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Virtual Scroll

In the past, we have provided an ion-virtual-scroll component in Ionic Framework to help with list virtualization. At the time this was not available in Angular, but recently Angular has provided its own solution via the @angular/cdk package.

Setup

To setup the CDK Scroller, first install @angular/cdk:

npm add @angular/cdk

This provides a collection of different utilities, but we'll focus on ScrollingModule for now.

When we want to use the CDK Scroller, we'll need to import the module in our component. For example, in a tabs starter project, we can add our import to the tabs1.module.ts file.

  import { IonicModule } from '@ionic/angular';
  import { NgModule } from '@angular/core';
  import { CommonModule } from '@angular/common';
  import { FormsModule } from '@angular/forms';
  import { Tab1Page } from './tab1.page';
  import { ExploreContainerComponentModule } from '../explore-container/explore-container.module';
+ import { ScrollingModule } from '@angular/cdk/scrolling';
  import { Tab1PageRoutingModule } from './tab1-routing.module';
  @NgModule({
    imports: [
      IonicModule,
      CommonModule,
      FormsModule,
      ExploreContainerComponentModule,
      Tab1PageRoutingModule,
+     ScrollingModule
    ],
    declarations: [Tab1Page]
  })
  export class Tab1PageModule {}
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With this added, we have access to the Virtual Scroller in the Tab1Page component.

Usage

The CDK Virtual Scroller can be added to a component by adding the cdk-virtual-scroll-viewport to a component's template.

<ion-content>
  <cdk-virtual-scroll-viewport> </cdk-virtual-scroll-viewport>
</ion-content>
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cdk-virtual-scroll-viewport becomes the root of our scrollable content and is responsible for recycling DOM nodes as they scroll out of view.

The DOM nodes at this point can be any content needed for an app. The difference is that when we want to iterate over a collection, *cdkVirtualFor is used instead of *ngFor.

<ion-content>
  <cdk-virtual-scroll-viewport>
    <ion-list>
      <ion-item *cdkVirtualFor="let item of items">
        <ion-avatar slot="start">
          <img src="https://loremflickr.com/40/40" />
        </ion-avatar>
        <ion-label>
          {{item }}
        </ion-label>
      </ion-item>
    </ion-list>
  </cdk-virtual-scroll-viewport>
</ion-content>
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Here, items is an array, but it can be an array, Observable<Array>, or DataSource. DataSource is an abstract class that can provide the data needed as well as utility methods. For more details, check out the CDK Virtual Scrolling docs.

The component is not complete yet as the cdk-virtual-scroll-viewport needs to know how big each node will be as well as the min/max buffer sizes.

At the moment, CDK Virtual Scroller only supports fixed sized elements, but dynamic sized elements are planned for the future. For the Tab1Page component, since it is only rendering an item, it can be hard-coded to a fixed size.

The min/max buffer size tells the scroller "render as many nodes as it takes to meet this minimum height, but not over this".

<cdk-virtual-scroll-viewport
  itemSize="56"
  minBufferPx="900"
  maxBufferPx="1350"
></cdk-virtual-scroll-viewport>
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For this case, the cdk-virtual-scroll-viewport will render cells at a height 56px until it reaches a height of 900px, but no more at 1350px. These numbers are arbitrary, so be sure to test out what values will work in a real use case.

Putting everything together, the final HTML should look like:

<ion-content>
  <cdk-virtual-scroll-viewport
    itemSize="56"
    minBufferPx="900"
    maxBufferPx="1350"
  >
    <ion-list>
      <ion-item *cdkVirtualFor="let item of items">
        <ion-avatar slot="start">
          <img src="https://loremflickr.com/40/40" />
        </ion-avatar>
        <ion-label>
          {{item }}
        </ion-label>
      </ion-item>
    </ion-list>
  </cdk-virtual-scroll-viewport>
</ion-content>
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The last piece needed is a some CSS to size the viewport correctly. In the tab1.page.scss file, add the following

cdk-virtual-scroll-viewport {
  height: 100%;
  width: 100%;
}
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Since the viewport is built to fit various use cases, the default sizing is not set and is up to developers to set.

A Note on Ionic Components

Certain Ionic Framework functionality is currently not compatible with virtual scrolling. Features such as collapsible large titles, ion-infinite-scroll, and ion-refresher rely on being able to scroll on ion-content itself, and as a result will not work when using virtual scrolling.

We are working to improve compatibility between these components and virtual scrolling solutions. You can follow progress and give feedback here: https://github.com/ionic-team/ionic-framework/issues/23437.

Further Reading

This only covers a small portion of what the CDK Virtual Scroller is capable of. For more details, please see the Angular CDK Virtual Scrolling docs.

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