resources/src/jquery/jquery.hidpi.js

   1 /**
   2  * Responsive images based on `srcset` and `window.devicePixelRatio` emulation where needed.
   3  *
   4  * Call `.hidpi()` on a document or part of a document to proces image srcsets within that section.
   5  *
   6  * `$.devicePixelRatio()` can be used as a substitute for `window.devicePixelRatio`.
   7  * It provides a familiar interface to retrieve the pixel ratio for browsers that don't
   8  * implement `window.devicePixelRatio` but do have a different way of getting it.
   9  *
  10  * @class jQuery.plugin.hidpi
  11  */
  12 ( function ( $ ) {
  13
  14 /**
  15  * Get reported or approximate device pixel ratio.
  16  *
  17  * - 1.0 means 1 CSS pixel is 1 hardware pixel
  18  * - 2.0 means 1 CSS pixel is 2 hardware pixels
  19  * - etc.
  20  *
  21  * Uses `window.devicePixelRatio` if available, or CSS media queries on IE.
  22  *
  23  * @static
  24  * @inheritable
  25  * @return {number} Device pixel ratio
  26  */
  27 $.devicePixelRatio = function () {
  28         if ( window.devicePixelRatio !== undefined ) {
  29                 // Most web browsers:
  30                 // * WebKit (Safari, Chrome, Android browser, etc)
  31                 // * Opera
  32                 // * Firefox 18+
  33                 return window.devicePixelRatio;
  34         } else if ( window.msMatchMedia !== undefined ) {
  35                 // Windows 8 desktops / tablets, probably Windows Phone 8
  36                 //
  37                 // IE 10 doesn't report pixel ratio directly, but we can get the
  38                 // screen DPI and divide by 96. We'll bracket to [1, 1.5, 2.0] for
  39                 // simplicity, but you may get different values depending on zoom
  40                 // factor, size of screen and orientation in Metro IE.
  41                 if ( window.msMatchMedia( '(min-resolution: 192dpi)' ).matches ) {
  42                         return 2;
  43                 } else if ( window.msMatchMedia( '(min-resolution: 144dpi)' ).matches ) {
  44                         return 1.5;
  45                 } else {
  46                         return 1;
  47                 }
  48         } else {
  49                 // Legacy browsers...
  50                 // Assume 1 if unknown.
  51                 return 1;
  52         }
  53 };
  54
  55 /**
  56  * Implement responsive images based on srcset attributes, if browser has no
  57  * native srcset support.
  58  *
  59  * @return {jQuery} This selection
  60  * @chainable
  61  */
  62 $.fn.hidpi = function () {
  63         var $target = this,
  64                 // @todo add support for dpi media query checks on Firefox, IE
  65                 devicePixelRatio = $.devicePixelRatio(),
  66                 testImage = new Image();
  67
  68         if ( devicePixelRatio > 1 && testImage.srcset === undefined ) {
  69                 // No native srcset support.
  70                 $target.find( 'img' ).each( function () {
  71                         var $img = $( this ),
  72                                 srcset = $img.attr( 'srcset' ),
  73                                 match;
  74                         if ( typeof srcset === 'string' && srcset !== '' ) {
  75                                 match = $.matchSrcSet( devicePixelRatio, srcset );
  76                                 if ( match !== null ) {
  77                                         $img.attr( 'src', match );
  78                                 }
  79                         }
  80                 } );
  81         }
  82
  83         return $target;
  84 };
  85
  86 /**
  87  * Match a srcset entry for the given device pixel ratio
  88  *
  89  * Exposed for testing.
  90  *
  91  * @private
  92  * @static
  93  * @param {number} devicePixelRatio
  94  * @param {string} srcset
  95  * @return {Mixed} null or the matching src string
  96  */
  97 $.matchSrcSet = function ( devicePixelRatio, srcset ) {
  98         var candidates,
  99                 candidate,
 100                 bits,
 101                 src,
 102                 i,
 103                 ratioStr,
 104                 ratio,
 105                 selectedRatio = 1,
 106                 selectedSrc = null;
 107         candidates = srcset.split( / *, */ );
 108         for ( i = 0; i < candidates.length; i++ ) {
 109                 candidate = candidates[i];
 110                 bits = candidate.split( / +/ );
 111                 src = bits[0];
 112                 if ( bits.length > 1 && bits[1].charAt( bits[1].length - 1 ) === 'x' ) {
 113                         ratioStr = bits[1].slice( 0, -1 );
 114                         ratio = parseFloat( ratioStr );
 115                         if ( ratio <= devicePixelRatio && ratio > selectedRatio ) {
 116                                 selectedRatio = ratio;
 117                                 selectedSrc = src;
 118                         }
 119                 }
 120         }
 121         return selectedSrc;
 122 };
 123
 124 /**
 125  * @class jQuery
 126  * @mixins jQuery.plugin.hidpi
 127  */
 128
 129 }( jQuery ) );