Adding caching to Range getMillisecondsPerPixel function (#3154)

lib/timeline/Range.js

@@ -16,7 +16,8 @@ function Range(body, options) {
   var now = moment().hours(0).minutes(0).seconds(0).milliseconds(0);
   var start = now.clone().add(-3, 'days').valueOf();
   var end = now.clone().add(3, 'days').valueOf(); 
-
+  this.millisecondsPerPixelCache = undefined;
+  
   if(options === undefined) {
     this.start = start;
     this.end = end;
@@ -200,6 +201,7 @@ Range.prototype.setRange = function(start, end, options, callback) {
   var finalStart = start != undefined ? util.convert(start, 'Date').valueOf() : null;
   var finalEnd   = end != undefined   ? util.convert(end, 'Date').valueOf()   : null;
   this._cancelAnimation();
+  this.millisecondsPerPixelCache = undefined;
 
   if (options.animation) { // true or an Object
     var initStart = this.start;
@@ -280,7 +282,10 @@ Range.prototype.setRange = function(start, end, options, callback) {
  * Get the number of milliseconds per pixel.
  */
 Range.prototype.getMillisecondsPerPixel = function() {
-  return (this.end - this.start) / this.body.dom.center.clientWidth;
+  if (this.millisecondsPerPixelCache === undefined) {
+    this.millisecondsPerPixelCache = (this.end - this.start) / this.body.dom.center.clientWidth;
+  }
+  return this.millisecondsPerPixelCache;
 }
 
 /**