js动态计算移动端rem

在做移动端web app的时候，众所周知，移动设备分辨率五花八门，虽然我们可以通过CSS3的media query来实现适配，例如下面这样：

 html {
         font-size : 20px;
     }
     @media only screen and (min-width: 401px){
         html {
             font-size: 25px !important;
         }
     }
     @media only screen and (min-width: 428px){
         html {
             font-size: 26.75px !important;
         }
     }
     @media only screen and (min-width: 481px){
         html {
             font-size: 30px !important;
         }
     }
     @media only screen and (min-width: 569px){
         html {
             font-size: 35px !important;
         }
     }
     @media only screen and (min-width: 641px){
         html {
             font-size: 40px !important;
         }
     }

但是这种做法并不能适配所有设备，于是就有了实现全适配的JS解决方案，例如下面这样:

  ;(function (doc, win, undefined) {
           var docEl = doc.documentElement,
             resizeEvt = 'orientationchange' in win? 'orientationchange' : 'resize',
             recalc = function () {
               var clientWidth = docEl.clientWidth;
               if (clientWidth === undefined) return;
               docEl.style.fontSize = 20 * (clientWidth / 320) + 'px';
             };
           if (doc.addEventListener === undefined) return;
           win.addEventListener(resizeEvt, recalc, false);
           doc.addEventListener('DOMContentLoaded', recalc, false)
         })(document, window);
        

另外附上淘宝移动端适配解决方案flexible.js源码:

 ;(function(win, lib) {
     var doc = win.document;
     var docEl = doc.documentElement;
     var metaEl = doc.querySelector('meta[name="viewport"]');
     var flexibleEl = doc.querySelector('meta[name="flexible"]');
     var dpr = 0;
     var scale = 0;
     var tid;
     var flexible = lib.flexible || (lib.flexible = {});

     if (metaEl) {
         //将根据已有的meta标签来设置缩放比例
         var match = metaEl.getAttribute('content').match(/initial\-scale=([\d\.]+)/);
         if (match) {
             scale = parseFloat(match[1]);
             dpr = parseInt(1 / scale);
         }
     } else if (flexibleEl) {
         var content = flexibleEl.getAttribute('content');
         if (content) {
             var initialDpr = content.match(/initial\-dpr=([\d\.]+)/);
             var maximumDpr = content.match(/maximum\-dpr=([\d\.]+)/);
             if (initialDpr) {
                 dpr = parseFloat(initialDpr[1]);
                 scale = parseFloat((1 / dpr).toFixed(2));
             }
             if (maximumDpr) {
                 dpr = parseFloat(maximumDpr[1]);
                 scale = parseFloat((1 / dpr).toFixed(2));
             }
         }
     }

     if (!dpr && !scale) {
         var isAndroid = win.navigator.appVersion.match(/android/gi);
         var isIPhone = win.navigator.appVersion.match(/iphone/gi);
         var devicePixelRatio = win.devicePixelRatio;
         if (isIPhone) {
             // iOS下，对于2和3的屏，用2倍的方案，其余的用1倍方案
             if (devicePixelRatio >= 3 && (!dpr || dpr >= 3)) {
                 dpr = 3;
             } else if (devicePixelRatio >= 2 && (!dpr || dpr >= 2)){
                 dpr = 2;
             } else {
                 dpr = 1;
             }
         } else {
             // 其他设备下，仍旧使用1倍的方案
             dpr = 1;
         }
         scale = 1 / dpr;
     }

     docEl.setAttribute('data-dpr', dpr);
     if (!metaEl) {
         metaEl = doc.createElement('meta');
         metaEl.setAttribute('name', 'viewport');
         metaEl.setAttribute('content', 'initial-scale=' + scale + ', maximum-scale=' + scale + ', minimum-scale=' + scale + ', user-scalable=no');
         if (docEl.firstElementChild) {
             docEl.firstElementChild.appendChild(metaEl);
         } else {
             var wrap = doc.createElement('div');
             wrap.appendChild(metaEl);
             doc.write(wrap.innerHTML);
         }
     }

     function refreshRem(){
         var width = docEl.getBoundingClientRect().width;
         if (width / dpr > 540) {
             width = 540 * dpr;
         }
         var rem = width / 10;
         docEl.style.fontSize = rem + 'px';
         flexible.rem = win.rem = rem;
     }

     win.addEventListener('resize', function() {
         clearTimeout(tid);
         tid = setTimeout(refreshRem, 300);
     }, false);
     win.addEventListener('pageshow', function(e) {
         if (e.persisted) {
             clearTimeout(tid);
             tid = setTimeout(refreshRem, 300);
         }
     }, false);

     if (doc.readyState === 'complete') {
         doc.body.style.fontSize = 12 * dpr + 'px';
     } else {
         doc.addEventListener('DOMContentLoaded', function(e) {
             doc.body.style.fontSize = 12 * dpr + 'px';
         }, false);
     }

     refreshRem();

     flexible.dpr = win.dpr = dpr;
     flexible.refreshRem = refreshRem;
     flexible.rem2px = function(d) {
         var val = parseFloat(d) * this.rem;
         if (typeof d === 'string' && d.match(/rem$/)) {
             val += 'px';
         }
         return val;
     }
     flexible.px2rem = function(d) {
         var val = parseFloat(d) / this.rem;
         if (typeof d === 'string' && d.match(/px$/)) {
             val += 'rem';
         }
         return val;
     }

 })(window, window['lib'] || (window['lib'] = {}));

flexible.js