Freeze time in the background.

Rather than periodically flushing the timer queue in the background, freeze time
such that timers remain blissfully unaware of their sleep when returning to the
foreground. Fixes d3/d3-transition#29.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "d3-timer",
-  "version": "0.3.1",
+  "version": "0.3.2",
   "description": "An efficient queue capable of managing thousands of concurrent animations.",
   "keywords": [
     "d3",

src/timer.js

@@ -1,16 +1,17 @@
 var frame = 0, // is an animation frame pending?
     timeout = 0, // is a timeout pending?
     interval = 0, // are any timers active?
-    maxDelay = 5000, // the longest we’ll sleep if there are active timers
+    pokeDelay = 1000, // how frequently we check for clock skew
     taskHead,
     taskTail,
     clockLast = 0,
     clockNow = 0,
+    clockSkew = 0,
     clock = typeof performance === "object" ? performance : Date,
     setFrame = typeof requestAnimationFrame === "function" ? requestAnimationFrame : function(callback) { return setTimeout(callback, 17); };
 
 export function now() {
-  return clockNow || (setFrame(clearNow), clockNow = clock.now());
+  return clockNow || (setFrame(clearNow), clockNow = clock.now() + clockSkew);
 }
 
 function clearNow() {
@@ -69,7 +70,7 @@ export function timerFlush() {
 }
 
 function wake(time) {
-  clockLast = clockNow = time || clock.now();
+  clockNow = (clockLast = time || clock.now()) + clockSkew;
   frame = timeout = 0;
   try {
     timerFlush();
@@ -81,9 +82,8 @@ function wake(time) {
 }
 
 function poke() {
-  if (clock.now() - clockLast > maxDelay) {
-    wake();
-  }
+  var now = clock.now(), delay = now - clockLast;
+  if (delay > pokeDelay) clockSkew -= delay, clockLast = now;
 }
 
 function nap() {
@@ -106,9 +106,9 @@ function sleep(time) {
   var delay = time - clockNow;
   if (delay > 24) {
     if (time < Infinity) timeout = setTimeout(wake, delay);
-    else if (interval) interval = clearInterval(interval);
+    if (interval) interval = clearInterval(interval);
   } else {
-    if (!interval) interval = setInterval(poke, maxDelay);
+    if (!interval) interval = setInterval(poke, pokeDelay);
     frame = 1, setFrame(wake);
   }
 }