Bug 1015509 - [B2G][Vertical Homescreen] Home screen scroll speed doe…

…s not change based on how close icon is to edge of screen r=kgrandon

shared/elements/gaia_grid/js/grid_dragdrop.js

@@ -12,6 +12,13 @@
    * within a length of a page edge configured by this value */
   const edgePageThreshold = 50;
 
+  const screenHeight = window.innerHeight;
+
+  const scrollStep = Math.round(screenHeight / edgePageThreshold);
+
+  /* The scroll step will be 10 times bigger over the edge */
+  const maxScrollStepFactor = 10;
+
   function DragDrop(gridView) {
     this.gridView = gridView;
     this.container = gridView.element;
@@ -107,6 +114,22 @@
       this.container.classList.remove('dragging');
     },
 
+    /**
+     * The closer to edge the faster (bigger step).
+     ** Distance 0px -> 10 times faster
+     ** Distance 25px -> 5 times faster
+     ** Distance 50px (edgePageThreshold) -> 0 times
+     */
+    getScrollStep: function(distanceToEdge) {
+      var factor = maxScrollStepFactor;
+
+      if (distanceToEdge > 0) {
+        factor *= ((edgePageThreshold - distanceToEdge) / edgePageThreshold);
+      }
+
+      return Math.round(scrollStep * factor);
+    },
+
     /**
      * Scrolls the page if needed.
      * The page is scrolled via javascript if an icon is being moved,
@@ -120,9 +143,6 @@
         return;
       }
 
-      var screenHeight = window.innerHeight;
-      var scrollStep = Math.round(screenHeight / edgePageThreshold);
-
       function doScroll(amount) {
         /* jshint validthis:true */
         this.isScrolling = true;
@@ -133,18 +153,19 @@
       }
 
       var docScroll = document.documentElement.scrollTop;
-      if (touch.pageY - docScroll > screenHeight - edgePageThreshold) {
+      var distanceFromTop = Math.abs(touch.pageY - docScroll);
+      if (distanceFromTop > screenHeight - edgePageThreshold) {
         var maxY = this.maxScroll;
+        var scrollStep = this.getScrollStep(screenHeight - distanceFromTop);
         // We cannot exceed the maximum scroll value
         if (touch.pageY >= maxY || maxY - touch.pageY < scrollStep) {
           this.isScrolling = false;
           return;
         }
 
         doScroll.call(this, scrollStep);
-      } else if (touch.pageY > 0 &&
-                 touch.pageY - docScroll < edgePageThreshold) {
-        doScroll.call(this, 0 - scrollStep);
+      } else if (touch.pageY > 0 && distanceFromTop < edgePageThreshold) {
+        doScroll.call(this, 0 - this.getScrollStep(distanceFromTop));
       } else {
         this.isScrolling = false;
       }