60 lines
1.4 KiB
JavaScript
60 lines
1.4 KiB
JavaScript
import { Store } from 'svelte/store.js'
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class VirtualListStore extends Store {
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}
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const virtualListStore = new VirtualListStore({
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items: [],
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itemHeights: {},
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})
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virtualListStore.compute('visibleItems',
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['items', 'scrollTop', 'itemHeights', 'offsetHeight'],
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(items, scrollTop, itemHeights, offsetHeight) => {
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let renderBuffer = 3 * offsetHeight
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let visibleItems = []
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let totalOffset = 0
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let len = items.length
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let i = -1
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while (++i < len) {
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let { props, key } = items[i]
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let height = itemHeights[key] || 0
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let currentOffset = totalOffset
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totalOffset += height
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let isBelowViewport = (currentOffset < scrollTop)
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if (isBelowViewport) {
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if (scrollTop - renderBuffer > currentOffset) {
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continue // below the area we want to render
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}
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} else {
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if (currentOffset > (scrollTop + height + renderBuffer)) {
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break // above the area we want to render
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}
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}
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visibleItems.push({
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offset: currentOffset,
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props: props,
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key: key,
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index: i
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})
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}
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return visibleItems
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})
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virtualListStore.compute('height', ['items', 'itemHeights'], (items, itemHeights) => {
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let sum = 0
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let i = -1
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let len = items.length
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while (++i < len) {
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sum += itemHeights[items[i].key] || 0
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}
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return sum
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})
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if (process.browser && process.env.NODE_ENV !== 'production') {
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window.virtualListStore = virtualListStore
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}
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export {
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virtualListStore
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} |