Fix #21 - expose dx and dy on drag event.

README.md

@@ -213,6 +213,8 @@ When a [drag event listener](#drag_on) is invoked, [d3.event](https://github.com
 * `subject` - the drag subject, defined by [*drag*.subject](#drag_subject).
 * `x` - the *x*-coordinate of the subject; see [*drag*.x](#drag_x) and [*drag*.container](#drag_container).
 * `y` - the *y*-coordinate of the subject; see [*drag*.y](#drag_y) and [*drag*.container](#drag_container).
+* `dx` - the change in *x*-coordinate since the previous drag event.
+* `dy` - the change in *y*-coordinate since the previous drag event.
 * `identifier` - the string “mouse”, or a numeric [touch identifier](https://www.w3.org/TR/touch-events/#widl-Touch-identifier).
 * `active` - the number of currently active drag gestures (on start and end, not including this one).
 * `sourceEvent` - the underlying input event, such as mousemove or touchmove.

src/drag.js

@@ -112,25 +112,25 @@ export default function(started) {
   }
 
   function beforestart(id, container, point, that, args) {
-    var p0 = point(container, id), dx, dy,
+    var p = point(container, id), dx, dy,
         sublisteners = listeners.copy(),
         node;
 
-    if (!customEvent(new DragEvent("beforestart", node, id, active, p0[0], p0[1], sublisteners), function() {
+    if (!customEvent(new DragEvent("beforestart", node, id, active, p[0], p[1], sublisteners), function() {
       if ((event.subject = node = subject.apply(that, args)) == null) return false;
-      dx = x.apply(that, args) - p0[0] || 0;
-      dy = y.apply(that, args) - p0[1] || 0;
+      dx = x.apply(that, args) - p[0] || 0;
+      dy = y.apply(that, args) - p[1] || 0;
       return true;
     })) return;
 
     return function gesture(type) {
-      var p, n;
+      var p0 = p, n;
       switch (type) {
-        case "start": p = p0, gestures[id] = gesture, n = active++; break;
+        case "start": gestures[id] = gesture, n = active++; break;
         case "end": delete gestures[id], --active; // nobreak
         case "drag": p = point(container, id), n = active; break;
       }
-      customEvent(new DragEvent(type, node, id, n, p[0] + dx, p[1] + dy, sublisteners), sublisteners.apply, sublisteners, [type, that, args]);
+      customEvent(new DragEvent(type, node, id, n, p[0] + dx, p[1] + dy, p[0] - p0[0], p[1] - p0[1], sublisteners), sublisteners.apply, sublisteners, [type, that, args]);
     };
   }
 

src/event.js

@@ -1,10 +1,12 @@
-export default function DragEvent(type, subject, id, active, x, y, dispatch) {
+export default function DragEvent(type, subject, id, active, x, y, dx, dy, dispatch) {
   this.type = type;
   this.subject = subject;
   this.identifier = id;
   this.active = active;
   this.x = x;
   this.y = y;
+  this.dx = dx;
+  this.dy = dy;
   this._ = dispatch;
 }