Simple frames-per-second displayer.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@4585 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/as/com/threerings/util/FPSDisplay.as

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+package com.threerings.util {
+
+import flash.events.Event;
+
+import flash.text.TextField;
+
+import flash.utils.getTimer; // function import
+
+public class FPSDisplay extends TextField
+{
+    public function FPSDisplay (framesToTrack :int = 150)
+    {
+        background = true;
+        text = String(Number.MIN_VALUE);
+        height = textHeight + 4;
+
+        _framesToTrack = framesToTrack;
+
+        addEventListener(Event.ENTER_FRAME, handleEnterFrame);
+    }
+
+    protected function handleEnterFrame (event :Event) :void
+    {
+        var curStamp :Number = getTimer();
+        _frameStamps.push(curStamp);
+        if (_frameStamps.length > _framesToTrack) {
+            _frameStamps.shift(); // forget the oldest timestamps
+        }
+
+        var firstStamp :Number = Number(_frameStamps[0]);
+        var seconds :Number = (curStamp - firstStamp) / 1000;
+        // subtract one from the frames, since we're measuring the time
+        // elapsed over the frames (when there are two timestamps, that's
+        // the difference between 1 frame)
+        var frames :Number = _frameStamps.length - 1;
+
+        if (frames > 0) {
+            this.text = String(frames / seconds);
+        }
+    }
+
+    /** Timestamps of past ENTER_FRAME events. */
+    protected var _frameStamps :Array = [];
+
+    /** The number of running frames we track. */
+    protected var _framesToTrack :int;
+}
+}