Respond.js – 让不懂爱的 IE6-8 支持 CSS3 Media Query

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/* 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,
				p = doc.createElement('p'),
				body = doc.body,
				originalHTMLFontSize = docElem.style.fontSize,
				originalBodyFontSize = body && body.style.fontSize,
				fakeUsed = false;

			p.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( p );

			if( fakeUsed ){
				docElem.insertBefore( body, docElem.firstChild );
			}

			ret = p.offsetWidth;

			if( fakeUsed ){
				docElem.removeChild( body );
			}
			else {
				body.removeChild( p );
			}

			// 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);