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：

 1 html {
 2         font-size : 20px;
 3     }
 4     @media only screen and (min-width: 401px){
 5         html {
 6             font-size: 25px !important;
 7         }
 8     }
 9     @media only screen and (min-width: 428px){
10         html {
11             font-size: 26.75px !important;
12         }
13     }
14     @media only screen and (min-width: 481px){
15         html {
16             font-size: 30px !important; 
17         }
18     }
19     @media only screen and (min-width: 569px){
20         html {
21             font-size: 35px !important; 
22         }
23     }
24     @media only screen and (min-width: 641px){
25         html {
26             font-size: 40px !important; 
27         }
28     }

 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:

 1  ;(function (doc, win, undefined) {
 2           var docEl = doc.documentElement,
 3             resizeEvt = 'orientationchange' in win? 'orientationchange' : 'resize',
 4             recalc = function () {
 5               var clientWidth = docEl.clientWidth;
 6               if (clientWidth === undefined) return;
 7               docEl.style.fontSize = 20 * (clientWidth / 320) + 'px';
 8             };
 9           if (doc.addEventListener === undefined) return;
10           win.addEventListener(resizeEvt, recalc, false);
11           doc.addEventListener('DOMContentLoaded', recalc, false)
12         })(document, window);
13        

另外附上淘宝移动端适配解决方案flexible.js源码:

  1 ;(function(win, lib) {
  2     var doc = win.document;
  3     var docEl = doc.documentElement;
  4     var metaEl = doc.querySelector('meta[name="viewport"]');
  5     var flexibleEl = doc.querySelector('meta[name="flexible"]');
  6     var dpr = 0;
  7     var scale = 0;
  8     var tid;
  9     var flexible = lib.flexible || (lib.flexible = {});
 10     
 11     if (metaEl) {
 12         //将根据已有的meta标签来设置缩放比例
 13         var match = metaEl.getAttribute('content').match(/initial\-scale=([\d\.]+)/);
 14         if (match) {
 15             scale = parseFloat(match[1]);
 16             dpr = parseInt(1 / scale);
 17         }
 18     } else if (flexibleEl) {
 19         var content = flexibleEl.getAttribute('content');
 20         if (content) {
 21             var initialDpr = content.match(/initial\-dpr=([\d\.]+)/);
 22             var maximumDpr = content.match(/maximum\-dpr=([\d\.]+)/);
 23             if (initialDpr) {
 24                 dpr = parseFloat(initialDpr[1]);
 25                 scale = parseFloat((1 / dpr).toFixed(2));    
 26             }
 27             if (maximumDpr) {
 28                 dpr = parseFloat(maximumDpr[1]);
 29                 scale = parseFloat((1 / dpr).toFixed(2));    
 30             }
 31         }
 32     }
 33 
 34     if (!dpr && !scale) {
 35         var isAndroid = win.navigator.appVersion.match(/android/gi);
 36         var isIPhone = win.navigator.appVersion.match(/iphone/gi);
 37         var devicePixelRatio = win.devicePixelRatio;
 38         if (isIPhone) {
 39             // iOS下，对于2和3的屏，用2倍的方案，其余的用1倍方案
 40             if (devicePixelRatio >= 3 && (!dpr || dpr >= 3)) {                
 41                 dpr = 3;
 42             } else if (devicePixelRatio >= 2 && (!dpr || dpr >= 2)){
 43                 dpr = 2;
 44             } else {
 45                 dpr = 1;
 46             }
 47         } else {
 48             // 其他设备下，仍旧使用1倍的方案
 49             dpr = 1;
 50         }
 51         scale = 1 / dpr;
 52     }
 53 
 54     docEl.setAttribute('data-dpr', dpr);
 55     if (!metaEl) {
 56         metaEl = doc.createElement('meta');
 57         metaEl.setAttribute('name', 'viewport');
 58         metaEl.setAttribute('content', 'initial-scale=' + scale + ', maximum-scale=' + scale + ', minimum-scale=' + scale + ', user-scalable=no');
 59         if (docEl.firstElementChild) {
 60             docEl.firstElementChild.appendChild(metaEl);
 61         } else {
 62             var wrap = doc.createElement('div');
 63             wrap.appendChild(metaEl);
 64             doc.write(wrap.innerHTML);
 65         }
 66     }
 67 
 68     function refreshRem(){
 69         var width = docEl.getBoundingClientRect().width;
 70         if (width / dpr > 540) {
 71             width = 540 * dpr;
 72         }
 73         var rem = width / 10;
 74         docEl.style.fontSize = rem + 'px';
 75         flexible.rem = win.rem = rem;
 76     }
 77 
 78     win.addEventListener('resize', function() {
 79         clearTimeout(tid);
 80         tid = setTimeout(refreshRem, 300);
 81     }, false);
 82     win.addEventListener('pageshow', function(e) {
 83         if (e.persisted) {
 84             clearTimeout(tid);
 85             tid = setTimeout(refreshRem, 300);
 86         }
 87     }, false);
 88 
 89     if (doc.readyState === 'complete') {
 90         doc.body.style.fontSize = 12 * dpr + 'px';
 91     } else {
 92         doc.addEventListener('DOMContentLoaded', function(e) {
 93             doc.body.style.fontSize = 12 * dpr + 'px';
 94         }, false);
 95     }
 96     
 97 
 98     refreshRem();
 99 
100     flexible.dpr = win.dpr = dpr;
101     flexible.refreshRem = refreshRem;
102     flexible.rem2px = function(d) {
103         var val = parseFloat(d) * this.rem;
104         if (typeof d === 'string' && d.match(/rem$/)) {
105             val += 'px';
106         }
107         return val;
108     }
109     flexible.px2rem = function(d) {
110         var val = parseFloat(d) / this.rem;
111         if (typeof d === 'string' && d.match(/px$/)) {
112             val += 'rem';
113         }
114         return val;
115     }
116 
117 })(window, window['lib'] || (window['lib'] = {}));

flexible.js