Version: v6

ion-virtual-scroll

Angular
React
Vue
Approximate Widths and Heights
Images Within Virtual Scroll
Virtual Scroll Performance Tips
Usage
Properties
Events
Methods
CSS Shadow Parts
CSS Custom Properties
Slots

note

This component has been deprecated in favor of using virtual scrolling libraries provided by each JavaScript Framework. See below for alternatives.

Virtual Scroll displays a virtual, "infinite" list. An array of records is passed to the virtual scroll containing the data to create templates for. The template created for each record, referred to as a cell, can consist of items, headers, and footers. For performance reasons, not every record in the list is rendered at once; instead a small subset of records (enough to fill the viewport) are rendered and reused as the user scrolls.

This guide will go over the recommended virtual scrolling packages for each framework integration as well as documentation for the deprecated ion-virtual-scroll component for Ionic Angular. We recommend using the framework-specific solutions listed below, but the ion-virtual-scroll documentation is available below for developers who are still using that component.

Angular

For virtual scrolling options in Ionic Angular, please see Angular Virtual Scroll Guide.

React

For virtual scrolling options in Ionic React, please see React Virtual Scroll Guide.

Vue

For virtual scrolling options in Ionic Vue, please see Vue Virtual Scroll Guide.

The following documentation applies to the ion-virtual-scroll component.

Approximate Widths and Heights

If the height of items in the virtual scroll are not close to the default size of 40px, it is extremely important to provide a value for the approxItemHeight property. An exact pixel-perfect size is not necessary, but without an estimate the virtual scroll will not render correctly.

The approximate width and height of each template is used to help determine how many cells should be created, and to help calculate the height of the scrollable area. Note that the actual rendered size of each cell comes from the app's CSS, whereas this approximation is only used to help calculate initial dimensions.

It's also important to know that Ionic's default item sizes have slightly different heights between platforms, which is perfectly fine.

Images Within Virtual Scroll

HTTP requests, image decoding, and image rendering can cause jank while scrolling. In order to better control images, Ionic provides <ion-img> to manage HTTP requests and image rendering. While scrolling through items quickly, <ion-img> knows when and when not to make requests, when and when not to render images, and only loads the images that are viewable after scrolling. Read more about ion-img.

It's also important for app developers to ensure image sizes are locked in, and after images have fully loaded they do not change size and affect any other element sizes. Simply put, to ensure rendering bugs are not introduced, it's vital that elements within a virtual item does not dynamically change.

For virtual scrolling, the natural effects of the <img> are not desirable features. We recommend using the <ion-img> component over the native <img> element because when an <img> element is added to the DOM, it immediately makes a HTTP request for the image file. Additionally, <img> renders whenever it wants which could be while the user is scrolling. However, <ion-img> is governed by the containing ion-content and does not render images while scrolling quickly.

Virtual Scroll Performance Tips

iOS Cordova WKWebView

When deploying to iOS with Cordova, it's highly recommended to use the WKWebView plugin in order to take advantage of iOS's higher performing webview. Additionally, WKWebView is superior at scrolling efficiently in comparison to the older UIWebView.

Lock in element dimensions and locations

In order for virtual scroll to efficiently size and locate every item, it's very important every element within each virtual item does not dynamically change its dimensions or location. The best way to ensure size and location does not change, it's recommended each virtual item has locked in its size via CSS.

Use `ion-img` for images

When including images within Virtual Scroll, be sure to use ion-img rather than the standard <img> HTML element. With ion-img, images are lazy loaded so only the viewable ones are rendered, and HTTP requests are efficiently controlled while scrolling.

Set Approximate Widths and Heights

As mentioned above, all elements should lock in their dimensions. However, virtual scroll isn't aware of the dimensions until after they have been rendered. For the initial render, virtual scroll still needs to set how many items should be built. With "approx" property inputs, such as approxItemHeight, we're able to give virtual scroll an approximate size, therefore allowing virtual scroll to decide how many items should be created.

Changing dataset should use `trackBy`

It is possible for the identities of elements in the iterator to change while the data does not. This can happen, for example, if the iterator produced from an RPC to the server, and that RPC is re-run. Even if the "data" hasn't changed, the second response will produce objects with different identities, and Ionic will tear down the entire DOM and rebuild it. This is an expensive operation and should be avoided if possible.

Each virtual item must stay extremely efficient, but one way to really kill its performance is to perform any DOM operations within section header and footer functions. These functions are called for every record in the dataset, so please make sure they're performant.

Usage

<ion-content>
  <ion-virtual-scroll [items]="items" approxItemHeight="320px">
    <ion-card *virtualItem="let item; let itemBounds = bounds;">
      <div>
        <ion-img [src]="item.imgSrc" [height]="item.imgHeight" [alt]="item.name"></ion-img>
      </div>
    <ion-card-header>
      <ion-card-title>{{ item.name }}</ion-card-title>
    </ion-card-header>
    <ion-card-content>{{ item.content }}</ion-card-content>
    </ion-card>
  </ion-virtual-scroll>
</ion-content>

export class VirtualScrollPageComponent {
  items: any[] = [];

  constructor() {
    for (let i = 0; i < 1000; i++) {
      this.items.push({
        name: i + ' - ' + images[rotateImg],
        imgSrc: getImgSrc(),
        avatarSrc: getImgSrc(),
        imgHeight: Math.floor(Math.random() * 50 + 150),
        content: lorem.substring(0, Math.random() * (lorem.length - 100) + 100)
      });

      rotateImg++;
      if (rotateImg === images.length) {
        rotateImg = 0;
      }
    }
  }
}

const lorem = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, seddo eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.';

const images = [
  'bandit',
  'batmobile',
  'blues-brothers',
  'bueller',
  'delorean',
  'eleanor',
  'general-lee',
  'ghostbusters',
  'knight-rider',
  'mirth-mobile'
];

function getImgSrc() {
  const src = 'https://dummyimage.com/600x400/${Math.round( Math.random() * 99999)}/fff.png';
  rotateImg++;
  if (rotateImg === images.length) {
    rotateImg = 0;
  }
  return src;
}

let rotateImg = 0;

Basic

The array of records should be passed to the items property on the ion-virtual-scroll element. The data given to the items property must be an array. An item template with the *virtualItem property is required in the ion-virtual-scroll. The *virtualItem property can be added to any element.

<ion-virtual-scroll [items]="items">
  <ion-item *virtualItem="let item">
    {{ item }}
  </ion-item>
</ion-virtual-scroll>

Section Headers and Footers

Section headers and footers are optional. They can be dynamically created from developer-defined functions. For example, a large list of contacts usually has a divider for each letter in the alphabet. Developers provide their own custom function to be called on each record. The logic in the custom function should determine whether to create the section template and what data to provide to the template. The custom function should return null if a template shouldn't be created.

<ion-virtual-scroll [items]="items" [headerFn]="myHeaderFn">
  <ion-item-divider *virtualHeader="let header">
    {{ header }}
  </ion-item-divider>
  <ion-item *virtualItem="let item">
    Item: {{ item }}
  </ion-item>
</ion-virtual-scroll>

Below is an example of a custom function called on every record. It gets passed the individual record, the record's index number, and the entire array of records. In this example, after every 20 records a header will be inserted. So between the 19th and 20th records, between the 39th and 40th, and so on, a <ion-item-divider> will be created and the template's data will come from the function's returned data.

myHeaderFn(record, recordIndex, records) {
  if (recordIndex % 20 === 0) {
    return 'Header ' + recordIndex;
  }
  return null;
}

Custom Components

If a custom component is going to be used within Virtual Scroll, it's best to wrap it with a <div> to ensure the component is rendered correctly. Since each custom component's implementation and internals can be quite different, wrapping within a <div> is a safe way to make sure dimensions are measured correctly.

<ion-virtual-scroll [items]="items">
  <div *virtualItem="let item">
    <my-custom-item [item]="item">
      {{ item }}
    </my-custom-item>
  </div>
</ion-virtual-scroll>

Properties

approxFooterHeight

Description	The approximate width of each footer template's cell. This dimension is used to help determine how many cells should be created when initialized, and to help calculate the height of the scrollable area. This height value can only use `px` units. Note that the actual rendered size of each cell comes from the app's CSS, whereas this approximation is used to help calculate initial dimensions before the item has been rendered.
Attribute	`approx-footer-height`
Type	`number`
Default	`30`

approxHeaderHeight

Description	The approximate height of each header template's cell. This dimension is used to help determine how many cells should be created when initialized, and to help calculate the height of the scrollable area. This height value can only use `px` units. Note that the actual rendered size of each cell comes from the app's CSS, whereas this approximation is used to help calculate initial dimensions before the item has been rendered.
Attribute	`approx-header-height`
Type	`number`
Default	`30`

approxItemHeight

Description	It is important to provide this if virtual item height will be significantly larger than the default The approximate height of each virtual item template's cell. This dimension is used to help determine how many cells should be created when initialized, and to help calculate the height of the scrollable area. This height value can only use `px` units. Note that the actual rendered size of each cell comes from the app's CSS, whereas this approximation is used to help calculate initial dimensions before the item has been rendered.
Attribute	`approx-item-height`
Type	`number`
Default	`45`

footerFn

Description	Section footers and the data used within its given template can be dynamically created by passing a function to `footerFn`. The logic within the footer function can decide if the footer template should be used, and what data to give to the footer template. The function must return `null` if a footer cell shouldn't be created.
Attribute	`undefined`
Type	`((item: any, index: number, items: any[]) => string ｜ null ｜ undefined) ｜ undefined`
Default	`undefined`

footerHeight

Description	An optional function that maps each item footer within their height.
Attribute	`undefined`
Type	`((item: any, index: number) => number) ｜ undefined`
Default	`undefined`

headerFn

Description	Section headers and the data used within its given template can be dynamically created by passing a function to `headerFn`. For example, a large list of contacts usually has dividers between each letter in the alphabet. App's can provide their own custom `headerFn` which is called with each record within the dataset. The logic within the header function can decide if the header template should be used, and what data to give to the header template. The function must return `null` if a header cell shouldn't be created.
Attribute	`undefined`
Type	`((item: any, index: number, items: any[]) => string ｜ null ｜ undefined) ｜ undefined`
Default	`undefined`

headerHeight

Description	An optional function that maps each item header within their height.
Attribute	`undefined`
Type	`((item: any, index: number) => number) ｜ undefined`
Default	`undefined`

itemHeight

Description	An optional function that maps each item within their height. When this function is provides, heavy optimizations and fast path can be taked by `ion-virtual-scroll` leading to massive performance improvements. This function allows to skip all DOM reads, which can be Doing so leads to massive performance
Attribute	`undefined`
Type	`((item: any, index: number) => number) ｜ undefined`
Default	`undefined`

items

Description	The data that builds the templates within the virtual scroll. It's important to note that when this data has changed, then the entire virtual scroll is reset, which is an expensive operation and should be avoided if possible.
Attribute	`undefined`
Type	`any[] ｜ undefined`
Default	`undefined`

nodeRender

Description	NOTE: only Vanilla JS API.
Attribute	`undefined`
Type	`((el: HTMLElement ｜ null, cell: Cell, domIndex: number) => HTMLElement) ｜ undefined`
Default	`undefined`

renderFooter

Description	NOTE: only JSX API for stencil. Provide a render function for the footer to be rendered. Returns a JSX virtual-dom.
Attribute	`undefined`
Type	`((item: any, index: number) => any) ｜ undefined`
Default	`undefined`

renderHeader

Description	NOTE: only JSX API for stencil. Provide a render function for the header to be rendered. Returns a JSX virtual-dom.
Attribute	`undefined`
Type	`((item: any, index: number) => any) ｜ undefined`
Default	`undefined`

renderItem

Description	NOTE: only JSX API for stencil. Provide a render function for the items to be rendered. Returns a JSX virtual-dom.
Attribute	`undefined`
Type	`((item: any, index: number) => any) ｜ undefined`
Default	`undefined`

Events

No events available for this component.

Methods

checkEnd

Description	This method marks the tail the items array as dirty, so they can be re-rendered. It's equivalent to calling: `js virtualScroll.checkRange(lastItemLen);`
Signature	`checkEnd() => Promise<void>`

checkRange

Description	This method marks a subset of items as dirty, so they can be re-rendered. Items should be marked as dirty any time the content or their style changes. The subset of items to be updated can are specifying by an offset and a length.
Signature	`checkRange(offset: number, len?: number) => Promise<void>`

positionForItem

Description	Returns the position of the virtual item at the given index.
Signature	`positionForItem(index: number) => Promise<number>`

CSS Shadow Parts

No CSS shadow parts available for this component.

CSS Custom Properties

No CSS custom properties available for this component.

Slots

No slots available for this component.

Contents

note

Angular​

React​

Vue​

Approximate Widths and Heights​

Images Within Virtual Scroll​

Virtual Scroll Performance Tips​

iOS Cordova WKWebView​

Lock in element dimensions and locations​

Use ion-img for images​

Set Approximate Widths and Heights​

Changing dataset should use trackBy​

Efficient headers and footer functions​

Usage​

Basic​

Section Headers and Footers​

Custom Components​

Properties​

approxFooterHeight​

approxHeaderHeight​

approxItemHeight​

footerFn​

footerHeight​

headerFn​

headerHeight​

itemHeight​

items​

nodeRender​

renderFooter​

renderHeader​

renderItem​

Events​

Methods​

checkEnd​

checkRange​

positionForItem​

CSS Shadow Parts​

CSS Custom Properties​

Slots​

Angular

React

Vue

Approximate Widths and Heights

Images Within Virtual Scroll

Virtual Scroll Performance Tips

iOS Cordova WKWebView

Lock in element dimensions and locations

Use `ion-img` for images

Set Approximate Widths and Heights

Changing dataset should use `trackBy`

Efficient headers and footer functions

Usage

Basic

Section Headers and Footers

Custom Components

Properties

approxFooterHeight

approxHeaderHeight

approxItemHeight

footerFn

footerHeight

headerFn

headerHeight

itemHeight

items

nodeRender

renderFooter

renderHeader

renderItem

Events

Methods

checkEnd

checkRange

positionForItem

CSS Shadow Parts

CSS Custom Properties

Slots