VirtualScroll

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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.

The Basics

The array of records should be passed to the virtualScroll property. The data given to the virtualScroll property must be an array. An item template with the *virtualItem property is required in the virtualScroll. The virtualScroll and *virtualItem properties can be added to any element.

<ion-list [virtualScroll]="items">

  <ion-item *virtualItem="let item">
    {{ item }}
  </ion-item>

</ion-list>

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-list [virtualScroll]="items" [headerFn]="myHeaderFn">

  <ion-item-divider *virtualHeader="let header">
    Header: {{ header }}
  </ion-item-divider>

  <ion-item *virtualItem="let item">
    Item: {{ item }}
  </ion-item>

</ion-list>

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;
}

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 approxItemHeight height. 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.

<ion-list [virtualScroll]="items">

  <ion-item *virtualItem="let item">
    <ion-avatar item-start>
      <ion-img [src]="item.avatarUrl"></ion-img>
    </ion-avatar>
    {{ item.firstName }} {{ item.lastName }}
  </ion-item>

</ion-list>

Custom Components

If a custom component is going to be used within Virtual Scroll, it's best to wrap it with a good old <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-list [virtualScroll]="items">

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

</ion-list>

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 performimg webview. Additionally, WKWebView is superior at scrolling efficiently in comparision 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 virtualTrackBy

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.

Efficient headers and footer functions

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.

Input Properties