關(guān)于性能優(yōu)化的那點(diǎn)事——BigRender首屏渲染優(yōu)化

玉伯指出：
頁(yè)面下載完畢后, 要經(jīng)過(guò)Tokenization - Tree Construction - Rendering. 要讓首屏盡快出來(lái), 得給瀏覽器減輕渲染首屏的工作量. 可以從兩方面入手:

減少DOM節(jié)點(diǎn)數(shù), 節(jié)點(diǎn)數(shù)越少, 意味著Tokenization, Rendering等操作耗費(fèi)的時(shí)間越少.(對(duì)于典型的淘寶商品詳情頁(yè),經(jīng)測(cè)試發(fā)現(xiàn), 每增加一個(gè)DOM節(jié)點(diǎn), 會(huì)導(dǎo)致首屏渲染時(shí)間延遲約0.5ms)

減少腳本執(zhí)行時(shí)間. 腳本執(zhí)行和UI Update共享一個(gè)thread, 腳本耗的時(shí)間約少, UI Update就能越發(fā)提前.

* 減少首屏DOM渲染,
* 加快首屏加載速度
* 分塊加載js/css(使用于模塊區(qū)分度高的網(wǎng)站)

* 需要更改DOM結(jié)構(gòu)
* 可能引起一些重排和重繪
* 沒有開啟js功能的用戶將看不到延遲加載的內(nèi)容
* 額外性能損耗(渲染前的textarea里面的html代碼,在服務(wù)端把html代碼保存在隱藏的textarea里面
  所以在服務(wù)端會(huì)把html代碼轉(zhuǎn)義, 尖括號(hào)等都被轉(zhuǎn)義了, 會(huì)增加服務(wù)端的壓力, 而且這個(gè)改造只是前端
  的渲染, 服務(wù)器依舊是一次計(jì)算所有的數(shù)據(jù), 輸出所有的數(shù)據(jù). 一般使用都是由后端拼接成html字符串
  然后塞入textarea標(biāo)簽, 吐給前端)
* 不利于SEO(在搜索引擎看來(lái)網(wǎng)頁(yè)也缺少了關(guān)鍵的DOM節(jié)點(diǎn), 原本信息量豐富的網(wǎng)頁(yè)內(nèi)容被放入單個(gè)的
  里面, 使搜索引擎認(rèn)為網(wǎng)頁(yè)內(nèi)容貧乏, 大幅影響排名情況)
</pre>
<b>SEO解決方案</b>
<p>關(guān)于美團(tuán)BigRender技術(shù)的SEO解決方案：</p>
<p>如果放棄BigRender手段, 雖然可以提升SEO效果, 但也會(huì)因?yàn)榫W(wǎng)頁(yè)打開變慢使用戶體驗(yàn)受損.和技術(shù)權(quán)衡后嘗試了一種解決方案, 將原有的大量團(tuán)購(gòu)單鏈接分別置于多個(gè)<textarea>內(nèi)部.測(cè)試證明有效, 搜索引擎認(rèn)為多個(gè)<textarea>構(gòu)成的網(wǎng)頁(yè)仍是信息豐富的, 排名有非常顯著的提升.</p>
<b>BigRender 完整示例:</b>
<b>css:</b>
<pre>ul {
    width: 300px;
    padding: 0;
    list-style: none;
}

.lazy {
    width: 300px;
    height: 168px;
    margin-bottom: 100px;
    background: #0cf;
}

.datalazyload {
    width: 300px;
    height: 168px;
}</pre>
<b>html:</b>
<pre><ul>
    <div   id="dlpxffr"   class="">
        <li id="xndtl5j"    class="lazy"></li>
    </div>
    <div   id="9tbvnzp"   class="">
        <li id="vfxbhhx"    class="lazy"></li>
    </div>
    <div   id="tvfnttf"   class="">
        <li id="brz5rjb"    class="lazy"></li>
    </div>
    <div   id="ptx5trr"   class="loading">
        <textarea class="datalazyload" style="visibility: hidden;">
            <style>.lazy {width: 300px; height: 168px; background: #333;}</style>
            <script>console.log("eval success");</script>
            <li id="pfz77hj"    class="lazy"></li>
        
    

;(function(win, doc) {

    // 兼容低版本 IE
    Function.prototype.bind = Function.prototype.bind || function(context) {
        var that = this;
        return function() {
            return that.apply(context, arguments);
        };
    };

    // 工具方法 begin
    var Util = {
        getElementsByClassName: function(cls) {
            if (doc.getElementsByClassName) {
                return doc.getElementsByClassName(cls);
            }

            var o = doc.getElementsByTagName("*"),
                rs = [];

            for (var i = 0, t, len = o.length; i < len; i++) {
                (t = o[i]) && ~t.className.indexOf(cls) && rs.push(t);
            }

            return rs;
        },
        addEvent: function(ele, type, fn) {
            ele.attachEvent ? ele.attachEvent("on" + type, fn) : ele.addEventListener(type, fn, false);
        },
        removeEvent: function(ele, type, fn) {
            ele.detachEvent ? ele.detachEvent("on" + type, fn) : ele.removeEventListener(type, fn, false);
        },
        getPos: function(ele) {
            var pos = {
                x: 0,
                y: 0
            };

            while (ele.offsetParent) {
                pos.x += ele.offsetLeft;
                pos.y += ele.offsetTop;
                ele = ele.offsetParent;
            }

            return pos;
        },
        getViewport: function() {
            var html = doc.documentElement;

            return {
                w: !window.innerWidth ? html.clientHeight : window.innerWidth,
                h: !window.innerHeight ? html.clientHeight : window.innerHeight
            };
        },
        getScrollHeight: function() {
            html = doc.documentElement, bd = doc.body;
            return Math.max(window.pageYOffset || 0, html.scrollTop, bd.scrollTop);
        },
        getEleSize: function(ele) {
            return {
                w: ele.offsetWidth,
                h: ele.offsetHeight
            };
        }
    };
    // 工具方法 end

    var Datalazyload = {
        threshold: 0,  // {number} 閾值，預(yù)加載高度，單位(px)
        els: null,  // {Array} 延遲加載元素集合(數(shù)組)
        fn: null,   // {Function} scroll、resize、touchmove 所綁定方法，即為 pollTextareas()

        evalScripts: function(code) {
            var head = doc.getElementsByTagName("head")[0],
                js = doc.createElement("script");

            js.text = code;
            head.insertBefore(js, head.firstChild);
            head.removeChild(js);
        },

        evalStyles: function(code) {
            var head = doc.getElementsByTagName("head")[0],
                css = doc.createElement("style");

            css.type = "text/css";

            try {
                css.appendChild(doc.createTextNode(code));
            } catch (e) {
                css.styleSheet.cssText = code;
            }

            head.appendChild(css);
        },

        extractCode: function(str, isStyle) {
            var cata = isStyle ? "style" : "script",
                scriptFragment = "<" + cata + "[^>]*>([Ss]*?)",
                matchAll = new RegExp(scriptFragment, "img"),
                matchOne = new RegExp(scriptFragment, "im"),
                matchResults = str.match(matchAll) || [],
                ret = [];

            for (var i = 0, len = matchResults.length; i < len; i++) {
                var temp = (matchResults[i].match(matchOne) || [ "", "" ])[1];
                temp && ret.push(temp);
            }
            return ret;
        },

        decodeHTML: function(str) {
            return str.replace(//g, ">").replace(/&/g, "&");
        },

        insert: function(ele) {
            var parent = ele.parentNode,
                txt = this.decodeHTML(ele.innerHTML),
                matchStyles = this.extractCode(txt, true),
                matchScripts = this.extractCode(txt);
            // console.log(txt)
            console.log(matchStyles);
            console.log(matchScripts);

            parent.innerHTML = txt
                .replace(new RegExp("]*>([Ss]*?)", "img"), "")
                .replace(new RegExp("]*>([Ss]*?)", "img"), "");

            if (matchStyles.length) {
                for (var i = matchStyles.length; i--;) {
                    this.evalStyles(matchStyles[i]);
                }
            }


            // 如果延遲部分需要做 loading 效果
            parent.className = parent.className.replace("loading", "");

            if (matchScripts.length) {
                for (var i = 0, len = matchScripts.length; i < len; i++) {
                    this.evalScripts(matchScripts[i]);
                }
            }
        },

        inView: function(ele) {
          var top = Util.getPos(ele).y
            , viewVal = Util.getViewport().h
            , scrollVal = Util.getScrollHeight()
            , eleHeight = Util.getEleSize(ele).h;

          if (top >= scrollVal  - eleHeight - this.threshold && top <= scrollVal + viewVal + this.threshold) {
            return true;
          }

          return false;
        },

        pollTextareas: function() {
            // 需延遲加載的元素已經(jīng)全部加載完
            if (!this.els.length) {
                Util.removeEvent(window, "scroll", this.fn);
                Util.removeEvent(window, "resize", this.fn);
                Util.removeEvent(doc.body, "touchMove", this.fn);
                return;
            }

            // 判斷是否需要加載
            for (var i = this.els.length; i--; ) {
                var ele = this.els[i];

                if (!this.inView(ele)) {
                    continue;
                }

                this.insert(ele);
                this.els.splice(i, 1);
            }
        },

        init: function(config) {
            var cls = config.cls;
            this.threshold = config.threshold ? config.threshold : 0;

            this.els = Array.prototype.slice.call(Util.getElementsByClassName(cls));
            this.fn = this.pollTextareas.bind(this);

            this.fn();
            Util.addEvent(window, "scroll", this.fn);
            Util.addEvent(window, "resize", this.fn);
            Util.addEvent(doc.body, "touchMove", this.fn);
        }
    };

    win["datalazyload"] = Datalazyload;
})(window, document);


// demo:
datalazyload.init({
    cls: "datalazyload",    // 需要延遲加載的類，即 textarea 的類名
    threshold: 100          // 距離底部多高，進(jìn)行延遲加載的閾值
});

參考原文