【求助】新手学flash，求大神指点

按钮代码：
on (release)
{
Selection.setFocus(PIN);
var str:String = PIN.text;
trace(PIN.text);
var txt:String = "";
switch (_root.step)
{
case 1:
{
txt = _root.PIN.text;
trace(123);
trace(txt);
trace(123);
break;
}
} // End of switch
txt = txt.substring(0, txt.length - 1);
//Selection.setFocus(PIN);
if (txt.length == 0)
{
Selection.setFocus(PIN);
} // end if
trace("输出txt:"+txt);
_root.date.point.gotoAndStop(_root.step);
switch (_root.step)
{
case 1:
{
_root.PIN.text = txt;
trace(5);
trace("输出PIN.text:"+PIN.text);
trace("输出_root.step:"+_root.step);
break;
}
}
}