Buffered writing to a file with Swift OutputStreamType and GCD i/o channel

FileOutputStream.swift

/// An `OutputStreamType` which uses GCD to write to a file
class FileOutputStream : OutputStreamType
{
  private let filepath:NSURL
  private let channel:dispatch_io_t!
  
  /**
   Initializes output stream to write to `filename` in the user's Documents directory.
   */
  convenience init?(filename:String) {
    let pathURL = NSFileManager.defaultManager().URLsForDirectory(NSSearchPathDirectory.DocumentDirectory, inDomains:.UserDomainMask).first!.URLByAppendingPathComponent(filename)
    self.init(filepath:pathURL)
  }

  /**
   Initialized an output stream that writes to `filepath`.
   
   - requires: `filepath` must be a file path URL, for a file that can be created.

   */
  init?(filepath f:NSURL) {
    self.filepath = f
    let iotype:dispatch_io_type_t = DISPATCH_IO_STREAM

    if
      let path = f.path,
      let cpath = path.cStringUsingEncoding(NSUTF8StringEncoding) {
        let outputflag:Int32 = O_CREAT | O_WRONLY               // create, write-only
        let mode:mode_t = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH // permissions: u+rw, g+r, o+r
        self.channel = dispatch_io_create_with_path(iotype,cpath,outputflag,mode,
          dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)) { (errcode:Int32) -> Void in
            if errcode != 0 {
              NSLog("FileOutputStream: error creating io channel")
            }
        }
    }
    else {
      self.channel = nil
      return nil
    }
  }
  
  // MARK: OutputStreamType
  
  func write(string: String) {
    if
      let dataString = string.dataUsingEncoding(NSUTF8StringEncoding),
      let dispatchData:dispatch_data_t = dispatch_data_create(dataString.bytes, dataString.length, nil, nil)
    {
      dispatch_io_write(self.channel, 0, dispatchData, dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)) { (done:Bool, data:dispatch_data_t!, errorCode:Int32) -> Void in
        // handle progress reporting here
        if errorCode != 0 {
          NSLog("FileOutputStream: error writing data to channel")
        }
      }
    }
  }

  deinit {
    let flags:dispatch_io_close_flags_t = 0
    dispatch_io_close(self.channel, flags)
  }
}