From f0f1ea086753455ee883b2694eff9ea3f8976b05 Mon Sep 17 00:00:00 2001
From: ahocevar <andreas.hocevar@gmail.com>
Date: Mon, 17 Dec 2012 14:54:08 +0100
Subject: [PATCH] Skip frames when minimum frame rate is not reached

The new minFrameRate option is used to make sure that an animation does not
run longer than the time calculated from that frame rate. Time is made up
by skipping frames, i.e. skipping execution of the eachStep callback.
---
 lib/OpenLayers/Tween.js | 22 +++++++++++++++++++++-
 tests/Tween.html        | 30 ++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/lib/OpenLayers/Tween.js b/lib/OpenLayers/Tween.js
index 4e504381a5..14e77210b8 100644
--- a/lib/OpenLayers/Tween.js
+++ b/lib/OpenLayers/Tween.js
@@ -52,6 +52,22 @@ OpenLayers.Tween = OpenLayers.Class({
      */
     time: null,
     
+    /**
+     * APIProperty: minFrameRate
+     * {Number} The minimum framerate for animations. 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.
+     */
+    minFrameRate: 30,
+
+    /**
+     * 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
@@ -92,6 +108,7 @@ OpenLayers.Tween = OpenLayers.Class({
         this.duration = duration;
         this.callbacks = options.callbacks;
         this.time = 0;
+        this.startTime = new Date().getTime();
         OpenLayers.Animation.stop(this.animationId);
         this.animationId = null;
         if (this.callbacks && this.callbacks.start) {
@@ -139,7 +156,10 @@ 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) {
diff --git a/tests/Tween.html b/tests/Tween.html
index fdd8fa7016..5c52b84e26 100644
--- a/tests/Tween.html
+++ b/tests/Tween.html
@@ -74,6 +74,36 @@
         tween.stop();
         t.ok(tween.animationId != null, "stop method doesn't do anything if tween isn't running");
     }
+    
+    function test_Tween_skip(t) {
+        t.plan(2);
+        
+        var tween = new OpenLayers.Tween();
+        var log1 = 0;
+        tween.start({count: 0}, {count: 10}, 10, {
+            callbacks: {
+                eachStep: function() {
+                    log1++;
+                }
+            },
+            minFrameRate: 10000
+        });
+        
+        var log2 = 0;
+        tween.start({count: 0}, {count: 10}, 10, {
+            callbacks: {
+                eachStep: function() {
+                    log2++;
+                }
+            },
+            minFrameRate: 1
+        });
+
+        t.delay_call(0.8, function() {
+            t.eq(log1, 0, 'all frames skipped at a frame rate of 10000');            
+            t.eq(log2, 11, 'no frames skipped at a frame rate of 1');            
+        });
+    }
 
     </script> 
 </head>