使sprite以某个点为中心旋转

package com.everydayflash.util {
       
        import flash.geom.Point;

        /**
         * This class can be used as a replacement for the DisplayObject.rotation property.
         *
         * It rotates a DisplayObject, but it does not limit itself to rotate around the it's
         * registration point, instead it can rotate the object around any point. The point is
         * defined in the objects parent coodrinate system.
         *
         * @author Bartek Drozdz (http://www.everydayflash.com)
         * @version 1.0
         */
        public class Rotator {
               
                private var target:Object;
               
                /**
                 * A value that is based on the initial rotation of the display object itself, and
                 * the angle between the registration point of the display object and of the rotator
                 */
                private var offset:Number;
               
                /**
                 * Registration point - the point around which the rotation takse place
                 */
                private var point:Point;
               
                /**
                 * Distance between the registration point of the display object and the registration
                 * point of the rotator
                 */
                private var dist:Number;

                /**
                 * Registers a DisplayObject that will be rotated and an registration Point around which it will be rotated.
                 *
                 * @param       target DisplayObject to rotate
                 * @param       registrationPoint Point containing the coodrinates around which the object should be rotated
                 *          (in the targets parent coordinate space) If omitted, the displays object x and y coordinates are used
                 */
                public function Rotator(target:Object, registrationPoint:Point=null) {
                        this.target = target;
                        setRegistrationPoint(registrationPoint);
                }
               
                /**
                 * Once set in the constructor, the rotation registration point can be modified an any moment
                 *
                 * @param       registrationPoint, if null defaults to targets x and y coordinates
                 */
                public function setRegistrationPoint(registrationPoint:Point=null):void {
                        if (registrationPoint == null) point = new Point(target.x, target.y);
                        else point = registrationPoint;
                       
                        var dx:Number = point.x - target.x;
                        var dy:Number = point.y - target.y;
                        dist = Math.sqrt( dx * dx + dy * dy );
                       
                        var a:Number = Math.atan2(dy, dx) * 180 / Math.PI;
                        offset = 180 - a + target.rotation;
                }
               
                /**
                 * Sets the rotation to the angle passed as parameter.
                 *
                 * Since it uses a getter/setter Rotator can easily be used with Tween or Tweener classes.
                 */
                public function set rotation(angle:Number):void {
                        var tp:Point = new Point(target.x, target.y);

                        var ra:Number = (angle - offset) * Math.PI / 180;
                       
                        target.x = point.x + Math.cos(ra) * dist;
                        target.y = point.y + Math.sin(ra) * dist;
                       
                        target.rotation =  angle;
                }
               
                /**
                 * Returns current rotation of the target in degrees
                 */
                public function get rotation():Number {
                        return target.rotation;
                }
               
                /**
                 * Rotates the target by the angle passed as parameter.
                 * Works the same as Rotator.rotation += angle;
                 *
                 * @param angle angle by which to rotate the target DisplayObject
                 */
                public function rotateBy(angle:Number):void {
                        var tp:Point = new Point(target.x, target.y);

                        var ra:Number = (target.rotation + angle - offset) * Math.PI / 180;
                       
                        target.x = point.x + Math.cos(ra) * dist;
                        target.y = point.y + Math.sin(ra) * dist;
                       
                        target.rotation =  target.rotation + angle;
                }
        }
}