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| 1 | +import { Fragment, h } from 'preact'; |
| 2 | +import { memo } from 'preact/compat'; |
| 3 | +import styles from './VirtualizedList.module.css'; |
| 4 | +import { useEffect, useLayoutEffect, useRef, useState } from 'preact/hooks'; |
| 5 | + |
| 6 | +/** |
| 7 | + * @template T |
| 8 | + * @typedef RenderProps |
| 9 | + * @property {T} item |
| 10 | + * @property {number} index |
| 11 | + * @property {string} cssClassName |
| 12 | + * @property {number} itemTopOffset |
| 13 | + * @property {Record<string, any>} style - inline styles to apply to your element |
| 14 | + * |
| 15 | + */ |
| 16 | + |
| 17 | +/** |
| 18 | + * @template T |
| 19 | + * @param {object} props |
| 20 | + * @param {T[]} props.items |
| 21 | + * @param {number[]} props.heights - a list of known heights for every item. This prevents needing to measure on the fly |
| 22 | + * @param {number} props.overscan - how many items should be loaded before and after the current set on screen |
| 23 | + * @param {string} props.scrollingElement - a CSS selector matching a parent element that will scroll |
| 24 | + * @param {(arg: RenderProps<T>) => import("preact").ComponentChild} props.renderItem - A function to render individual items. |
| 25 | + */ |
| 26 | +export function VirtualizedList({ items, heights, overscan, scrollingElement, renderItem }) { |
| 27 | + const { start, end } = useVisibleRows(items, heights, scrollingElement, overscan); |
| 28 | + const subset = items.slice(start, end + 1); |
| 29 | + return ( |
| 30 | + <Fragment> |
| 31 | + {subset.map((item, rowIndex) => { |
| 32 | + const originalIndex = start + rowIndex; |
| 33 | + const itemTopOffset = heights.slice(0, originalIndex).reduce((acc, item) => acc + item, 0); |
| 34 | + return renderItem({ |
| 35 | + item, |
| 36 | + index: originalIndex, |
| 37 | + cssClassName: styles.listItem, |
| 38 | + itemTopOffset, |
| 39 | + style: { |
| 40 | + transform: `translateY(${itemTopOffset}px)`, |
| 41 | + }, |
| 42 | + }); |
| 43 | + })} |
| 44 | + </Fragment> |
| 45 | + ); |
| 46 | +} |
| 47 | + |
| 48 | +export const VisibleItems = memo(VirtualizedList); |
| 49 | + |
| 50 | +/** |
| 51 | + * @param {Array} rows - The array of rows to be virtually rendered. Each row represents an item in the list. |
| 52 | + * @param {number[]} heights - index lookup for known element heights |
| 53 | + * @param {string} scrollerSelector - A CSS selector for tracking the scrollable area |
| 54 | + * @param {number} overscan - how many items to fetch outside the window |
| 55 | + * @return {Object} An object containing the calculated `start` and `end` indices of the visible rows. |
| 56 | + */ |
| 57 | +function useVisibleRows(rows, heights, scrollerSelector, overscan = 5) { |
| 58 | + // set the start/end indexes of the elements |
| 59 | + const [{ start, end }, setVisibleRange] = useState({ start: 0, end: 1 }); |
| 60 | + |
| 61 | + // hold a mutable value that we update on resize |
| 62 | + const mainScrollerRef = useRef(/** @type {Element|null} */ (null)); |
| 63 | + const scrollingSize = useRef(/** @type {number|null} */ (null)); |
| 64 | + |
| 65 | + /** |
| 66 | + * When called, make the expensive calls to `getBoundingClientRect` to measure things |
| 67 | + */ |
| 68 | + function updateGlobals() { |
| 69 | + if (!mainScrollerRef.current) return; |
| 70 | + const rec = mainScrollerRef.current.getBoundingClientRect(); |
| 71 | + scrollingSize.current = rec.height; |
| 72 | + } |
| 73 | + |
| 74 | + /** |
| 75 | + * decide which the start/end indexes should be, based on scroll position. |
| 76 | + * NOTE: this is called on scroll, so must not incur expensive checks/measurements - math only! |
| 77 | + */ |
| 78 | + function setVisibleRowsForOffset() { |
| 79 | + if (!mainScrollerRef.current) return console.warn('cannot access mainScroller ref'); |
| 80 | + if (scrollingSize.current === null) return console.warn('need height'); |
| 81 | + const scrollY = mainScrollerRef.current?.scrollTop ?? 0; |
| 82 | + const next = calcVisibleRows(heights || [], scrollingSize.current, scrollY); |
| 83 | + |
| 84 | + const withOverScan = { |
| 85 | + start: Math.max(next.startIndex - overscan, 0), |
| 86 | + end: next.endIndex + overscan, |
| 87 | + }; |
| 88 | + |
| 89 | + // don't set state if the offset didn't change |
| 90 | + setVisibleRange((prev) => { |
| 91 | + if (withOverScan.start !== prev.start || withOverScan.end !== prev.end) { |
| 92 | + // todo: find a better place to emit this! |
| 93 | + window.dispatchEvent(new CustomEvent('range-change', { detail: { start: withOverScan.start, end: withOverScan.end } })); |
| 94 | + return { start: withOverScan.start, end: withOverScan.end }; |
| 95 | + } |
| 96 | + return prev; |
| 97 | + }); |
| 98 | + } |
| 99 | + |
| 100 | + useLayoutEffect(() => { |
| 101 | + mainScrollerRef.current = document.querySelector(scrollerSelector) || document.documentElement; |
| 102 | + if (!mainScrollerRef.current) console.warn('missing elements'); |
| 103 | + |
| 104 | + // always update globals first |
| 105 | + updateGlobals(); |
| 106 | + |
| 107 | + // and set visible rows once the size is known |
| 108 | + setVisibleRowsForOffset(); |
| 109 | + |
| 110 | + const controller = new AbortController(); |
| 111 | + |
| 112 | + // when the main area is scrolled, update the visible offset for the rows. |
| 113 | + mainScrollerRef.current?.addEventListener('scroll', setVisibleRowsForOffset, { signal: controller.signal }); |
| 114 | + |
| 115 | + return () => { |
| 116 | + controller.abort(); |
| 117 | + }; |
| 118 | + }, [rows, heights, scrollerSelector]); |
| 119 | + |
| 120 | + useEffect(() => { |
| 121 | + let lastWindowHeight = window.innerHeight; |
| 122 | + function handler() { |
| 123 | + if (lastWindowHeight === window.innerHeight) return; |
| 124 | + lastWindowHeight = window.innerHeight; |
| 125 | + updateGlobals(); |
| 126 | + setVisibleRowsForOffset(); |
| 127 | + } |
| 128 | + window.addEventListener('resize', handler); |
| 129 | + return () => { |
| 130 | + return window.removeEventListener('resize', handler); |
| 131 | + }; |
| 132 | + }, [heights, rows]); |
| 133 | + |
| 134 | + return { start, end }; |
| 135 | +} |
| 136 | + |
| 137 | +/** |
| 138 | + * @param {number[]} heights - an array of integers that represents a 1:1 mapping to `rows` - each value is pixels |
| 139 | + * @param {number} space - the height in pixels that we have to fill |
| 140 | + * @param {number} scrollOffset - the y offset in pixels representing scrolling |
| 141 | + * @return {{startIndex: number, endIndex: number}} |
| 142 | + */ |
| 143 | +function calcVisibleRows(heights, space, scrollOffset) { |
| 144 | + let startIndex = 0; |
| 145 | + let endIndex = 0; |
| 146 | + let currentHeight = 0; |
| 147 | + |
| 148 | + // Adjust startIndex for the scrollOffset |
| 149 | + for (let i = 0; i < heights.length; i++) { |
| 150 | + if (currentHeight + heights[i] > scrollOffset) { |
| 151 | + startIndex = i; |
| 152 | + break; |
| 153 | + } |
| 154 | + currentHeight += heights[i]; |
| 155 | + } |
| 156 | + |
| 157 | + // Start calculating endIndex from the adjusted startIndex |
| 158 | + currentHeight = 0; |
| 159 | + for (let i = startIndex; i < heights.length; i++) { |
| 160 | + if (currentHeight + heights[i] > space) { |
| 161 | + endIndex = i; |
| 162 | + break; |
| 163 | + } |
| 164 | + currentHeight += heights[i]; |
| 165 | + endIndex = i; |
| 166 | + } |
| 167 | + |
| 168 | + return { startIndex, endIndex }; |
| 169 | +} |
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