var error:NSError?
let manager = NSFileManager.defaultManager()
let docURL = manager.URLForDirectory(.DocumentDirectory,inDomain:.UserDomainMask,appropriateForURL:nil,create:true,error:&error)
docURL.URLByAppendingPathComponent("/RicFile.txt") <-- doesn't work

通过调试器：

file:///Users/Ric/Library/Developer/CoreSimulator/Devices/
<device id>/data/Containers/Data/Application/<app id>/Documents/

使用docURL将字符串写入文件不起作用,因为缺少文件名.

原因(通过错误)：

“The operation couldn’t be completed. Is a directory”

所以问题：为什么以下不起作用？

docURL.URLByAppendingPathComponent("/RicFile.txt")