Merge pull request #803 from ahocevar/tween-framerate

Tween: skip frames when minimum frame rate is not reached. r=@bartvde

lib/OpenLayers/Tween.js

@@ -52,6 +52,22 @@ OpenLayers.Tween = OpenLayers.Class({
      */
     time: null,
     
+    /**
+     * APIProperty: minFrameRate
+     * {Number} The minimum framerate for animations in frames per second. After
+     * each step, the time spent in the animation is compared to the calculated
+     * time at this frame rate. If the animation runs longer than the calculated
+     * time, the next step is skipped. Default is 30.
+     */
+    minFrameRate: null,
+
+    /**
+     * Property: startTime
+     * {Number} The timestamp of the first execution step. Used for skipping
+     * frames
+     */
+    startTime: null,
+    
     /**
      * Property: animationId
      * {int} Loop id returned by OpenLayers.Animation.start
@@ -83,7 +99,8 @@ OpenLayers.Tween = OpenLayers.Class({
      * begin - {Object} values to start the animation with
      * finish - {Object} values to finish the animation with
      * duration - {int} duration of the tween (number of steps)
-     * options - {Object} hash of options (for example callbacks (start, eachStep, done))
+     * options - {Object} hash of options (callbacks (start, eachStep, done),
+     *     minFrameRate)
      */
     start: function(begin, finish, duration, options) {
         this.playing = true;
@@ -91,7 +108,9 @@ OpenLayers.Tween = OpenLayers.Class({
         this.finish = finish;
         this.duration = duration;
         this.callbacks = options.callbacks;
+        this.minFrameRate = options.minFrameRate || 30;
         this.time = 0;
+        this.startTime = new Date().getTime();
         OpenLayers.Animation.stop(this.animationId);
         this.animationId = null;
         if (this.callbacks && this.callbacks.start) {
@@ -139,7 +158,10 @@ OpenLayers.Tween = OpenLayers.Class({
         this.time++;
         
         if (this.callbacks && this.callbacks.eachStep) {
-            this.callbacks.eachStep.call(this, value);
+            // skip frames if frame rate drops below threshold
+            if ((new Date().getTime() - this.startTime) / this.time <= 1000 / this.minFrameRate) {
+                this.callbacks.eachStep.call(this, value);
+            }
         }
         
         if (this.time > this.duration) {