100 lines
2.4 KiB
JavaScript
100 lines
2.4 KiB
JavaScript
import { Store } from 'svelte/store.js'
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import { mark, stop } from '../_utils/marks'
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const VIEWPORT_RENDER_FACTOR = 4
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class VirtualListStore extends Store {
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constructor(state) {
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super(state)
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this._batches = {}
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}
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batchUpdate(key, subKey, value) {
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let batch = this._batches[key]
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if (!batch) {
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batch = this._batches[key] = {}
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}
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batch[subKey] = value
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requestAnimationFrame(() => {
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let batch = this._batches[key]
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if (!batch) {
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return
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}
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let updatedKeys = Object.keys(batch)
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if (!updatedKeys.length) {
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return
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}
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mark('batchUpdate()')
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let obj = this.get(key)
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for (let otherKey of updatedKeys) {
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obj[otherKey] = batch[otherKey]
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}
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delete this._batches[key]
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let toSet = {}
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toSet[key] = obj
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this.set(toSet)
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stop('batchUpdate()')
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})
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}
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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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mark('compute visibleItems')
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let renderBuffer = VIEWPORT_RENDER_FACTOR * 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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stop('compute visibleItems')
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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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virtualListStore.compute('numItems', ['items'], (items) => items.length)
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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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} |