| 1 | import { useCallback, useEffect, useMemo, useRef, useState } from 'react' |
| 2 | |
| 3 | function limitVisibleIds(ids: ReadonlySet<string>, limit: number): Set<string> { |
| 4 | if (limit <= 0) return new Set() |
| 5 | if (ids.size <= limit) return new Set(ids) |
| 6 | return new Set(Array.from(ids).slice(-limit)) |
| 7 | } |
| 8 | |
| 9 | export function useVisibleItemIds( |
| 10 | itemIds: ReadonlySet<string>, |
| 11 | limit: number |
| 12 | ): { |
| 13 | visibleIds: Set<string> |
| 14 | setItemRef: (itemId: string) => (element: HTMLElement | null) => void |
| 15 | } { |
| 16 | const [intersectingIds, setIntersectingIds] = useState<Set<string>>(() => new Set()) |
| 17 | const observerRef = useRef<IntersectionObserver | null>(null) |
| 18 | const itemRefsRef = useRef<Map<string, HTMLElement>>(new Map()) |
| 19 | const itemIdsByElementRef = useRef<WeakMap<Element, string>>(new WeakMap()) |
| 20 | |
| 21 | useEffect(() => { |
| 22 | setIntersectingIds((current) => { |
| 23 | const next = new Set(Array.from(current).filter((id) => itemIds.has(id))) |
| 24 | return next.size === current.size ? current : next |
| 25 | }) |
| 26 | }, [itemIds]) |
| 27 | |
| 28 | useEffect(() => { |
| 29 | if (itemIds.size === 0) return |
| 30 | |
| 31 | const observer = new IntersectionObserver( |
| 32 | (entries) => { |
| 33 | setIntersectingIds((current) => { |
| 34 | const next = new Set(current) |
| 35 | let changed = false |
| 36 | for (const entry of entries) { |
| 37 | const id = itemIdsByElementRef.current.get(entry.target) |
| 38 | if (!id) continue |
| 39 | if (entry.isIntersecting) { |
| 40 | next.delete(id) |
| 41 | next.add(id) |
| 42 | changed = true |
| 43 | } else if (next.delete(id)) { |
| 44 | changed = true |
| 45 | } |
| 46 | } |
| 47 | return changed ? next : current |
| 48 | }) |
| 49 | }, |
| 50 | { rootMargin: '80px 40px', threshold: 0 } |
| 51 | ) |
| 52 | observerRef.current = observer |
| 53 | for (const element of itemRefsRef.current.values()) observer.observe(element) |
| 54 | |
| 55 | return () => { |
| 56 | observer.disconnect() |
| 57 | observerRef.current = null |
| 58 | } |
| 59 | }, [itemIds.size]) |
| 60 | |
| 61 | const visibleIds = useMemo( |
| 62 | () => limitVisibleIds(intersectingIds, limit), |
| 63 | [intersectingIds, limit] |
| 64 | ) |
| 65 | |
| 66 | const setItemRef = useCallback( |
| 67 | (itemId: string) => (element: HTMLElement | null) => { |
| 68 | const itemRefs = itemRefsRef.current |
| 69 | if (element) { |
| 70 | itemRefs.set(itemId, element) |
| 71 | itemIdsByElementRef.current.set(element, itemId) |
| 72 | observerRef.current?.observe(element) |
| 73 | return |
| 74 | } |
| 75 | |
| 76 | const previous = itemRefs.get(itemId) |
| 77 | if (previous) observerRef.current?.unobserve(previous) |
| 78 | itemRefs.delete(itemId) |
| 79 | }, |
| 80 | [] |
| 81 | ) |
| 82 | |
| 83 | return { visibleIds, setItemRef } |
| 84 | } |
| 85 |