virtual-scroll: emit the currently scrolled to index

src/cdk-experimental/scrolling/auto-size-virtual-scroll.ts

@@ -9,6 +9,7 @@
 import {coerceNumberProperty} from '@angular/cdk/coercion';
 import {ListRange} from '@angular/cdk/collections';
 import {Directive, forwardRef, Input, OnChanges} from '@angular/core';
+import {Observable} from 'rxjs';
 import {VIRTUAL_SCROLL_STRATEGY, VirtualScrollStrategy} from './virtual-scroll-strategy';
 import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
 
@@ -65,6 +66,13 @@ export class ItemSizeAverager {
 
 /** Virtual scrolling strategy for lists with items of unknown or dynamic size. */
 export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
+  /** @docs-private Implemented as part of VirtualScrollStrategy. */
+  scrolledIndexChange = Observable.create(() => {
+    // TODO(mmalerba): Implement.
+    throw Error('cdk-virtual-scroll: scrolledIndexChange is currently not supported for the' +
+        ' autosize scroll strategy');
+  });
+
   /** The attached viewport. */
   private _viewport: CdkVirtualScrollViewport | null = null;
 
@@ -154,7 +162,7 @@ export class AutoSizeVirtualScrollStrategy implements VirtualScrollStrategy {
   /** Scroll to the offset for the given index. */
   scrollToIndex(): void {
     // TODO(mmalerba): Implement.
-    throw new Error('cdk-virtual-scroll: scrollToIndex is currently not supported for the autosize'
+    throw Error('cdk-virtual-scroll: scrollToIndex is currently not supported for the autosize'
         + ' scroll strategy');
   }
 

src/cdk-experimental/scrolling/fixed-size-virtual-scroll.ts

@@ -9,12 +9,19 @@
 import {coerceNumberProperty} from '@angular/cdk/coercion';
 import {ListRange} from '@angular/cdk/collections';
 import {Directive, forwardRef, Input, OnChanges} from '@angular/core';
+import {Observable, Subject} from 'rxjs';
+import {distinctUntilChanged} from 'rxjs/operators';
 import {VIRTUAL_SCROLL_STRATEGY, VirtualScrollStrategy} from './virtual-scroll-strategy';
 import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
 
 
 /** Virtual scrolling strategy for lists with items of known fixed size. */
 export class FixedSizeVirtualScrollStrategy implements VirtualScrollStrategy {
+  private _scrolledIndexChange = new Subject<number>();
+
+  /** @docs-private Implemented as part of VirtualScrollStrategy. */
+  scrolledIndexChange: Observable<number> = this._scrolledIndexChange.pipe(distinctUntilChanged());
+
   /** The attached viewport. */
   private _viewport: CdkVirtualScrollViewport | null = null;
 
@@ -45,6 +52,7 @@ export class FixedSizeVirtualScrollStrategy implements VirtualScrollStrategy {
 
   /** Detaches this scroll strategy from the currently attached viewport. */
   detach() {
+    this._scrolledIndexChange.complete();
     this._viewport = null;
   }
 
@@ -113,6 +121,8 @@ export class FixedSizeVirtualScrollStrategy implements VirtualScrollStrategy {
             this._bufferSize);
     this._viewport.setRenderedRange(range);
     this._viewport.setRenderedContentOffset(this._itemSize * range.start);
+
+    this._scrolledIndexChange.next(firstVisibleIndex);
   }
 
   /**

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

@@ -6,8 +6,9 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
 import {InjectionToken} from '@angular/core';
+import {Observable} from 'rxjs';
+import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
 
 
 /** The injection token used to specify the virtual scrolling strategy. */
@@ -17,6 +18,9 @@ export const VIRTUAL_SCROLL_STRATEGY =
 
 /** A strategy that dictates which items should be rendered in the viewport. */
 export interface VirtualScrollStrategy {
+  /** Emits when the index of the first element visible in the viewport changes. */
+  scrolledIndexChange: Observable<number>;
+
   /**
    * Attaches this scroll strategy to a viewport.
    * @param viewport The viewport to attach this strategy to.

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

@@ -206,6 +206,21 @@ describe('CdkVirtualScrollViewport', () => {
       expect(viewport.getRenderedRange()).toEqual({start: 2, end: 6});
     }));
 
+    it('should output scrolled index', fakeAsync(() => {
+      finishInit(fixture);
+      triggerScroll(viewport, testComponent.itemSize * 2 - 1);
+      fixture.detectChanges();
+      flush();
+
+      expect(testComponent.scrolledToIndex).toBe(1);
+
+      triggerScroll(viewport, testComponent.itemSize * 2);
+      fixture.detectChanges();
+      flush();
+
+      expect(testComponent.scrolledToIndex).toBe(2);
+    }));
+
     it('should update viewport as user scrolls down', fakeAsync(() => {
       finishInit(fixture);
 
@@ -608,7 +623,8 @@ function triggerScroll(viewport: CdkVirtualScrollViewport, offset?: number) {
   template: `
     <cdk-virtual-scroll-viewport
         [itemSize]="itemSize" [bufferSize]="bufferSize" [orientation]="orientation"
-        [style.height.px]="viewportHeight" [style.width.px]="viewportWidth">
+        [style.height.px]="viewportHeight" [style.width.px]="viewportWidth"
+        (scrolledIndexChange)="scrolledToIndex = $event">
       <div class="item"
            *cdkVirtualFor="let item of items; let i = index; trackBy: trackBy; \
                            templateCacheSize: templateCacheSize"
@@ -642,6 +658,8 @@ class FixedSizeVirtualScroll {
   @Input() trackBy;
   @Input() templateCacheSize = 20;
 
+  scrolledToIndex = 0;
+
   get viewportWidth() {
     return this.orientation == 'horizontal' ? this.viewportSize : this.viewportCrossSize;
   }

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

@@ -18,11 +18,12 @@ import {
   NgZone,
   OnDestroy,
   OnInit,
+  Output,
   ViewChild,
   ViewEncapsulation,
 } from '@angular/core';
 import {animationFrameScheduler, fromEvent, Observable, Subject} from 'rxjs';
-import {sampleTime, takeUntil} from 'rxjs/operators';
+import {sample, sampleTime, takeUntil} from 'rxjs/operators';
 import {CdkVirtualForOf} from './virtual-for-of';
 import {VIRTUAL_SCROLL_STRATEGY, VirtualScrollStrategy} from './virtual-scroll-strategy';
 
@@ -54,9 +55,22 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
   /** Emits when the rendered range changes. */
   private _renderedRangeSubject = new Subject<ListRange>();
 
+  /** Emits when a change detection cycle completes. */
+  private _changeDetectionComplete = new Subject<void>();
+
   /** The direction the viewport scrolls. */
   @Input() orientation: 'horizontal' | 'vertical' = 'vertical';
 
+  // Note: we don't use the typical EventEmitter here because we need to subscribe to the scroll
+  // strategy lazily (i.e. only if the user is actually listening to the events). We do this because
+  // depending on how the strategy calculates the scrolled index, it may come at a cost to
+  // performance.
+  /** Emits when the index of the first element visible in the viewport changes. */
+  @Output() scrolledIndexChange: Observable<number> =
+      Observable.create(observer => this._scrollStrategy.scrolledIndexChange
+          .pipe(sample(this._changeDetectionComplete))
+          .subscribe(observer));
+
   /** The element that wraps the rendered content. */
   @ViewChild('contentWrapper') _contentWrapper: ElementRef<HTMLElement>;
 
@@ -139,6 +153,7 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
     // Complete all subjects
     this._renderedRangeSubject.complete();
     this._detachedSubject.complete();
+    this._changeDetectionComplete.complete();
     this._destroyed.complete();
   }
 
@@ -363,5 +378,7 @@ export class CdkVirtualScrollViewport implements OnInit, OnDestroy {
     for (const fn of runAfterChangeDetection) {
       fn();
     }
+
+    this._ngZone.run(() => this._changeDetectionComplete.next());
   }
 }

src/demo-app/virtual-scroll/virtual-scroll-demo.html

@@ -56,8 +56,12 @@ <h2>Fixed size</h2>
 <button mat-button (click)="viewport1.scrollToIndex(scrollToIndex, scrollToBehavior)">
   Go to index
 </button>
+<p>
+  Currently scrolled to item: {{scrolledIndex}}
+</p>
 
-<cdk-virtual-scroll-viewport class="demo-viewport" [itemSize]="50" #viewport1>
+<cdk-virtual-scroll-viewport class="demo-viewport" [itemSize]="50" #viewport1
+                             (scrolledIndexChange)="scrolled($event)">
   <div *cdkVirtualFor="let size of fixedSizeData; let i = index" class="demo-item"
        [style.height.px]="size">
     Item #{{i}} - ({{size}}px)

src/demo-app/virtual-scroll/virtual-scroll-demo.ts

@@ -28,6 +28,7 @@ export class VirtualScrollDemo {
   scrollToOffset = 0;
   scrollToIndex = 0;
   scrollToBehavior: ScrollBehavior = 'auto';
+  scrolledIndex = 0;
   fixedSizeData = Array(10000).fill(50);
   increasingSizeData = Array(10000).fill(0).map((_, i) => (1 + Math.floor(i / 1000)) * 20);
   decreasingSizeData = Array(10000).fill(0)
@@ -110,4 +111,8 @@ export class VirtualScrollDemo {
       return 0;
     }));
   }
+
+  scrolled(index: number) {
+    this.scrolledIndex = index;
+  }
 }