react lazyload

思路： 
DOM加载时，<img> 标签里，添加data-src路径 = src 路径， src路径 = 本地默认图片路径，

DOM加载完成后，监听页面可视区域，有data-src时，就将src路径替换成data-src路径，进行图片加载。

/**
* 页面
*/
import LazyLoad from '../libs/LazyLoad';

imglazyload(htmlStr) {
    let imgList = [];
    const src = [];
    const dataSrc = [];
    const imgSrc = [];
    let newHtmlStr = htmlStr;
    imgList = newHtmlStr.match(/<img [^>]* src=['"]([^'"]+)[^>]*>/gi, function (match, capture) {
    });
    /*
     匹配图片（g表示匹配所有结果i表示区分大小写）
     const imgReg = /<img.*>/gi;
     匹配src属性
    */
    const srcReg = / src=['"]?([^'"]*)['"]?/gi;
    /* 匹配data-src属性
       var dataSrcReg = / data-src=['"]?([^'"]*)['"]?/gi; */
    if (imgList) {
        for (let i = 0; i < imgList.length; i++) {
            src[i] = imgList[i].match(srcReg);
            /* dataSrc[i] = imgList[i].match(dataSrcReg); */
            /* dataSrc赋值src,且不随src的改变而改变 */
            dataSrc[i] = src[i].slice(0);
            /* 替换src属性 */
            dataSrc[i][0] = dataSrc[i][0].replace(/src/gi, 'data-src');
            /* 组装 src data-src */
            imgSrc[i] = '<img class="news_img_back"' + dataSrc[i][0];
            /* 获取图片地址 */
            newHtmlStr = newHtmlStr.replace(/<img [^>]* src=['"]?([^'"]*)['"]?/gi, imgSrc[i]);
        }

　　　　　　//newHtmlStr = newHtmlStr.replace(/(<img[^>]+)src=(['"]?([^'"]*)['"]?)/gi, "$1 data-src= $2"); //功能与绿色字体部分相同

        /* 进程的最后执行，即在return之后执行 */
        setTimeout(LazyLoad, 0);
    }

    return newHtmlStr;
}

render(){

　　<div dangerouslySetInnerHTML={{ __html: this.imglazyload(news.content)}} />
}

/* lazyload.js 
 * expects a list of:
 * `<img src="blank.gif" data-src="my_image.png" width="600" height="400" class="lazy">`
 */

const $q = function (q, res) {
        if (document.querySelectorAll) {
            res = document.querySelectorAll(q);
        } else {
            const d = document, a = d.styleSheets[0] || d.createStyleSheet();
            a.addRule(q, 'f:b');
            for (let l = d.all, b = 0, c = [], f = l.length; b < f; b++) {
                l[b].currentStyle.f && c.push(l[b]);
            }
            a.removeRule(0);
            res = c;
        }
        return res;
    },

    addEventListener = function (evt, fn) {
         window.addEventListener ? window.addEventListener(evt, fn, false)
                : (window.attachEvent) ? this.attachEvent('on' + evt, fn)
                : window['on' + evt] = fn;
    },
    _has = function (obj, key) {
        return Object.prototype.hasOwnProperty.call(obj, key);
    };

/* src 与data-src 替换 */
function loadImage(el, fn) {
    const img = new Image(),
        /* getAttribute() 方法返回指定属性名的属性值 */
        src = el.getAttribute('data-src');
        /* img.onload: img元素加载完后执行 */
        img.onload = function () {
        if (!! el.parent) {
            el.parent.replaceChild(img, el);
        } else {
            el.src = src;
        }
        fn ? fn() : null;
    };
    img.src = src;
}

function elementInViewport(el) {
    /* 获取元素各边距离页面各边的距离 */
    const rect = el.getBoundingClientRect();
    return (
        rect.top >= 0
        && rect.left >= 0
        && rect.top <= (window.innerHeight || document.documentElement.clientHeight)
    );
}

export default function () {
    const images = [],
        query = $q('[data-src]'),
        processScroll = function () {
            for (let i = 0; i < images.length; i++) {
                if (elementInViewport(images[i])) {
                    loadImage(images[i], function () {
                        images.splice(i, i);
                    });
                }
            }
        };
/* Array.prototype.slice.call is not callable under our lovely IE8 */
    for (let i = 0; i < query.length; i++) {
        images.push(query[i]);
    }
    processScroll();
    addEventListener('scroll', processScroll);
}