pinafore/routes/_utils/virtualListStore.js
2018-01-17 00:59:15 -08:00

63 lines
1.5 KiB
JavaScript

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