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jquery.scrolldepth.js

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+/*
+ * jquery.scrolldepth.js
+ * Copyright (c) 2012 Rob Flaherty (@robflaherty)
+ * Licensed under the MIT and GPL licenses.
+ */
+;(function ( $, window, document, undefined ) {
+
+  "use strict";
+
+  var defaults = {
+    elements: [],
+    minHeight: 0,
+    offset: 0,
+    percentage: true
+  },
+
+  $window = $(window),
+  cache = [];
+
+  function sendEvent(action, label) {
+    _gaq.push(['_trackEvent', 'Scroll Depth', action, label]);
+  }
+
+  function calculateMarks(docHeight) {
+    return {
+      '25%' : parseInt(docHeight * 0.25, 10),          
+      '50%' : parseInt(docHeight * 0.50, 10),          
+      '75%' : parseInt(docHeight * 0.75, 10),
+      '100%': docHeight
+    };
+  }
+
+  function checkMarks(marks, scrollDistance) {
+    // Check each active mark
+    $.each(marks, function(key, val) {
+      if ( $.inArray(key, cache) === -1 && scrollDistance >= val ) {
+        sendEvent('Percentage', key);
+        cache.push(key);
+      }
+    });
+  }
+
+  function checkElements(elements, scrollDistance) {
+    $.each(elements, function(index, elem) {
+      if ( $.inArray(elem, cache) === -1) {
+        var position = $(elem).offset().top;
+
+        if (scrollDistance >= position) {
+          sendEvent('Elements', elem);
+          cache.push(elem);
+        }
+      }
+    });
+  }
+
+  $.scrollDepth = function(options) {
+
+    options = $.extend({}, defaults, options);
+
+    if ( $(document).height() < options.minHeight ) {
+      return;
+    }
+
+    // Establish baseline (0% scroll)
+    sendEvent('Percentage', 'Baseline');
+
+    $window.on('scroll.scrollDepth', function() {
+      var docHeight = $(document).height(),
+        winHeight = $window.height(),
+        scrollDistance = $window.scrollTop() + winHeight,
+        offset = parseInt(winHeight * (options.offset / 100), 10),
+        marks = calculateMarks(docHeight);
+
+      // If all marks already hit, unbind scroll event
+      if (cache.length >= marks.length + options.elements.length) {
+        $window.off('scroll.scrollDepth');
+        return;
+      }
+
+      // Check specified DOM elements
+      if (options.elements) {
+        checkElements(options.elements, scrollDistance);
+      }
+
+      // Check standard marks
+      if (options.percentage) {        
+        checkMarks(marks, scrollDistance);        
+      }
+    });
+
+  };
+
+})( jQuery, window, document );