svelte-window

Svelte components for efficiently rendering large, scrollable lists and tabular data. Port of react-window to Svelte.

Svelte window works by only rendering part of a large data set (just enough to fill the viewport). This helps address some common performance bottlenecks:

1. It reduces the amount of work (and time) required to render the initial view and to process updates.
2. It reduces the memory footprint by avoiding over-allocation of DOM nodes.

Install

npm install --save-dev svelte-window

Usage

This library is a port of react-window, here are some examples: react-window.now.sh.

This is how to render a basic list:

<script>
    import { FixedSizeList as List, styleString as sty } from 'svelte-window';
</script>
<List
height={150}
itemCount={1000}
itemSize={35}
width={300}
let:items>
    {#each items as it (it.key)}
        <div style={sty(it.style)}>Row {it.index}</div>        
    {/each}
</List>

Here is another example with a fixed size grid with a scroll-to button, scrolling indicators:

<script>
  import { FixedSizeGrid as Grid, styleString as sty } from 'svelte-window'

  let grid

  const click = () => {
    if (grid) {
      grid.scrollToItem({
        align: 'start',
        columnIndex: 150,
        rowIndex: 300,
      })
    }
  }
</script>

<Grid
  bind:this={grid}
  columnCount={1000}
  columnWidth={100}
  height={150}
  rowCount={1000}
  rowHeight={35}
  width={300}
  useIsScrolling
  let:items>
  {#each items as it (it.key)}
    <div style={sty(it.style)}>
      {it.isScrolling ? 'Scrolling' : `Row ${it.rowIndex} - Col ${it.columnIndex}`}
    </div>
  {/each}
</Grid>

<button on:click={click}> To row 300, column 150 </button>

Calendar example with VariableSizeGrid

Here is a more complex example. Each column represents a date, workday coluns are wider than weekend columns. A sparse dataset is called.

<script lang="ts">
  import { VariableSizeGrid as Grid, styleString as sty } from 'svelte-window';
  const startDate = new Date("1/1/2022");
  const dateCell = (rowIndex:number,columnIndex:number)=>{
    return new Date(startDate.getTime() + 
      1000 * 60 * 60 * 8  + // start at 8 o'clock
      1000 * 60 * 60 * rowIndex + // each row adds an hour
      1000 * 60 * 60 * 24 * columnIndex // each column adds a day
    )
  }
 
  const isWorkday = (index)=>{
    const offset = startDate.getDay();
    const d = (index + offset) % 7;
    //return true on monday to friday
    return d!==0 && d !== 6; 
  }

  const columnWidth = (index)=>isWorkday(index) ? 200 : 100

  //round date to full hours
  const formatDate = (date:Date)=>Math.round(date.getTime() / (1000 * 60 * 60 )) 

  //sparse event data
  const rawEvents = [
    {date:new Date("1/4/2022 10:00"), event:"Pilates"},
    {date:new Date("1/11/2022 11:00"), event:"Zumba"},
    {date:new Date("1/18/2022 11:00"), event:"Pilates"},
    {date:new Date("1/25/2022 10:00"), event:"Zumba"},
    //etc.
  ]
  
  //format for quick access
  const events = rawEvents.reduce(
    (p,v)=>{
      p[formatDate(v.date)] = v
      return p
    },{}
  )
  
</script>

<Grid
  columnCount={1000}
  {columnWidth}
  height={150}
  rowCount={12}
  rowHeight={()=>95}
  width={300}
  let:items>
  {#each items as it (it.key)}
    <div style={sty(it.style)}>
      {dateCell(it.rowIndex, it.columnIndex)}
      {#if events[formatDate(dateCell(it.rowIndex,it.columnIndex))]}
        {events[formatDate(dateCell(it.rowIndex,it.columnIndex))].event}
      {/if}
    </div>
  {/each}
</Grid>

SvelteKit

This section is probably obsolete. SvelteKit is pretty stable today, so no problems should occur.

SvelteKit is in public beta, so a 100% compatibility cannot be guaranteed. Since version 1.2.0 svelte-window should work with SvelteKit when imported as a devDependency (npm i --save-dev svelte-window). By design, svelte-window is a client side library. Normal components like FixedSizeList need to be guarded from server-side-rendering (eg. with a {#if mounted}... clause). For convenience, there are SSR counterparts to all four components, which handle guarding within the library: FixedSizeListSSR, FixedSizeGridSSR, VariableSizeListSSR, VariableSizeGridSSR. In the examples above, just change eg.:

<script>
    import { FixedSizeList as List, styleString as sty } from 'svelte-window';
</script>
...

to

<script>
    import { FixedSizeListSSR as List, styleString as sty } from 'svelte-window';
</script>
...

---

Bundle Size

If you don't use all of svelte-windows components on a page, you can minimize the bundle size by using direct imports from the lib folder. Eg. you can change imports like this

import { FixedSizeListSSR as List, styleString as sty } from 'svelte-window';

to

import List from 'svelte-window/lib/FixedSizeListSSR.svelte';
import {styleString as sty} from 'svelte-window/src/styleString';

Differences to the React library

1. Grids and lists don't actively render the children. Instead, an array of item information is passed down via item props. You can use the let:item to access it and render with the each block.
2. Styles are passed down as objects, like in the React library, but Svelte only accepts strings for style. A helper function styleString is exported from svelte-window, to convert the style object to string
3. Variable sized variants have utilities to reset the cache, when row/cell sizes change. These are not directly added to the class, but to a member called instance. Eg. instead of

list.resetAfterIndex(...)

use

list.instance.resetAfterIndex(...)

Affected functions:

VariableSizeList:

• resetAfterIndex

VariableSizeGrid:

• resetAfterIndices
• resetAfterColumnIndex
• resetAfterRowIndex

Related libraries

• svelte-virtualized-auto-sizer: HOC that grows to fit all of the available space and passes the width and height values to its child.

More information

Here is a blog post about how the library was ported from React to Svelte.