d3.touch defaults to changedTouches.

This way, it’s easier to tell whether the touch changed during the event. This
also fixes #1600 because the drag behavior now only dispatches a drag event on
elements that moved, even if multiple touches are active.

d3.js

@@ -1144,7 +1144,10 @@
           type: "dragstart"
         });
         function moved() {
-          var position1 = position(parent, dragId), dx = position1[0] - position0[0], dy = position1[1] - position0[1];
+          var position1 = position(parent, dragId), dx, dy;
+          if (!position1) return;
+          dx = position1[0] - position0[0];
+          dy = position1[1] - position0[1];
           dragged |= dx | dy;
           position0 = position1;
           dispatch({
@@ -1156,7 +1159,7 @@
           });
         }
         function ended() {
-          if (dragId != null && position(parent, dragId)) return;
+          if (!position(parent, dragId)) return;
           dragSubject.on(move + dragName, null).on(end + dragName, null);
           dragRestore(dragged && d3.event.target === dragTarget);
           dispatch({
@@ -2050,7 +2053,7 @@
   d3.csv = d3.dsv(",", "text/csv");
   d3.tsv = d3.dsv("	", "text/tab-separated-values");
   d3.touch = function(container, touches, identifier) {
-    if (arguments.length < 3) identifier = touches, touches = d3_eventSource().touches;
+    if (arguments.length < 3) identifier = touches, touches = d3_eventSource().changedTouches;
     if (touches) for (var i = 0, n = touches.length, touch; i < n; ++i) {
       if ((touch = touches[i]).identifier === identifier) {
         return d3_mousePoint(container, touch);

src/behavior/drag.js

@@ -41,10 +41,11 @@ d3.behavior.drag = function() {
       dispatch({type: "dragstart"});
 
       function moved() {
-        var position1 = position(parent, dragId),
-            dx = position1[0] - position0[0],
-            dy = position1[1] - position0[1];
+        var position1 = position(parent, dragId), dx, dy;
+        if (!position1) return; // this touch didn’t move
 
+        dx = position1[0] - position0[0];
+        dy = position1[1] - position0[1];
         dragged |= dx | dy;
         position0 = position1;
 
@@ -58,7 +59,7 @@ d3.behavior.drag = function() {
       }
 
       function ended() {
-        if (dragId != null && position(parent, dragId)) return; // this touch still active
+        if (!position(parent, dragId)) return; // this touch didn’t end
         dragSubject.on(move + dragName, null).on(end + dragName, null);
         dragRestore(dragged && d3.event.target === dragTarget);
         dispatch({type: "dragend"});

src/event/touch.js

@@ -2,7 +2,7 @@ import "event";
 import "mouse";
 
 d3.touch = function(container, touches, identifier) {
-  if (arguments.length < 3) identifier = touches, touches = d3_eventSource().touches;
+  if (arguments.length < 3) identifier = touches, touches = d3_eventSource().changedTouches;
   if (touches) for (var i = 0, n = touches.length, touch; i < n; ++i) {
     if ((touch = touches[i]).identifier === identifier) {
       return d3_mousePoint(container, touch);