NSSpeechSynthesizer Saving to URL - multithreading

Question

I'm creating a MacOS application that generates dialog files from a tab-delimited text file. The text file is formatted (output filename) [tab] (text to be spoken to that file). I got the program working perfectly running in the main thread, but this blocked the UI, of course. When I moved the meat of the application to a User Initiated thread, I unblocked the UI, but now it occasionally and randomly speaks a line out the speaker instead of to a file. A file is still created, but it's of zero duration. If the script contains 1,000 lines, it might get through the whole thing perfectly, or it might "speak" ten or more of them out the speaker. It's almost always different lines each time.
This is the code that speaks to a file:
// Set URL to save file to disk
if let speechFileURL = NSURL(string: speechFilePath) {
speechSynth.startSpeakingString(spokenText, toURL: speechFileURL) // Speak to file
}
else {
dialogAlert("Invalid File: ", text: speechFileName)
}
while(NSSpeechSynthesizer.isAnyApplicationSpeaking() == true) {
if self.gUserClickedStop {
speechSynth.stopSpeaking()
self.gIsSpeaking = false
break
}
}
This is the call to the function that parses the script and generates the speech files:
dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)) { [unowned self] in
self.parseTheScript()
}
As mentioned above, calling parseTheScript() without wrapping it in the Grand Central Dispatch call makes it work perfectly, except for the blocking issue.
I'm new to programming in Swift and the idea of multithreaded programming, so I'm sure I'm doing something wrong, but I'm stumped as to how to fix this problem.
Thank you in advance for your help.
Edit to add...
This is the entire function that parses the text file and generates the dialog files:
// This opens the text file and generates the spoken audio files
func parseTheScript() {
let filemgr = NSFileManager.defaultManager()
print("Chosen path: \(scriptFileTextField.stringValue)") // debug
if filemgr.fileExistsAtPath(scriptFileTextField.stringValue) {
print("File exists")
do {
outputFilename.stringValue = ""
scriptToSpeak.stringValue = ""
outputFilename.hidden = false
scriptToSpeak.hidden = false
progressBar.minValue = 0
progressBar.doubleValue = 0.0
progressBar.hidden = false
filenameLabel.hidden = false
let text = try String(contentsOfFile: scriptFileTextField.stringValue, encoding: NSUTF8StringEncoding)
let lines: [String] = text.componentsSeparatedByString("\n")
var progressEndValue = Double(lines.count)
progressBar.maxValue = progressEndValue
let speechSynth = NSSpeechSynthesizer.init()
let theVoice = gVoiceList[voiceListPopup.indexOfSelectedItem - 1] // Not sure why it's off by one
speechSynth.setVoice(theVoice)
for line in lines {
let thisLine: [String] = line.componentsSeparatedByString("\t") // Split the line at the tab character
if (thisLine.count == 2) { // Did the line split into two parts?
let speechFileName = thisLine[0]
let spokenText = thisLine[1]
outputFilename.stringValue = speechFileName
scriptToSpeak.stringValue = spokenText
let speechFilePath = destinationFolderTextField.stringValue + speechFileName + ".aif" // Build the path string for the output speech file
print("Filename: \(speechFilePath)")
print("SpokenText: \(spokenText)")
if(gSpeakToFile) {
// Set URL to save file to disk
if let speechFileURL = NSURL(string: speechFilePath) {
speechSynth.startSpeakingString(spokenText, toURL: speechFileURL) // Speak to file
}
else {
dialogAlert("Invalid File: ", text: speechFileName)
}
}
else {
speechSynth.startSpeakingString(spokenText) // Speak to audio output
}
while(NSSpeechSynthesizer.isAnyApplicationSpeaking() == true) {
if self.gUserClickedStop {
speechSynth.stopSpeaking()
self.gIsSpeaking = false
break
}
}
progressBar.incrementBy(1.0)
}
if gUserClickedStop {
gUserClickedStop = false
gIsSpeaking = false
progressEndValue = 0
generateButton.title = "Start"
break
}
}
gIsSpeaking = false
progressBar.doubleValue = progressEndValue
generateButton.title = "Start"
filenameLabel.hidden = true
outputFilename.stringValue = ""
scriptToSpeak.stringValue = ""
} catch let error as NSError {
dialogAlert("Error:", text: String(error.localizedDescription))
print("Error: \(error)")
}
} else {
print("File does not exist")
}
}