63 lines
1.5 KiB
JavaScript
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
|
|
} |