flex 判断文件是否存在
var loader:Loader = new Loader();
loader.contentLoaderInfo.addEventListener(IOErrorEvent.IO_ERROR, ioErrorHandler);
var request:URLRequest = new URLRequest("a.jpg");
loader.load(request);
private function ioErrorHandler(event:IOErrorEvent):void {

trace("文件不存在，ioErrorHandler: " + event);

}