From d9f27087cad28e52dc504c019965654fc4c4f8a7 Mon Sep 17 00:00:00 2001
From: Mashhood Rastgar <mashhoodr@gmail.com>
Date: Wed, 10 Dec 2014 10:38:41 +0500
Subject: [PATCH] added option for delay

The image downloading starts after a delay (in ms), so if the user scroll past several images to see the ones at the end, it will load only those at the end.
---
 jquery.unveil.js | 75 +++++++++++++++++++++++++++---------------------
 1 file changed, 42 insertions(+), 33 deletions(-)

diff --git a/jquery.unveil.js b/jquery.unveil.js
index 8cfeb25..e6346e6 100644
--- a/jquery.unveil.js
+++ b/jquery.unveil.js
@@ -10,47 +10,56 @@
 
 ;(function($) {
 
-  $.fn.unveil = function(threshold, callback) {
+    $.fn.unveil = function(threshold, definedDelay, callback) {
 
-    var $w = $(window),
-        th = threshold || 0,
-        retina = window.devicePixelRatio > 1,
-        attrib = retina? "data-src-retina" : "data-src",
-        images = this,
-        loaded;
+        var $w = $(window),
+            th = threshold || 0,
+            retina = window.devicePixelRatio > 1,
+            attrib = retina? "data-src-retina" : "data-src",
+            images = this,
+            delay = definedDelay || 0,
+            lastScroll = (new Date()).getTime(),
+            loaded;
 
-    this.one("unveil", function() {
-      var source = this.getAttribute(attrib);
-      source = source || this.getAttribute("data-src");
-      if (source) {
-        this.setAttribute("src", source);
-        if (typeof callback === "function") callback.call(this);
-      }
-    });
+        this.one("unveil", function() {
+            var source = this.getAttribute(attrib);
+            source = source || this.getAttribute("data-src");
+            if (source) {
+                this.setAttribute("src", source);
+                if (typeof callback === "function") callback.call(this);
+            }
+        });
 
-    function unveil() {
-      var inview = images.filter(function() {
-        var $e = $(this);
-        if ($e.is(":hidden")) return;
+        function unveil() {
+            if(((new Date()).getTime() - lastScroll) < delay)
+                return; // do not process if the page has scrolled again since last time
+            
+            var inview = images.filter(function() {
+                var $e = $(this);
+                if ($e.is(":hidden")) return;
 
-        var wt = $w.scrollTop(),
-            wb = wt + $w.height(),
-            et = $e.offset().top,
-            eb = et + $e.height();
+                var wt = $w.scrollTop(),
+                    wb = wt + $w.height(),
+                    et = $e.offset().top,
+                    eb = et + $e.height();
 
-        return eb >= wt - th && et <= wb + th;
-      });
+                return eb >= wt - th && et <= wb + th;
+            });
 
-      loaded = inview.trigger("unveil");
-      images = images.not(loaded);
-    }
+            loaded = inview.trigger("unveil");
+            images = images.not(loaded);
+        }
 
-    $w.on("scroll.unveil resize.unveil lookup.unveil", unveil);
+        $w.on("scroll.unveil resize.unveil lookup.unveil", function () {
+            lastScroll = (new Date()).getTime();
+            setTimeout(unveil, delay);
+        });
 
-    unveil();
+        setTimeout(unveil, delay);
 
-    return this;
-
-  };
+        return this;
+    };
 
 })(window.jQuery || window.Zepto);
+
+