字蛛是一个 WebFont 智能压缩工具，它能自动化分析页面中所使用的 WebFont 并进行按需压缩，通常好几 MB 的中文字体可以被压缩成几 KB 大小。
字蛛主页： http://font-spider.org
字蛛从 2014 年 7 月诞生以来，时隔近两年，终于发布了 v1.0.0 正式版本，改进如下：
- 支持绝大多数的中英文 Truetype 字体
- 支持开源图标字体库 （New: v1.0.0新特性）
- 支持 CSS 伪元素解析，支持 content: "string" 与 content: attr(value) 表达式
- 支持远程页面解析，并支持资源映射
- 支持四种样式规则：
- 支持四种调用方式：命令行、Gulp、Grunt、JS Api
- 性能、稳定性大幅提高
新特性：图标字体库压缩
得益于对 CSS 伪元素的支持，除了常规中英文字体压缩之外，v1.0.0 还带来了万众期待的——图标字体压缩支持，能够支持业界流行的开源图标字库。
以 Font Awesome 为例，它是一个典型的开源图标字体项目，目前包含有 628 个图标，并且还不断在添加中。虽然它已经做了很多优化，但字库的体积在移动端来说依然偏大，会影响页面载入速度。使用字蛛可以删除掉字体中没有用到的图标，将字体瘦身。例如一个使用 Font Awesome 的示例页面：
输入 font-spider 命令，启动字蛛进行字体压缩：
经过字蛛分析与压缩处理后， Font Awesome 字体中只保留了页面所用到的 20 个图标，ttf 格式字体体积由 142 KB 降为 6 KB，如果再配合使用 Webpack 等前端工具将字体 Base64 编码后内嵌到 CSS 中，载入速度可以进一步提升。
爬虫实现原理
为什么字蛛能够找到字体中没有使用的字形数据？这里就涉及到对 HTML 与 CSS 的静态分析。
虚拟浏览器技术
字蛛 v1.0.0 版本使用了虚拟浏览器技术来实现 HTML 与 CSS 加载与解析，爬虫模块所依赖的浏览器相关 API 均为它提供。
- 处理
标签以及资源定位 - 加载 标签或 @import 语句导入的 CSS 文件
- 处理 CSS Unicode 字符
- 管理网络请求，处理资源映射配置
- 支持 CSS3 选择器、样式表树、文本节点读取
由于虚拟浏览器部分涉及到太多的东西且不是本文重点，所以本文将会略过这部分细节。这部分代码已经分离出来并开源，有兴趣可以去了解： https://github.com/aui/browser-x
操作样式语法树
字蛛是通过解析样式表语法树（CSSOM）来获得 WebFont 信息，在浏览器中可以通过 document.styleSheets 来访问 CSS 的语法树，遍历 CSS 规则的函数实现：
// 遍历 CSS 的规则var eachCssRuleList = (function() { // 遍历 CSSRuleList function cssRuleListFor(cssRuleList, callback) { var index = -1; var length = cssRuleList.length; var cssRule, cssStyleSheet; while (++index < length) { cssRule = cssRuleList[index]; // 导入的样式规则 if (cssRule instanceof CSSImportRule) { cssStyleSheet = cssRule.styleSheet; cssRuleListFor(cssStyleSheet.cssRules || [], callback); // CSS 媒体查询规则 } else if (cssRule instanceof CSSMediaRule) { cssRuleListFor(cssRule.cssRules || [], callback); // 普通的规则 } else { callback(cssRule); } } } return function(callback) { var index = -1; var styleSheetList = document.styleSheets; var length = styleSheetList.length; var cssStyleSheet, cssRuleList; // 遍历 StyleSheetList while (++index < length) { cssStyleSheet = styleSheetList[index]; cssRuleList = cssStyleSheet.cssRules || []; cssRuleListFor(cssRuleList, callback); } };})();
注：浏览器环境不允许访问跨域后的 CSSOM，但虚拟浏览器没有做此限制
查找字体
遍历样式表每一个规则，收集 CSSFontFaceRule 信息：
// 字体信息var webFonts = {};// 字体对应的元素列表var elements = {};// 找到 webFonteachCssRuleList(function(cssRule) { if (cssRule instanceof CSSFontFaceRule) { var style = cssRule.style; var family = style['font-family']; var src = style.src; // 保存使用此字体的所有元素列表 elements[family] = []; // 保存字体信息 webFonts[family] = { family: family, src: src, chars: '' }; }}); 以如下页面作为示例：
<!DOCTYPE html><html lang="zh-CN"><head> <meta charset="UTF-8"> <title>font-spider</title> <style> @font-face { font-family: 'demo-font'; src: url('./demo-font.ttf'); } h1.title { font-family: 'demo-font'; } h1.title::after { content: '——海子'; } </style></head><body> <h1 id="面朝大海-春暖花开">面朝大海，春暖花开</h1></body></html> 得到 webFonts ：
{ "demo-font": { "family": "demo-font", "src": "url(\"file:///Users/aui/Documents/demo-font.ttf\")", "chars": "" }} 查找字符
利用 document.querySelectorAll() 来获取使用 WebFont 的字符：
// 获取当前节点所使用的 webFontfunction matchFontFamily(cssRule) { var style = cssRule.style; var family = style['font-family']; return webFonts[family];}// 将 fontFace 与元素、字符关联起来eachCssRuleList(function(cssRule) { if (cssRule instanceof CSSStyleRule) { var selector = cssRule.selectorText; var webfont = matchFontFamily(cssRule); if (webfont) { // 根据选择器来查找元素 var elems = document.querySelectorAll(selector); Array.prototype.forEach.call(elems, function(element) { // 获取元素的文本 webfont.chars += element.textContent; // 将元素与字体关联起来 elements[webfont.family].push(element); }); } }}); 此时 webFonts ：
{ "demo-font": { "family": "demo-font", "src": "url(\"file:///Users/aui/Documents/demo-font.ttf\")", "chars": "面朝大海，春暖花开" }} 伪元素
// 处理伪元素，找到继承的 webFonteachCssRuleList(function(cssRule) { if (cssRule instanceof CSSStyleRule) { var selector = cssRule.selectorText; var pseudoName = /\:\:?(?:before|after)$/i; if (!pseudoName.test(selector)) { return; } // 查找伪元素所在的节点 selector = selector.replace(pseudoName, ''); var elems = document.querySelectorAll(selector); // 获取伪元素 content 值 var content = cssRule.style.content.replace(/^["']|["']$/g, ''); for (var i = 0; i < elems.length; i ++) { var elem = elems[i]; for (var family in webFonts) { // 从伪元素自身不断冒泡，直到找到继承的字体 while (elem) { if (elements[family].indexOf(elem) !== -1) { webFonts[family].chars += content; break; } elem = elem.parentNode; } } } }}); 此时 WebFont：
{ "demo-font": { "family": "demo-font", "src": "url(\"file:///Users/aui/Documents/demo-font.ttf\")", "chars": "面朝大海，春暖花开————海子" }} 完整代码在线演示： https://jsfiddle.net/9ont96c4/2
至此，以上例子已经成功演示了字蛛爬虫查找字体、查找文本的工作原理。实际上 HTML 与 CSS 远比上面示例页面复杂，需要处理：
- 伪类选择器
- font 缩写
- 行内样式
- 完整的字体匹配算法
由于篇幅有限，上述细节部分可以参见 字蛛爬虫模块源码 。
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