pinafore/routes/_components/VirtualListItem.html

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<div class="virtual-list-item {{shown ? 'shown' : ''}}"
virtual-list-key="{{key}}"
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ref:node
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style="transform: translate3d(0, {{offset}}px, 0);"
>
<:Component {component} virtualProps="{{props}}" />
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</div>
<style>
.virtual-list-item {
position: absolute;
top: 0;
opacity: 0;
pointer-events: none;
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/* will-change: transform; */ /* causes jank in mobile Firefox */
}
.shown {
opacity: 1;
pointer-events: auto;
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}
</style>
<script>
import { virtualListStore } from '../_utils/virtualListStore'
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import { mark, stop } from '../_utils/marks'
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let updateItemHeights = {}
let promise = Promise.resolve()
let onIntersectionCallbacks = {}
let intersectionObserver = new IntersectionObserver(entries => {
entries.forEach(entry => {
let key = entry.target.getAttribute('virtual-list-key')
onIntersectionCallbacks[key](entry)
})
})
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export default {
oncreate() {
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let key = this.get('key')
onIntersectionCallbacks[key] = entry => {
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let rect = entry.boundingClientRect
updateItemHeights[key] = rect.height
promise.then(() => {
// update all item heights in one microtask batch for better perf
let updatedKeys = Object.keys(updateItemHeights)
if (!updatedKeys.length) {
return
}
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mark('batch update VirtualListItem')
// batch all updates to itemHeights for better perf
let itemHeights = this.store.get('itemHeights')
for (key of updatedKeys) {
itemHeights[key] = updateItemHeights[key]
}
this.store.set({
itemHeights: itemHeights
})
updateItemHeights = {}
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stop('batch update VirtualListItem')
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})
}
intersectionObserver.observe(this.refs.node)
},
ondestroy() {
intersectionObserver.unobserve(this.refs.node)
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},
store: () => virtualListStore,
computed: {
'shown': ($itemHeights, key) => $itemHeights[key] > 0
}
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}
</script>