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docs(virtual-scroll): add live examples (#13327)
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‎src/cdk/scrolling/scrolling.md‎

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same API as [`*ngFor`](https://angular.io/api/common/NgForOf). The simplest usage just specifies the
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list of items:
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```html
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<cdk-virtual-scroll-viewport itemSize="50">
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<div *cdkVirtualFor="let item of items">{{item}}</div>
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</cdk-virtual-scroll-viewport>
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```
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<!-- example(cdk-virtual-scroll-overview) -->
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`*cdkVirtualFor` makes the following context variables available to the template:
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All of these apply to the index of the item in the data source, not the index in the rendered
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portion of the data.
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```html
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<cdk-virtual-scroll-viewport itemSize="50">
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<div *cdkVirtualFor="let item of items;
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let index = index;
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let count = count">
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{{item}} ({{index}} of {{count}})
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</div>
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</cdk-virtual-scroll-viewport>
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```
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<!-- example(cdk-virtual-scroll-context) -->
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A `trackBy` function can be specified and works the same as the `*ngFor` `trackBy`. The `index`
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passed to the tracking function will be the index in the data source, not the index in the rendered
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property; setting this size to `0` disables caching. If your templates are expensive in terms of
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memory you may wish to reduce this number to avoid spending too much memory on the template cache.
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```html
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<cdk-virtual-scroll-viewport itemSize="50">
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<div *cdkVirtualFor="let item of items; templateCacheSize: 0">{{item}}</div>
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</cdk-virtual-scroll-viewport>
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```
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<!-- example(cdk-virtual-scroll-template-cache) -->
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##### Specifying data
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`*cdkVirtualFor` accepts data from an `Array`, `Observable<Array>`, or `DataSource`. The
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destroyed, which may be the right time to clean up any subscriptions that were registered during the
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connect process.
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<!-- example(cdk-virtual-scroll-data-source) -->
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#### Scrolling over fixed size items
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When all items are the same fixed size, you can use the `FixedSizeVirtualScrollStrategy`. This can
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be easily added to your viewport using the `itemSize` directive. The advantage of this constraint is
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that it allows for better performance, since items do not need to be measured as they are rendered.
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```html
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<cdk-virtual-scroll-viewport itemSize="50">
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...
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</cdk-virtual-scroll-viewport>
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```
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The fixed size strategy also supports setting a couple of buffer parameters that determine how much
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extra content is rendered beyond what is visible in the viewport. The first of these parameters is
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`minBufferPx`. The `minBufferPx` is the minimum amount of content buffer (in pixels) that the
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least enough buffer to get back to `maxBufferPx`. In this case, it renders 4 items (an additional
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`200px`) to bring the total buffer size to `290px`, back above `maxBufferPx`.
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```html
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<cdk-virtual-scroll-viewport itemSize="50" minBufferPx="100" maxBufferPx="250">
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...
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</cdk-virtual-scroll-viewport>
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```
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<!-- example(cdk-virtual-scroll-fixed-buffer) -->
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Other virtual scrolling strategies can be implemented by extending `VirtualScrollStrategy`. An
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autosize strategy that works on elements of differing sizes is currently being developed in
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`.cdk-virtual-scroll-content-wrapper` which is the wrapper element that contains the rendered
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content.
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```html
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<cdk-virtual-scroll-viewport itemSize="50" orientation="horizontal">
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...
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</cdk-virtual-scroll-viewport>
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```
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<!-- example(cdk-virtual-scroll-horizontal) -->
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### Elements with parent tag requirements
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Some HTML elements such as `<tr>` and `<li>` have limitations on the kinds of parent elements they
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that the parent does not introduce additional space (e.g. via `margin` or `padding`) as it will
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interfere with the scrolling.
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```html
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<cdk-virtual-scroll-viewport itemSize="50">
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<table>
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<tr *cdkVirtualFor="let row of rows">
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<td>{{row.first}}</td>
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<td>{{row.second}}</td>
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</tr>
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</table>
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</cdk-virtual-scroll-viewport>
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```
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<!-- example(cdk-virtual-scroll-dl) -->

‎src/material-examples/cdk-virtual-scroll-context/cdk-virtual-scroll-context-example.css‎

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.example-viewport {
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height: 200px;
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width: 200px;
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border: 1px solid black;
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}
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.example-item-detail {
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height: 18px;
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}
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.example-alternate {
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background: rgba(127, 127, 127, 0.3);
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}

‎src/material-examples/cdk-virtual-scroll-context/cdk-virtual-scroll-context-example.html‎

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<cdk-virtual-scroll-viewport [itemSize]="18 * 7" class="example-viewport">
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<div *cdkVirtualFor="let item of items;
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let index = index;
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let count = count;
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let first = first;
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let last = last;
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let even = even;
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let odd = odd;" [class.example-alternate]="odd">
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<div class="example-item-detail">Item: {{item}}</div>
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<div class="example-item-detail">Index: {{index}}</div>
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<div class="example-item-detail">Count: {{count}}</div>
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<div class="example-item-detail">First: {{first ? 'Yes' : 'No'}}</div>
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<div class="example-item-detail">Last: {{last ? 'Yes' : 'No'}}</div>
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<div class="example-item-detail">Event: {{even ? 'Yes' : 'No'}}</div>
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<div class="example-item-detail">Odd: {{odd ? 'Yes' : 'No'}}</div>
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</div>
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</cdk-virtual-scroll-viewport>

‎src/material-examples/cdk-virtual-scroll-context/cdk-virtual-scroll-context-example.ts‎

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import {ChangeDetectionStrategy, Component} from '@angular/core';
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/** @title Virtual scroll context variables */
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@Component({
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selector: 'cdk-virtual-scroll-context-example',
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styleUrls: ['cdk-virtual-scroll-context-example.css'],
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templateUrl: 'cdk-virtual-scroll-context-example.html',
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changeDetection: ChangeDetectionStrategy.OnPush,
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})
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export class CdkVirtualScrollContextExample {
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items = Array.from({length: 100000}).map((_, i) => `Item #${i}`);
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}

‎src/material-examples/cdk-virtual-scroll-data-source/cdk-virtual-scroll-data-source-example.css‎

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.example-viewport {
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height: 200px;
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width: 200px;
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border: 1px solid black;
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}
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.example-item {
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height: 50px;
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}

‎src/material-examples/cdk-virtual-scroll-data-source/cdk-virtual-scroll-data-source-example.html‎

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<cdk-virtual-scroll-viewport itemSize="50" class="example-viewport">
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<div *cdkVirtualFor="let item of ds" class="example-item">{{item || 'Loading...'}}</div>
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</cdk-virtual-scroll-viewport>

‎src/material-examples/cdk-virtual-scroll-data-source/cdk-virtual-scroll-data-source-example.ts‎

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import {CollectionViewer, DataSource} from '@angular/cdk/collections';
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import {ChangeDetectionStrategy, Component} from '@angular/core';
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import {BehaviorSubject, Observable, Subscription} from 'rxjs';
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/** @title Virtual scroll with a custom data source */
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@Component({
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selector: 'cdk-virtual-scroll-data-source-example',
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styleUrls: ['cdk-virtual-scroll-data-source-example.css'],
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templateUrl: 'cdk-virtual-scroll-data-source-example.html',
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changeDetection: ChangeDetectionStrategy.OnPush,
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})
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export class CdkVirtualScrollDataSourceExample {
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ds = new MyDataSource();
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}
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export class MyDataSource extends DataSource<string | undefined> {
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private length = 100000;
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private pageSize = 100;
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private cachedData = Array.from<string>({length: this.length});
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private fetchedPages = new Set<number>();
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private dataStream = new BehaviorSubject<(string | undefined)[]>(this.cachedData);
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private subscription = new Subscription();
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connect(collectionViewer: CollectionViewer): Observable<(string | undefined)[]> {
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this.subscription.add(collectionViewer.viewChange.subscribe(range => {
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const startPage = this.getPageForIndex(range.start);
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const endPage = this.getPageForIndex(range.end - 1);
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for (let i = startPage; i <= endPage; i++) {
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this.fetchPage(i);
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}
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}));
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return this.dataStream;
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}
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disconnect(): void {
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this.subscription.unsubscribe();
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}
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private getPageForIndex(index: number): number {
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return Math.floor(index / this.pageSize);
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}
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private fetchPage(page: number) {
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if (this.fetchedPages.has(page)) {
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return;
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}
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this.fetchedPages.add(page);
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// Use `setTimeout` to simulate fetching data from server.
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setTimeout(() => {
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this.cachedData.splice(page * this.pageSize, this.pageSize,
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...Array.from({length: this.pageSize})
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.map((_, i) => `Item #${page * this.pageSize + i}`));
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this.dataStream.next(this.cachedData);
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}, Math.random() * 1000 + 200);
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}
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}

‎src/material-examples/cdk-virtual-scroll-dl/cdk-virtual-scroll-dl-example.css‎

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.example-viewport {
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height: 200px;
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width: 200px;
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border: 1px solid black;
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}
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.example-dt {
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height: 30px;
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font-weight: bold;
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}
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.example-dd {
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height: 30px;
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}

‎src/material-examples/cdk-virtual-scroll-dl/cdk-virtual-scroll-dl-example.html‎

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<cdk-virtual-scroll-viewport class="example-viewport" itemSize="60">
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<dl class="example-dl">
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<ng-container *cdkVirtualFor="let state of states">
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<dt class="example-dt">{{state.name}}</dt>
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<dd class="example-dd">{{state.capital}}</dd>
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</ng-container>
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</dl>
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</cdk-virtual-scroll-viewport>

‎src/material-examples/cdk-virtual-scroll-dl/cdk-virtual-scroll-dl-example.ts‎

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import {ChangeDetectionStrategy, Component} from '@angular/core';
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/** @title Virtual scrolling `<dl>` */
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@Component({
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selector: 'cdk-virtual-scroll-dl-example',
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styleUrls: ['cdk-virtual-scroll-dl-example.css'],
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templateUrl: 'cdk-virtual-scroll-dl-example.html',
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changeDetection: ChangeDetectionStrategy.OnPush,
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})
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export class CdkVirtualScrollDlExample {
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states = [
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{name: 'Alabama', capital: 'Montgomery'},
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{name: 'Alaska', capital: 'Juneau'},
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{name: 'Arizona', capital: 'Phoenix'},
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{name: 'Arkansas', capital: 'Little Rock'},
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{name: 'California', capital: 'Sacramento'},
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{name: 'Colorado', capital: 'Denver'},
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{name: 'Connecticut', capital: 'Hartford'},
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{name: 'Delaware', capital: 'Dover'},
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{name: 'Florida', capital: 'Tallahassee'},
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{name: 'Georgia', capital: 'Atlanta'},
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{name: 'Hawaii', capital: 'Honolulu'},
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{name: 'Idaho', capital: 'Boise'},
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{name: 'Illinois', capital: 'Springfield'},
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{name: 'Indiana', capital: 'Indianapolis'},
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{name: 'Iowa', capital: 'Des Moines'},
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{name: 'Kansas', capital: 'Topeka'},
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{name: 'Kentucky', capital: 'Frankfort'},
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{name: 'Louisiana', capital: 'Baton Rouge'},
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{name: 'Maine', capital: 'Augusta'},
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{name: 'Maryland', capital: 'Annapolis'},
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{name: 'Massachusetts', capital: 'Boston'},
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{name: 'Michigan', capital: 'Lansing'},
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{name: 'Minnesota', capital: 'St. Paul'},
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{name: 'Mississippi', capital: 'Jackson'},
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{name: 'Missouri', capital: 'Jefferson City'},
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{name: 'Montana', capital: 'Helena'},
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{name: 'Nebraska', capital: 'Lincoln'},
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{name: 'Nevada', capital: 'Carson City'},
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{name: 'New Hampshire', capital: 'Concord'},
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{name: 'New Jersey', capital: 'Trenton'},
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{name: 'New Mexico', capital: 'Santa Fe'},
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{name: 'New York', capital: 'Albany'},
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{name: 'North Carolina', capital: 'Raleigh'},
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{name: 'North Dakota', capital: 'Bismarck'},
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{name: 'Ohio', capital: 'Columbus'},
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{name: 'Oklahoma', capital: 'Oklahoma City'},
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{name: 'Oregon', capital: 'Salem'},
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{name: 'Pennsylvania', capital: 'Harrisburg'},
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{name: 'Rhode Island', capital: 'Providence'},
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{name: 'South Carolina', capital: 'Columbia'},
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{name: 'South Dakota', capital: 'Pierre'},
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{name: 'Tennessee', capital: 'Nashville'},
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{name: 'Texas', capital: 'Austin'},
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{name: 'Utah', capital: 'Salt Lake City'},
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{name: 'Vermont', capital: 'Montpelier'},
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{name: 'Virginia', capital: 'Richmond'},
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{name: 'Washington', capital: 'Olympia'},
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{name: 'West Virginia', capital: 'Charleston'},
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{name: 'Wisconsin', capital: 'Madison'},
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{name: 'Wyoming', capital: 'Cheyenne'},
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];
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}

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