adding core.preloadImages()

jspsych.js

@@ -144,6 +144,33 @@
             return exp_start_time;
         };
 
+        // core.preloadImage will load images into the browser cache so that they appear quickly when
+        // used during a trial. 
+        //  images: array of paths to images
+        //  callback_complete: a function with no arguments that calls when loading is complete
+        //  callback_load: a function with a single argument that calls whenever an image is loaded
+        //                  argument is the number of images currently loaded.
+
+        core.preloadImages = function(images, callback_complete, callback_load){
+          var n_loaded = 0;
+          var loadfn = (typeof callback_load === 'undefined') ? function(){} : callback_load;
+          var finishfn = (typeof callback_complete === 'undefined') ? function(){} : callback_complete;
+          
+          for(var i=0;i<images.length;i++){
+              var img = new Image();
+              
+              img.onload = function(){
+                n_loaded++;
+                loadfn(n_loaded);
+                if(n_loaded == images.length){
+                    finishfn();
+                }
+              };
+              
+              img.src = images[i];
+          }
+        };
+
         //
         // private functions //
         //