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分析微信公众号-LocalStorage 缓存机制

· 5 min read

微信公众号的阅读体验非常流畅，作为一名程序员很好奇他具体实现套路是什么，相比于我们常规开发用了什么黑科技？ 于是下意识的打开浏览器控制台一探究 奇怪！Network里面没有发起任何请求，那文章数据哪里来呢？

假设：服务端渲染机制​

通过请求得到的HTML文件发现并不是，只拿到了标题，很显然这个假设否定❌ 继续查找发现js文件都没有返回，那么就只肯定是本地localStorage 我这菜鸟也暂时想不到其他的可能了， 看到 Local Storage 里密密麻麻的 js 文件,我就放心了这确实也是一种前端加载优化策略。

通过网上的相关文章和本地分析，梳理一下大概实现的方案​

缓存更新机制​

项目在迭代开发的过程中，难以避免需要更新资源文件，常用的方法有文件名${md5}.js或者在资源 url 后面加上特定后缀的方式 etc.，而在微信做法中以 __MOON__pages/report.js 为例，其版本信息使用 key 为 __MOON__pages/report.js_ver 的存储项保存：

__MOON__pages/report.js_ver: //res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/report3b8dd6.js

我们不直接使用这个 value 动态插入 script 节点来加载该文件，而是根据后端提供的配置信息，判断是选择使用缓存的 __MOON__pages/report.js 文件，还是重新发起加载请求。

搭建更新代码的脚手架​

（加载 combo 化）使用基于 localStorage 的缓存机制，就需要一个脚手架来管理资源文件的读取和写入，不难看出微信使用的是自己开发的脚手架 moon.js，阅读其源码代价较大，暂不分析。

资源配置信息​

前端在进行资源更新时需要后端提供一份依据供前端用于判断哪些资源需要更新，并且脚手架 moon.js 需要该资源配置信息才能正常工作，所以配置信息一定要在 moon.js 的 script 标签前输出：

window.moon_map = 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x.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/topic_tpl.html31ff31.js","pages/weapp_tpl.html.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/weapp_tpl.html36906d.js","biz_common/utils/monitor.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/utils/monitor3518c6.js","appmsg/weapp_common.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/weapp_common3af55a.js","pages/voice_tpl.html.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/voice_tpl.html38518d.js","pages/kugoumusic_ctrl.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/kugoumusic_ctrl393e3a.js","pages/qqmusic_ctrl.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/qqmusic_ctrl39b68c.js","pages/voice_component.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/voice_component3af14e.js","pages/qqmusic_tpl.html.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/qqmusic_tpl.html393e3a.js","new_video/ctl.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/new_video/ctl2d441f.js","a/testdata.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/a/testdata3b86a9.js","appmsg/reward_entry.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/reward_entry3b1cff.js","appmsg/comment.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/comment3944ad.js","appmsg/like.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/like375fea.js","pages/version4video.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/version4video3a9bef.js","a/a.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/a/a3b8ecd.js","rt/appmsg/getappmsgext.rt.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/rt/appmsg/getappmsgext.rt2c21f6.js","biz_wap/utils/storage.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/utils/storage34c264.js","biz_common/tmpl.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/tmpl3518c6.js","appmsg/share_tpl.html.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/share_tpl.html36906d.js","appmsg/img_copyright_tpl.html.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/img_copyright_tpl.html2a2c13.js","pages/video_ctrl.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/pages/video_ctrl36ebcf.js","biz_common/ui/imgonepx.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/ui/imgonepx3518c6.js","biz_common/utils/respTypes.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/utils/respTypes3518c6.js","biz_wap/utils/log.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/utils/log34c264.js","sougou/index.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/sougou/index36913b.js","biz_wap/safe/mutation_observer_report.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/safe/mutation_observer_report34c264.js","appmsg/fereport.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/fereport3b9457.js","appmsg/report.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/report3404b3.js","appmsg/report_and_source.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/report_and_source3a7477.js","appmsg/page_pos.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/page_pos3a95b8.js","appmsg/cdn_speed_report.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/cdn_speed_report3097b2.js","appmsg/wxtopic.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/wxtopic31a3be.js","appmsg/new_index.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/new_index36906d.js","appmsg/weapp.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/weapp3af55a.js","appmsg/weproduct.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/weproduct3af55a.js","appmsg/voicemsg.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/voicemsg3b1748.js","appmsg/autoread.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/autoread3af14e.js","appmsg/voice.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/voice38518d.js","appmsg/qqmusic.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/qqmusic39dc43.js","appmsg/iframe.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/iframe39ab71.js","appmsg/product.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/product393966.js","appmsg/review_image.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/review_image3af55a.js","appmsg/outer_link.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/outer_link275627.js","appmsg/copyright_report.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/copyright_report2ec4b2.js","appmsg/async.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/async3b27d5.js","biz_wap/ui/lazyload_img.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/ui/lazyload_img3af55a.js","biz_common/log/jserr.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/log/jserr3518c6.js","appmsg/share.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/share3b4418.js","appmsg/cdn_img_lib.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/cdn_img_lib38b7bb.js","biz_common/utils/url/parse.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/utils/url/parse36ebcf.js","page/appmsg/not_in_mm.css":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/style/page/appmsg/not_in_mm.css36906d.js","page/appmsg/page_mp_article_improve_combo.css":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/style/page/appmsg/page_mp_article_improve_combo.css3b86a9.js","page/appmsg_new/not_in_mm.css":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/style/page/appmsg_new/not_in_mm.css36f05c.js","page/appmsg_new/combo.css":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/style/page/appmsg_new/combo.css3b86a9.js","biz_wap/jsapi/core.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/jsapi/core3b0568.js","biz_common/dom/event.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/dom/event3a25e9.js","appmsg/test.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/test354009.js","biz_wap/utils/mmversion.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/utils/mmversion34c264.js","appmsg/max_age.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/max_age2fdd28.js","biz_common/dom/attr.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/dom/attr3518c6.js","biz_wap/utils/ajax.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/utils/ajax38c31a.js","appmsg/log.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/log300330.js","biz_common/dom/class.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/dom/class3518c6.js","biz_wap/utils/device.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_wap/utils/device34c264.js","biz_common/utils/string/html.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/biz_common/utils/string/html3518c6.js","appmsg/index.js":"//res.wx.qq.com/mmbizwap/zh_CN/htmledition/js/appmsg/index3b1748.js"};

存在 XSS 安全隐患​

在 __MOON__pages/report.js 文件的入口处加入代码 alert('Helo World'); 刷新页面后弹出提示 “Hello World”，说明使用基于 localStorage 的缓存机制存在一些安全隐患，并且微信尚未对这些攻击漏洞进行处理。

关于使用 localStorage 进行资源缓存其他思考​

  1. 如果先输出 html 然后用 js 从本地缓存读取样式再插入会出现严重的阻塞和闪烁问题
  2. 这种解决方案更加适合单页面应用否则容易产生冗余
  3. 存在浏览器兼容性问题（隐身模式 etc.）（微信具有自己的X5内核完美解决该问题）
  4. 网络速度快，协商缓存的响应延迟可能比 LS读取+eval 更小
  5. 浏览器对于单次 set 和对 LS（本质是 SQL lite）的总容量存在限制
  6. 可以节省流量，并且可以用于 A/B test
  7. 移动端的浏览器缓存经常会被清理且网络状态通常较差，更加适合这种解决方案

  大概就是这么个意思，不过要快速构建到已有的项目中还没仔细考虑，后面再讨论。 https://github.com/mtjs/mt  （腾讯开源专注于移动端的、带有增量更新特色的js模块管理框架）