曲線を描くやつ

ActionScript3.0アニメーション本から

DrawingCurve.as

package {
    import flash.display.Sprite;
    import flash.events.MouseEvent;
    public class DrawingCurves extends Sprite {
        private var x0:Number = 100;
        private var y0:Number = 200;
        private var x1:Number;
        private var y1:Number;
        private var x2:Number = 300;
        private var y2:Number = 200;

        public function DrawingCurves() {
            init();
        }

        private function init():void {
            stage.addEventListener(MouseEvent.MOUSE_MOVE, onMouseMove);
        }

        private function onMouseMove(event:MouseEvent):void {
            x1 = mouseX * 2 - (x0 + x2) / 2; // ここがキモらしい
            y1 = mouseY * 2 - (y0 + y2) / 2;
            graphics.clear();
            graphics.lineStyle(1);
            graphics.moveTo(x0, y0);
            graphics.curveTo(x1, y1, x2, y2);
        }
    }
}