HTML5 respond.js 解决IE6~8的响应式布局问题
响应式布局，理想状态是，对PC/移动各种终端进行响应。媒体查询的支持程度是IE9+以及其他现代的浏览器，但是IE8在市场当中仍然占据了比较大量的市场份额，使我们不得不进行IE低端浏览器的考虑。那么如何在IE6~8浏览器中兼容响应式布局呢？这里我们需要借助这样一个文件：respond.js。文件下载地址：https://github.com/scottjehl/Respond（或者百度搜索respond.js）。自己在阅读了官方文档之后，进行了一系列测试。友情提示各位朋友，关于respond.js的使用，有一些需要注意的地方，一旦不注意，在IE6-8中就无法显示出来。
respond.js 可以直接复制下面的用
/* Respond.js: min/max-width media query polyfill. (c) Scott Jehl. MIT Lic. j.mp/respondjs */
(function( w ){
"use strict";
//exposed namespace
var respond = {};
w.respond = respond;
//define update even in native-mq-supporting browsers, to avoid errors
respond.update = function(){};
//define ajax obj
var requestQueue = [],
xmlHttp = (function() {
var xmlhttpmethod = false;
try {
xmlhttpmethod = new w.XMLHttpRequest();
}
catch( e ){
xmlhttpmethod = new w.ActiveXObject( "Microsoft.XMLHTTP" );
}
return function(){
return xmlhttpmethod;
};
})(),
//tweaked Ajax functions from Quirksmode
ajax = function( url, callback ) {
var req = xmlHttp();
if (!req){
return;
}
req.open( "GET", url, true );
req.onreadystatechange = function () {
if ( req.readyState !== 4 || req.status !== 200 && req.status !== 304 ){
return;
}
callback( req.responseText );
};
if ( req.readyState === 4 ){
return;
}
req.send( null );
},
isUnsupportedMediaQuery = function( query ) {
return query.replace( respond.regex.minmaxwh, '' ).match( respond.regex.other );
};
//expose for testing
respond.ajax = ajax;
respond.queue = requestQueue;
respond.unsupportedmq = isUnsupportedMediaQuery;
respond.regex = {
media: /@media[^\{]+\{([^\{\}]*\{[^\}\{]*\})+/gi,
keyframes: /@(?:\-(?:o|moz|webkit)\-)?keyframes[^\{]+\{(?:[^\{\}]*\{[^\}\{]*\})+[^\}]*\}/gi,
comments: /\/\*[^*]*\*+([^/][^*]*\*+)*\//gi,
urls: /(url\()['"]?([^\/\)'"][^:\)'"]+)['"]?(\))/g,
findStyles: /@media *([^\{]+)\{([\S\s]+?)$/,
only: /(only\s+)?([a-zA-Z]+)\s?/,
minw: /\(\s*min\-width\s*:\s*(\s*[0-9\.]+)(px|em)\s*\)/,
maxw: /\(\s*max\-width\s*:\s*(\s*[0-9\.]+)(px|em)\s*\)/,
minmaxwh: /\(\s*m(in|ax)\-(height|width)\s*:\s*(\s*[0-9\.]+)(px|em)\s*\)/gi,
other: /\([^\)]*\)/g
};
//expose media query support flag for external use
respond.mediaQueriesSupported = w.matchMedia && w.matchMedia( "only all" ) !== null && w.matchMedia( "only all" ).matches;
//if media queries are supported, exit here
if( respond.mediaQueriesSupported ){
return;
}
//define vars
var doc = w.document,
docElem = doc.documentElement,
mediastyles = [],
rules = [],
appendedEls = [],
parsedSheets = {},
resizeThrottle = 30,
head = doc.getElementsByTagName( "head" )[0] || docElem,
base = doc.getElementsByTagName( "base" )[0],
links = head.getElementsByTagName( "link" ),
lastCall,
resizeDefer,
//cached container for 1em value, populated the first time it's needed
eminpx,
// returns the value of 1em in pixels
getEmValue = function() {
var ret,
div = doc.createElement('div'),
body = doc.body,
originalHTMLFontSize = docElem.style.fontSize,
originalBodyFontSize = body && body.style.fontSize,
fakeUsed = false;
div.style.cssText = "position:absolute;font-size:1em;width:1em";
if( !body ){
body = fakeUsed = doc.createElement( "body" );
body.style.background = "none";
}
// 1em in a media query is the value of the default font size of the browser
// reset docElem and body to ensure the correct value is returned
docElem.style.fontSize = "100%";
body.style.fontSize = "100%";
body.appendChild( div );
if( fakeUsed ){
docElem.insertBefore( body, docElem.firstChild );
}
ret = div.offsetWidth;
if( fakeUsed ){
docElem.removeChild( body );
}
else {
body.removeChild( div );
}
// restore the original values
docElem.style.fontSize = originalHTMLFontSize;
if( originalBodyFontSize ) {
body.style.fontSize = originalBodyFontSize;
}
//also update eminpx before returning
ret = eminpx = parseFloat(ret);
return ret;
},
//enable/disable styles
applyMedia = function( fromResize ){
var name = "clientWidth",
docElemProp = docElem[ name ],
currWidth = doc.compatMode === "CSS1Compat" && docElemProp || doc.body[ name ] || docElemProp,
styleBlocks = {},
lastLink = links[ links.length-1 ],
now = (new Date()).getTime();
//throttle resize calls
if( fromResize && lastCall && now - lastCall < resizeThrottle ){
w.clearTimeout( resizeDefer );
resizeDefer = w.setTimeout( applyMedia, resizeThrottle );
return;
}
else {
lastCall = now;
}
for( var i in mediastyles ){
if( mediastyles.hasOwnProperty( i ) ){
var thisstyle = mediastyles[ i ],
min = thisstyle.minw,
max = thisstyle.maxw,
minnull = min === null,
maxnull = max === null,
em = "em";
if( !!min ){
min = parseFloat( min ) * ( min.indexOf( em ) > -1 ? ( eminpx || getEmValue() ) : 1 );
}
if( !!max ){
max = parseFloat( max ) * ( max.indexOf( em ) > -1 ? ( eminpx || getEmValue() ) : 1 );
}
// if there's no media query at all (the () part), or min or max is not null, and if either is present, they're true
if( !thisstyle.hasquery || ( !minnull || !maxnull ) && ( minnull || currWidth >= min ) && ( maxnull || currWidth <= max ) ){
if( !styleBlocks[ thisstyle.media ] ){
styleBlocks[ thisstyle.media ] = [];
}
styleBlocks[ thisstyle.media ].push( rules[ thisstyle.rules ] );
}
}
}
//remove any existing respond style element(s)
for( var j in appendedEls ){
if( appendedEls.hasOwnProperty( j ) ){
if( appendedEls[ j ] && appendedEls[ j ].parentNode === head ){
head.removeChild( appendedEls[ j ] );
}
}
}
appendedEls.length = 0;
//inject active styles, grouped by media type
for( var k in styleBlocks ){
if( styleBlocks.hasOwnProperty( k ) ){
var ss = doc.createElement( "style" ),
css = styleBlocks[ k ].join( "\n" );
ss.type = "text/css";
ss.media = k;
//originally, ss was appended to a documentFragment and sheets were appended in bulk.
//this caused crashes in IE in a number of circumstances, such as when the HTML element had a bg image set, so appending beforehand seems best. Thanks to @dvelyk for the initial research on this one!
head.insertBefore( ss, lastLink.nextSibling );
if ( ss.styleSheet ){
ss.styleSheet.cssText = css;
}
else {
ss.appendChild( doc.createTextNode( css ) );
}
//push to appendedEls to track for later removal
appendedEls.push( ss );
}
}
},
//find media blocks in css text, convert to style blocks
translate = function( styles, href, media ){
var qs = styles.replace( respond.regex.comments, '' )
.replace( respond.regex.keyframes, '' )
.match( respond.regex.media ),
ql = qs && qs.length || 0;
//try to get CSS path
href = href.substring( 0, href.lastIndexOf( "/" ) );
var repUrls = function( css ){
return css.replace( respond.regex.urls, "$1" + href + "$2$3" );
},
useMedia = !ql && media;
//if path exists, tack on trailing slash
if( href.length ){ href += "/"; }
//if no internal queries exist, but media attr does, use that
//note: this currently lacks support for situations where a media attr is specified on a link AND
//its associated stylesheet has internal CSS media queries.
//In those cases, the media attribute will currently be ignored.
if( useMedia ){
ql = 1;
}
for( var i = 0; i < ql; i++ ){
var fullq, thisq, eachq, eql;
//media attr
if( useMedia ){
fullq = media;
rules.push( repUrls( styles ) );
}
//parse for styles
else{
fullq = qs[ i ].match( respond.regex.findStyles ) && RegExp.$1;
rules.push( RegExp.$2 && repUrls( RegExp.$2 ) );
}
eachq = fullq.split( "," );
eql = eachq.length;
for( var j = 0; j < eql; j++ ){
thisq = eachq[ j ];
if( isUnsupportedMediaQuery( thisq ) ) {
continue;
}
mediastyles.push( {
media : thisq.split( "(" )[ 0 ].match( respond.regex.only ) && RegExp.$2 || "all",
rules : rules.length - 1,
hasquery : thisq.indexOf("(") > -1,
minw : thisq.match( respond.regex.minw ) && parseFloat( RegExp.$1 ) + ( RegExp.$2 || "" ),
maxw : thisq.match( respond.regex.maxw ) && parseFloat( RegExp.$1 ) + ( RegExp.$2 || "" )
} );
}
}
applyMedia();
},
//recurse through request queue, get css text
makeRequests = function(){
if( requestQueue.length ){
var thisRequest = requestQueue.shift();
ajax( thisRequest.href, function( styles ){
translate( styles, thisRequest.href, thisRequest.media );
parsedSheets[ thisRequest.href ] = true;
// by wrapping recursive function call in setTimeout
// we prevent "Stack overflow" error in IE7
w.setTimeout(function(){ makeRequests(); },0);
} );
}
},
//loop stylesheets, send text content to translate
ripCSS = function(){
for( var i = 0; i < links.length; i++ ){
var sheet = links[ i ],
href = sheet.href,
media = sheet.media,
isCSS = sheet.rel && sheet.rel.toLowerCase() === "stylesheet";
//only links plz and prevent re-parsing
if( !!href && isCSS && !parsedSheets[ href ] ){
// selectivizr exposes css through the rawCssText expando
if (sheet.styleSheet && sheet.styleSheet.rawCssText) {
translate( sheet.styleSheet.rawCssText, href, media );
parsedSheets[ href ] = true;
} else {
if( (!/^([a-zA-Z:]*\/\/)/.test( href ) && !base) ||
href.replace( RegExp.$1, "" ).split( "/" )[0] === w.location.host ){
// IE7 doesn't handle urls that start with '//' for ajax request
// manually add in the protocol
if ( href.substring(0,2) === "//" ) { href = w.location.protocol + href; }
requestQueue.push( {
href: href,
media: media
} );
}
}
}
}
makeRequests();
};
//translate CSS
ripCSS();
//expose update for re-running respond later on
respond.update = ripCSS;
//expose getEmValue
respond.getEmValue = getEmValue;
//adjust on resize
function callMedia(){
applyMedia( true );
}
if( w.addEventListener ){
w.addEventListener( "resize", callMedia, false );
}
else if( w.attachEvent ){
w.attachEvent( "onresize", callMedia );
}
})(this);
第一步，书写基本样式
要想实现响应式布局，首先需要书写基本的响应式布局的样式。
html,body {
height: 100%;
}
@media only screen and (min-width: 480px){
body {
background: yellow;
}
}
@media only screen and (min-width: 640px) and (max-width: 1024px) {
body {
background: green;
}
}
@media screen and (min-width: 1024px){
body {
background: blue;
}
}
插件原理
接下来，需要理解respond.js的实现思路：
第一步，将head中所有外部引入的CSS文件路径取出来存储到一个数组当中；
第二步，遍历数组，并一个个发送AJAX请求；
第三步，AJAX回调后，分析response中的media query的min-width和max-width语法（注意，仅仅支持min-width和max-width），分析出viewport变化区间对应相应的css块；
第四步，页面初始化时和window.resize时，根据当前viewport使用相应的css块。
核心结论
那么此时，就可以根据基本的实现思路，得到一些书写代码时要注意的地方：
1、需要启动本地服务器（localhost），不能使用普通本地的url地址（file://开头）；
2、需要外部引入CSS文件，将CSS样式书写在style中是无效的；
3、由于respond插件是查找CSS文件，再进行处理，所以respond文件一定要放置在CSS文件的后面
4、另外，虽然把respond放置在head里还是在body后面都能够实现，但是建议放置在head中（具体原因在下面的文档提示中有提到）
5、最好不要为CSS设置utf-8的编码，使用默认（原因详见下面的文档提示部分）
欢迎沟通交流~独行冰海
文档提示
在官方文档当中的一些提示：
1、越早的引入respond.js文件，也就越可能避免IE下出现的闪屏。
2、不支持嵌套的媒体查询
3、utf-8的字符编码对respond.js文件的运行有影响
官方API原文：if CSS files are encoded in UTF-8 with Byte-Order-Mark, they will not work with Respond.js in IE7 or IE8.
基本含义就是：utf-8格式的CSS文件字符编码会对插件造成影响。
但是在我使用IE6-8进行测试的时候，都能够正常显示（无论是在css文件中增加charset设置还是在link标签中增加charset设置）。因此，并不是太清楚这个位置bug的含义。
4、跨域可能会出现闪屏（还没有测试，具体情况不详）
实例demo
html文件部分
<!doctype html> <html> <head> <meta charset="UTF-8"> <title>HTML5-响应式布局--respond.js-独行冰海</title> <link rel="stylesheet" href="test.css" charset="utf-8"> <script src="respond.min.js"></script> </head> <body> <div class="wrap" id="con"> 让IE6~8支持响应式布局——独行冰海 </div> </body> </html>
其他的支持响应式布局的插件-css3-mediaqueries-js
css3-mediaqueries-js官方文档和demo都没有，相对于respond.js css3-mediaqueries-js支持几乎所有的media query的语法。会出现闪屏。并不是很推荐使用，虽然能够支持全部的mediaqueries，但min-width和max-width其实就可以满足我们对响应式布局的需要。
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