Added a helpful Animation for smooth alpha transitions.

git-svn-id: svn+ssh://src.earth.threerings.net/nenya/trunk@317 ed5b42cb-e716-0410-a449-f6a68f950b19

src/as/com/threerings/flash/AlphaFade.as

@@ -0,0 +1,68 @@
+//
+// $Id: AnimationImpl.as 263 2007-06-14 18:20:16Z dhoover $
+//
+// Nenya library - tools for developing networked games
+// Copyright (C) 2002-2007 Three Rings Design, Inc., All Rights Reserved
+// http://www.threerings.net/code/nenya/
+//
+// This library is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License as published
+// by the Free Software Foundation; either version 2.1 of the License, or
+// (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+package com.threerings.flash {
+
+import flash.display.DisplayObject;
+
+/**
+ * An Animation that linearly transitions the alpha attribute of a given display object from
+ * one value to another and optionally executes a callback function when the transition is
+ * complete.
+ */
+public class AlphaFade extends AnimationImpl
+{
+    /**
+     * Constructs a new AlphaFade instance. The alpha values should lie in [0, 1] and the
+     * duration is measured in milliseconds.
+     */
+    public function AlphaFade (disp :DisplayObject, from :Number = 0, to :Number = 1,
+                               duration :Number = 1, done :Function = null)
+    {
+        _disp = disp;
+        _from = from;
+        _to = to;
+        _duration = duration;
+        _done = done;
+    }
+
+    override public function updateAnimation (elapsed :Number) :void
+    {
+        if (elapsed < _duration) {
+            _disp.alpha = _from + ((_to - _from) * elapsed) / _duration;
+            return;
+        }
+
+        _disp.alpha = _to;
+        stopAnimation();
+        if (_done != null) {
+            _done();
+        }
+    }
+
+    protected var _disp :DisplayObject;
+    protected var _from :Number;
+    protected var _to :Number;
+    protected var _duration :Number;
+    protected var _done :Function;
+    
+}
+}