pinafore/routes/_utils/virtualListStore.js

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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
class VirtualListStore extends Store {
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constructor(state) {
super(state)
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this._batches = {}
}
batchUpdate(key, subKey, value) {
let batch = this._batches[key]
if (!batch) {
batch = this._batches[key] = {}
}
batch[subKey] = value
requestAnimationFrame(() => {
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let batch = this._batches[key]
if (!batch) {
return
}
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let updatedKeys = Object.keys(batch)
if (!updatedKeys.length) {
return
}
mark('batchUpdate()')
let obj = this.get(key)
for (let otherKey of updatedKeys) {
obj[otherKey] = batch[otherKey]
}
delete this._batches[key]
let toSet = {}
toSet[key] = obj
this.set(toSet)
stop('batchUpdate()')
})
}
}
const virtualListStore = new VirtualListStore({
items: [],
itemHeights: {},
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showFooter: false,
footerHeight: 0
})
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virtualListStore.compute('visibleItems',
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['items', 'scrollTop', 'itemHeights', 'offsetHeight'],
(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
let len = items.length
let i = -1
while (++i < len) {
let { props, key } = items[i]
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let height = itemHeights[key] || 0
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let currentOffset = totalOffset
totalOffset += height
let isBelowViewport = (currentOffset < scrollTop)
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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} 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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visibleItems.push({
offset: currentOffset,
props: props,
key: key,
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index: i
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})
}
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stop('compute visibleItems')
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return visibleItems
})
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virtualListStore.compute('heightWithoutFooter',
['items', 'itemHeights'],
(items, itemHeights) => {
let sum = 0
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let i = -1
let len = items.length
while (++i < len) {
sum += itemHeights[items[i].key] || 0
}
return sum
})
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virtualListStore.compute('height',
['heightWithoutFooter', 'showFooter', 'footerHeight'],
(heightWithoutFooter, showFooter, footerHeight) => {
return showFooter ? (heightWithoutFooter + footerHeight) : heightWithoutFooter
})
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virtualListStore.compute('numItems', ['items'], (items) => items.length)
if (process.browser && process.env.NODE_ENV !== 'production') {
window.virtualListStore = virtualListStore
}
export {
virtualListStore
}