在 React 中实现无限滚动和图像延迟加载

 var intObserver = new IntersectionObserver(entries => { entries.forEach(entry => { console.log(entry) console.log(entry.isIntersecting) // returns true if the target intersects the root element }) }, { // default options } ); let target = document.querySelector('#targetId'); intObserver.observe(target); // start observation

 import React, { useEffect, useReducer } from 'react'; import './index.css'; function App() { const imgReducer = (state, action) => { switch (action.type) { case 'STACK_IMAGES': return { ...state, images: state.images.concat(action.images) } case 'FETCHING_IMAGES': return { ...state, fetching: action.fetching } default: return state; } } const [imgData, imgDispatch] = useReducer(imgReducer,{ images:[], fetching: true}) // next code block goes here }

 // make API calls useEffect(() => { imgDispatch({ type: 'FETCHING_IMAGES', fetching: true }) fetch('https://picsum.photos/v2/list?page=0&limit=10') .then(data => data.json()) .then(images => { imgDispatch({ type: 'STACK_IMAGES', images }) imgDispatch({ type: 'FETCHING_IMAGES', fetching: false }) }) .catch(e => { // handle error imgDispatch({ type: 'FETCHING_IMAGES', fetching: false }) return e }) }, [ imgDispatch ]) // next code block goes here

 return ( <div className=""> <nav className="navbar bg-light"> <div className="container"> <a className="navbar-brand" href="/#"> <h2>Infinite scroll + image lazy loading</h2> </a> </div> </navv <div id='images' className="container"> <div className="row"> {imgData.images.map((image, index) => { const { author, download_url } = image return ( <div key={index} className="card"> <div className="card-body "> <img alt={author} className="card-img-top" src={download_url} /> </div> <div className="card-footer"> <p className="card-text text-center text-capitalize text-primary">Shot by: {author}</p> </div> </div> ) })} </div> </div> </div> );

 // App.js const imgReducer = (state, action) => { ... } const pageReducer = (state, action) => { switch (action.type) { case 'ADVANCE_PAGE': return { ...state, page: state.page + 1 } default: return state; } } const [ pager, pagerDispatch ] = useReducer(pageReducer, { page: 0 })

 // App.js import React, { useEffect, useReducer, useCallback, useRef } from 'react'; useEffect(() => { ... }, [ imgDispatch, pager.page ]) // implement infinite scrolling with intersection observer let bottomBoundaryRef = useRef(null); const scrollObserver = useCallback( node => { new IntersectionObserver(entries => { entries.forEach(en => { if (en.intersectionRatio > 0) { pagerDispatch({ type: 'ADVANCE_PAGE' }); } }); }).observe(node); }, [pagerDispatch] ); useEffect(() => { if (bottomBoundaryRef.current) { scrollObserver(bottomBoundaryRef.current); } }, [scrollObserver, bottomBoundaryRef]);

 bottomBoundaryRef.current = <div></div>

 // App.js <div id='image'> ... </div> {imgData.fetching && ( <div className="text-center bg-secondary m-auto p-3"> <p className="m-0 text-white">Getting images</p> </div> )} <div id='page-bottom-boundary' style={{ border: '1px solid red' }} ref={bottomBoundaryRef}></div>

 // App.js // lazy loads images with intersection observer // only swap out the image source if the new url exists const imagesRef = useRef(null); const imgObserver = useCallback(node => { const intObs = new IntersectionObserver(entries => { entries.forEach(en => { if (en.intersectionRatio > 0) { const currentImg = en.target; const newImgSrc = currentImg.dataset.src; // only swap out the image source if the new url exists if (!newImgSrc) { console.error('Image source is invalid'); } else { currentImg.src = newImgSrc; } intObs.unobserve(node); // detach the observer when done } }); }) intObs.observe(node); }, []); useEffect(() => { imagesRef.current = document.querySelectorAll('.card-img-top'); if (imagesRef.current) { imagesRef.current.forEach(img => imgObserver(img)); } }, [imgObserver, imagesRef, imgData.images]);

 <img alt={author} data-src={download_url} className="card-img-top" src={'https://picsum.photos/id/870/300/300?grayscale&blur=2'} />

 // customHooks.js import { useEffect, useCallback, useRef } from 'react'; // make API calls and pass the returned data via dispatch export const useFetch = (data, dispatch) => { useEffect(() => { dispatch({ type: 'FETCHING_IMAGES', fetching: true }); fetch(`https://picsum.photos/v2/list?page=${data.page}&limit=10`) .then(data => data.json()) .then(images => { dispatch({ type: 'STACK_IMAGES', images }); dispatch({ type: 'FETCHING_IMAGES', fetching: false }); }) .catch(e => { dispatch({ type: 'FETCHING_IMAGES', fetching: false }); return e; }) }, [dispatch, data.page]) } // next code block here

 // infinite scrolling with intersection observer export const useInfiniteScroll = (scrollRef, dispatch) => { const scrollObserver = useCallback( node => { new IntersectionObserver(entries => { entries.forEach(en => { if (en.intersectionRatio > 0) { dispatch({ type: 'ADVANCE_PAGE' }); } }); }).observe(node); }, [dispatch] ); useEffect(() => { if (scrollRef.current) { scrollObserver(scrollRef.current); } }, [scrollObserver, scrollRef]); } // next code block here

 // lazy load images with intersection observer export const useLazyLoading = (imgSelector, items) => { const imgObserver = useCallback(node => { const intObs = new IntersectionObserver(entries => { entries.forEach(en => { if (en.intersectionRatio > 0) { const currentImg = en.target; const newImgSrc = currentImg.dataset.src; // only swap out the image source if the new url exists if (!newImgSrc) { console.error('Image source is invalid'); } else { currentImg.src = newImgSrc; } intObs.unobserve(node); // detach the observer when done } }); }) intObs.observe(node); }, []); const imagesRef = useRef(null); useEffect(() => { imagesRef.current = document.querySelectorAll(imgSelector); if (imagesRef.current) { imagesRef.current.forEach(img => imgObserver(img)); } }, [imgObserver, imagesRef, imgSelector, items]) }

 // App.js // import custom hooks import { useFetch, useInfiniteScroll, useLazyLoading } from './customHooks' const imgReducer = (state, action) => { ... } // retain this const pageReducer = (state, action) => { ... } // retain this const [pager, pagerDispatch] = useReducer(pageReducer, { page: 0 }) // retain this const [imgData, imgDispatch] = useReducer(imgReducer,{ images:[], fetching: true }) // retain this let bottomBoundaryRef = useRef(null); useFetch(pager, imgDispatch); useLazyLoading('.card-img-top', imgData.images) useInfiniteScroll(bottomBoundaryRef, pagerDispatch); // retain the return block return ( ... )