck, OnDestroy {
+export class CdkVirtualForOf<T> implements
+    CdkVirtualScrollRepeater<T>, CollectionViewer, DoCheck, OnDestroy {
   /** Emits when the rendered view of the data changes. */
   viewChange = new Subject<ListRange>();
 

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

@@ -0,0 +1,18 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Observable} from 'rxjs';
+import {ListRange} from '@angular/cdk/collections';
+
+/**
+ * An item to be repeated by the VirtualScrollViewport
+ */
+export interface CdkVirtualScrollRepeater<T> {
+  dataStream: Observable<T[] | ReadonlyArray<T>>;
+  measureRangeSize(range: ListRange, orientation: 'horizontal' | 'vertical'): number;
+}

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

@@ -34,9 +34,9 @@ import {
 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';
+import {CdkVirtualScrollRepeater} from './virtual-scroll-repeater';
 
 /** Checks if the given ranges are equal. */
 function rangesEqual(r1: ListRange, r2: ListRange): boolean {
@@ -131,8 +131,8 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
   /** The size of the viewport (in pixels). */
   private _viewportSize = 0;
 
-  /** the currently attached CdkVirtualForOf. */
-  private _forOf: CdkVirtualForOf<any> | null;
+  /** the currently attached CdkVirtualScrollRepeater. */
+  private _forOf: CdkVirtualScrollRepeater<any> | null;
 
   /** The last rendered content offset that was set. */
   private _renderedContentOffset = 0;
@@ -215,8 +215,8 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
     super.ngOnDestroy();
   }
 
-  /** Attaches a `CdkVirtualForOf` to this viewport. */
-  attach(forOf: CdkVirtualForOf<any>) {
+  /** Attaches a `CdkVirtualScrollRepeater` to this viewport. */
+  attach(forOf: CdkVirtualScrollRepeater<any>) {
     if (this._forOf) {
       throw Error('CdkVirtualScrollViewport is already attached.');
     }