var huabu:Sprite=new Sprite();
huabu.graphics.beginFill(0xFFFFFF);
huabu.graphics.drawRect(20,20,510,360);
huabu.graphics.endFill();
addChild(huabu);
var _drawing:Boolean;
var quxian:Sprite=new Sprite();
quxian.graphics.lineStyle( 2, 0xFF0000 );
huabu.addChild(quxian);
_drawing = false;
huabu.addEventListener( MouseEvent.MOUSE_DOWN, startDrawing );
huabu.addEventListener( MouseEvent.MOUSE_MOVE,moving);
huabu.addEventListener( MouseEvent.MOUSE_UP, stopDrawing );
stage.addEventListener(MouseEvent.MOUSE_UP,stopDrawing);
function startDrawing( event:MouseEvent ):void {
 quxian.graphics.moveTo( mouseX, mouseY );
 _drawing = true;
}
function moving( event:MouseEvent ):void {
 if ( _drawing ) {
  quxian.graphics.lineTo( mouseX, mouseY );
 }
}
function stopDrawing( event:MouseEvent ):void {
 _drawing = false;
}