feat(cdk/scrolling): add input to enable append-only mode in virtual scroll viewport

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

@@ -6,7 +6,10 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {coerceNumberProperty, NumberInput} from '@angular/cdk/coercion';
+import {
+  coerceNumberProperty,
+  NumberInput
+} from '@angular/cdk/coercion';
 import {Directive, forwardRef, Input, OnChanges} from '@angular/core';
 import {Observable, Subject} from 'rxjs';
 import {distinctUntilChanged} from 'rxjs/operators';

src/cdk/scrolling/scrolling.md

@@ -121,3 +121,10 @@ custom strategy by creating a class that implements the `VirtualScrollStrategy`
 providing it as the `VIRTUAL_SCROLL_STRATEGY` on the component containing your viewport.
 
 <!-- example(cdk-virtual-scroll-custom-strategy) -->
+
+### Append only mode
+Virtual scroll viewports that render nontrivial items may find it more performant to simply append
+to the list as the user scrolls without removing rendered views. The `appendOnly` input ensures
+views that are already rendered persist in the DOM after they scroll out of view.
+
+<!-- example(cdk-virtual-scroll-append-only) -->

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

@@ -928,6 +928,34 @@ describe('CdkVirtualScrollViewport', () => {
       expect(testComponent.trackBy).toHaveBeenCalled();
     }));
   });
+
+  describe('with append only', () => {
+    let fixture: ComponentFixture<VirtualScrollWithAppendOnly>;
+    let testComponent: VirtualScrollWithAppendOnly;
+    let viewport: CdkVirtualScrollViewport;
+
+    beforeEach(waitForAsync(() => {
+      TestBed.configureTestingModule({
+        imports: [ScrollingModule, CommonModule],
+        declarations: [VirtualScrollWithAppendOnly],
+      }).compileComponents();
+      fixture = TestBed.createComponent(VirtualScrollWithAppendOnly);
+      testComponent = fixture.componentInstance;
+      viewport = testComponent.viewport;
+    }));
+
+    it('should not remove item that have already been rendered', fakeAsync(() => {
+      finishInit(fixture);
+      viewport.setRenderedRange({start: 100, end: 200});
+      fixture.detectChanges();
+      flush();
+      viewport.setRenderedRange({start: 10, end: 50});
+      fixture.detectChanges();
+      flush();
+
+      expect(viewport.getRenderedRange()).toEqual({start: 0, end: 200});
+    }));
+  });
 });
 
 
@@ -1182,3 +1210,36 @@ class DelayedInitializationVirtualScroll {
   trackBy = jasmine.createSpy('trackBy').and.callFake((item: unknown) => item);
   renderVirtualFor = false;
 }
+
+@Component({
+  template: `
+    <cdk-virtual-scroll-viewport appendOnly itemSize="50">
+      <div class="item" *cdkVirtualFor="let item of items">{{item}}</div>
+    </cdk-virtual-scroll-viewport>
+  `,
+  styles: [`
+    .cdk-virtual-scroll-content-wrapper {
+      display: flex;
+      flex-direction: column;
+    }
+
+    .cdk-virtual-scroll-viewport {
+      width: 200px;
+      height: 200px;
+      background-color: #f5f5f5;
+    }
+
+    .item {
+      width: 100%;
+      height: 50px;
+      box-sizing: border-box;
+      border: 1px dashed #ccc;
+    }
+  `],
+  encapsulation: ViewEncapsulation.None
+})
+class VirtualScrollWithAppendOnly {
+  @ViewChild(CdkVirtualScrollViewport, {static: true}) viewport: CdkVirtualScrollViewport;
+  itemSize = 50;
+  items = Array(20000).fill(0).map((_, i) => i);
+}

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

@@ -37,6 +37,7 @@ import {CdkScrollable, ExtendedScrollToOptions} from './scrollable';
 import {VIRTUAL_SCROLL_STRATEGY, VirtualScrollStrategy} from './virtual-scroll-strategy';
 import {ViewportRuler} from './viewport-ruler';
 import {CdkVirtualScrollRepeater} from './virtual-scroll-repeater';
+import {BooleanInput, coerceBooleanProperty} from '@angular/cdk/coercion';
 
 /** Checks if the given ranges are equal. */
 function rangesEqual(r1: ListRange, r2: ListRange): boolean {
@@ -89,6 +90,19 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
   }
   private _orientation: 'horizontal' | 'vertical' = 'vertical';
 
+  /**
+   * Whether rendered items should persist in the DOM after scrolling out of view. By default, items
+   * will be removed.
+   */
+  @Input()
+  get appendOnly(): boolean {
+    return this._appendOnly;
+  }
+  set appendOnly(value: boolean) {
+    this._appendOnly = coerceBooleanProperty(value);
+  }
+  private _appendOnly = false;
+
   // 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
@@ -271,6 +285,9 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
   /** Sets the currently rendered range of indices. */
   setRenderedRange(range: ListRange) {
     if (!rangesEqual(this._renderedRange, range)) {
+      if (this.appendOnly) {
+        range = {start: 0, end: Math.max(this._renderedRange.end, range.end)};
+      }
       this._renderedRangeSubject.next(this._renderedRange = range);
       this._markChangeDetectionNeeded(() => this._scrollStrategy.onContentRendered());
     }
@@ -431,4 +448,6 @@ export class CdkVirtualScrollViewport extends CdkScrollable implements OnInit, O
     this._totalContentWidth =
         this.orientation === 'horizontal' ? `${this._totalContentSize}px` : '';
   }
+
+  static ngAcceptInputType_appendOnly: BooleanInput;
 }

src/components-examples/cdk/scrolling/cdk-virtual-scroll-append-only/cdk-virtual-scroll-append-only-example.css

@@ -0,0 +1,9 @@
+.example-viewport {
+  height: 200px;
+  width: 200px;
+  border: 1px solid black;
+}
+
+.example-item {
+  height: 50px;
+}

src/components-examples/cdk/scrolling/cdk-virtual-scroll-append-only/cdk-virtual-scroll-append-only-example.html

@@ -0,0 +1,3 @@
+<cdk-virtual-scroll-viewport appendOnly itemSize="50" class="example-viewport">
+  <div *cdkVirtualFor="let item of items" class="example-item">{{item}}</div>
+</cdk-virtual-scroll-viewport>

src/components-examples/cdk/scrolling/cdk-virtual-scroll-append-only/cdk-virtual-scroll-append-only-example.ts

@@ -0,0 +1,12 @@
+import {ChangeDetectionStrategy, Component} from '@angular/core';
+
+/** @title Virtual scroll with view recycling disabled. */
+@Component({
+  selector: 'cdk-virtual-scroll-append-only-example',
+  styleUrls: ['cdk-virtual-scroll-append-only-example.css'],
+  templateUrl: 'cdk-virtual-scroll-append-only-example.html',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+})
+export class CdkVirtualScrollAppendOnlyExample {
+  items = Array.from({length: 100000}).map((_, i) => `Item #${i}`);
+}

src/components-examples/cdk/scrolling/index.ts

@@ -1,5 +1,8 @@
 import {ScrollingModule} from '@angular/cdk/scrolling';
 import {NgModule} from '@angular/core';
+import {
+  CdkVirtualScrollAppendOnlyExample
+} from './cdk-virtual-scroll-append-only/cdk-virtual-scroll-append-only-example';
 import {
   CdkVirtualScrollContextExample
 } from './cdk-virtual-scroll-context/cdk-virtual-scroll-context-example';
@@ -24,6 +27,7 @@ import {
 } from './cdk-virtual-scroll-template-cache/cdk-virtual-scroll-template-cache-example';
 
 export {
+  CdkVirtualScrollAppendOnlyExample,
   CdkVirtualScrollContextExample,
   CdkVirtualScrollCustomStrategyExample,
   CdkVirtualScrollDataSourceExample,
@@ -35,6 +39,7 @@ export {
 };
 
 const EXAMPLES = [
+  CdkVirtualScrollAppendOnlyExample,
   CdkVirtualScrollContextExample,
   CdkVirtualScrollCustomStrategyExample,
   CdkVirtualScrollDataSourceExample,

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

@@ -170,3 +170,11 @@ <h2>Use with <code>&lt;table&gt;</code></h2>
     </tr>
   </table>
 </cdk-virtual-scroll-viewport>
+
+<h2>Append only</h2>
+<cdk-virtual-scroll-viewport class="demo-viewport" appendOnly [itemSize]="50">
+  <div *cdkVirtualFor="let size of fixedSizeData; let i = index" class="demo-item"
+       [style.height.px]="size">
+    Item #{{i}} - ({{size}}px)
+  </div>
+</cdk-virtual-scroll-viewport>

tools/public_api_guard/cdk/scrolling.d.ts

@@ -120,6 +120,8 @@ export declare class CdkVirtualScrollViewport extends CdkScrollable implements O
     _contentWrapper: ElementRef<HTMLElement>;
     _totalContentHeight: string;
     _totalContentWidth: string;
+    get appendOnly(): boolean;
+    set appendOnly(value: boolean);
     elementRef: ElementRef<HTMLElement>;
     get orientation(): 'horizontal' | 'vertical';
     set orientation(orientation: 'horizontal' | 'vertical');
@@ -143,7 +145,8 @@ export declare class CdkVirtualScrollViewport extends CdkScrollable implements O
     setRenderedContentOffset(offset: number, to?: 'to-start' | 'to-end'): void;
     setRenderedRange(range: ListRange): void;
     setTotalContentSize(size: number): void;
-    static ɵcmp: i0.ɵɵComponentDeclaration<CdkVirtualScrollViewport, "cdk-virtual-scroll-viewport", never, { "orientation": "orientation"; }, { "scrolledIndexChange": "scrolledIndexChange"; }, never, ["*"]>;
+    static ngAcceptInputType_appendOnly: BooleanInput;
+    static ɵcmp: i0.ɵɵComponentDeclaration<CdkVirtualScrollViewport, "cdk-virtual-scroll-viewport", never, { "orientation": "orientation"; "appendOnly": "appendOnly"; }, { "scrolledIndexChange": "scrolledIndexChange"; }, never, ["*"]>;
     static ɵfac: i0.ɵɵFactoryDeclaration<CdkVirtualScrollViewport, [null, null, null, { optional: true; }, { optional: true; }, null, null]>;
 }