Force radial x y accessors

README.md

@@ -457,8 +457,25 @@ The *radius* accessor is invoked for each [node](#simulation_nodes) in the simul
 
 <a name="radial_x" href="#radial_x">#</a> <i>radial</i>.<b>x</b>([<i>x</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/radial.js "Source")
 
-If *x* is specified, sets the *x*-coordinate of the circle center to the specified number and returns this force. If *x* is not specified, returns the current *x*-coordinate of the center, which defaults to zero.
+If *x* is specified, sets the *x*-coordinate accessor to the specified number or function, re-evaluates the *x*-accessor for each node, and returns this force. If *x* is not specified, returns the current *x*-accessor, which defaults to:
+
+```js
+function x() {
+  return 0;
+}
+```
+
+The *x*-accessor is invoked for each [node](#simulation_nodes) in the simulation, being passed the *node* and its zero-based *index*. The resulting number is then stored internally, such that the target *x*-coordinate of each node is only recomputed when the force is initialized or when this method is called with a new *x*, and not on every application of the force.
 
 <a name="radial_y" href="#radial_y">#</a> <i>radial</i>.<b>y</b>([<i>y</i>]) [<>](https://github.com/d3/d3-force/blob/master/src/radial.js "Source")
 
-If *y* is specified, sets the *y*-coordinate of the circle center to the specified number and returns this force. If *y* is not specified, returns the current *y*-coordinate of the center, which defaults to zero.
+If *y* is specified, sets the *y*-coordinate accessor to the specified number or function, re-evaluates the *y*-accessor for each node, and returns this force. If *y* is not specified, returns the current *y*-accessor, which defaults to:
+
+```js
+function y() {
+  return 0;
+}
+```
+
+The *y*-accessor is invoked for each [node](#simulation_nodes) in the simulation, being passed the *node* and its zero-based *index*. The resulting number is then stored internally, such that the target *y*-coordinate of each node is only recomputed when the force is initialized or when this method is called with a new *y*, and not on every application of the force.
+

src/radial.js

@@ -4,17 +4,19 @@ export default function(radius, x, y) {
   var nodes,
       strength = constant(0.1),
       strengths,
-      radiuses;
+      radiuses,
+      xs,
+      ys;
 
   if (typeof radius !== "function") radius = constant(+radius);
-  if (x == null) x = 0;
-  if (y == null) y = 0;
+  if (typeof x !== "function") x = constant(x == null ? 0 : +x);
+  if (typeof y !== "function") y = constant(y == null ? 0 : +y);
 
   function force(alpha) {
     for (var i = 0, n = nodes.length; i < n; ++i) {
       var node = nodes[i],
-          dx = node.x - x || 1e-6,
-          dy = node.y - y || 1e-6,
+          dx = node.x - xs[i] || 1e-6,
+          dy = node.y - ys[i] || 1e-6,
           r = Math.sqrt(dx * dx + dy * dy),
           k = (radiuses[i] - r) * strengths[i] * alpha / r;
       node.vx += dx * k;
@@ -27,8 +29,12 @@ export default function(radius, x, y) {
     var i, n = nodes.length;
     strengths = new Array(n);
     radiuses = new Array(n);
+    xs = new Array(n);
+    ys = new Array(n);
     for (i = 0; i < n; ++i) {
       radiuses[i] = +radius(nodes[i], i, nodes);
+      xs[i] = +x(nodes[i], i, nodes);
+      ys[i] = +y(nodes[i], i, nodes);
       strengths[i] = isNaN(radiuses[i]) ? 0 : +strength(nodes[i], i, nodes);
     }
   }
@@ -46,11 +52,11 @@ export default function(radius, x, y) {
   };
 
   force.x = function(_) {
-    return arguments.length ? (x = +_, force) : x;
+    return arguments.length ? (x = typeof _ === "function" ? _ : constant(+_), initialize(), force) : x;
   };
 
   force.y = function(_) {
-    return arguments.length ? (y = +_, force) : y;
+    return arguments.length ? (y = typeof _ === "function" ? _ : constant(+_), initialize(), force) : y;
   };
 
   return force;