why doesn't attrTween pass old datum to function tween()?

[Peter Reinhardt]

I'm working on an elliptical arc for a speedometer-type visualization. I see the example for tweening arcs at How-To: Update a Pie Chart (Part 2), but this._current doesn't seem a very satisfying solution.

From the documentation on the github wiki, tween functions get "passed the current datumd, the current index i and the current attribute value a, with the this context as the current DOM element."

function tween(d, i, a) {
  return d3.interpolate(a, String(value));
}

Why doesn't the tween function get the old datum as well? I'd rather not store an external cache of old datum values like this._current... any thoughts?

Thanks,

Peter

[Mike Bostock]

By the time you call selection.data, you've overwritten the old data
and thus it is not available when you create the transition later.

Also, this._current doesn't store the old data, but the current
interpolated value. (The interpolator returned by d3.interpolateObject
reuses the same object to reduce garbage collection, so the result of
`i(0)` is the same object later returned by `i(t)`.) If the transition
is interrupted, say by clicking twice quickly, a new transition
correctly resumes from the interrupted state.

Mike