Swift read data from file as its being written to live

I would like to read this video stream live and convert it to NSData. I am struggling with being able to read the stream live right now. I can only access it after the recording is finished. I found this old post but I do not understand it.

var outputFilePath: String = NSTemporaryDirectory().stringByAppendingPathComponent( "movie".stringByAppendingPathExtension("mov")!)


self.movieFileOutput!.startRecordingToOutputFileURL(NSURL.fileURLWithPath(outputFilePath), recordingDelegate: self)