Once set up, how are NSSpeechSynthesizerDelegate protocol methods called automatically? - swift

Question

I'm working through a tutorial that uses NSSpeechSynthesizer and two of its NSSpeechSynthesizerDelegate protocol methods. In my ViewController, I don't explicitly call the protocol methods so I'm curious as to what do I need to research in order to understand how these methods are called during runtime? The delegate methods are working as expected but I'm wondering how are they being called which makes this possible?
import Cocoa
class MainWindowController: NSWindowController, NSSpeechSynthesizerDelegate, NSWindowDelegate {
//Now MainWindowController is more powerful by having its own KITT being able to delegate powerful functionality and do less work. The delegate will do all the heavy lifting and return the results to MainWindowController instances.
// MARK: - Properties
#IBOutlet weak var textField: NSTextField!
#IBOutlet weak var speakButton: NSButton!
#IBOutlet weak var stopButton: NSButton!
let speechSynth = NSSpeechSynthesizer.init(voice: NSSpeechSynthesizer.VoiceName.init(rawValue: "com.apple.speech.synthesis.voice.Victoria"))
var isSpeaking: Bool = false {
didSet {
updateButtons()
}
}
// MARK: - Overriden Properties
override var windowNibName: NSNib.Name? {
return NSNib.Name("MainWindowController")
}
// MARK: - Overidden Methods
override func windowDidLoad() {
super.windowDidLoad()
updateButtons()
speechSynth?.delegate = self
}
// MARK: - UI methods
#IBAction func speakIt(sender: NSButton) {
//Get tuype-in text as a string
let string = textField.stringValue
if string.isEmpty {
print("string from \(textField) is empty")
} else {
speechSynth?.startSpeaking(string)
isSpeaking = true
}
}
#IBAction func stopIt(sender: NSButton) {
speechSynth?.stopSpeaking()
}
func updateButtons(){
if isSpeaking {
speakButton.isEnabled = false
stopButton.isEnabled = true
} else {
speakButton.isEnabled = true
stopButton.isEnabled = false
}
}
// MARK: - NSSpeechSynthesizerDelegate Methods
//this functionality is considered more powerful and is made possible due to the speechSynthesizer.delegate = self
//the delegate is doing the work and reporting that completed work to the MainWindowController instance
//so kinda like the delegate is providing the signature and its up to us as the developers based on what we do with those parameters inside the function in order for us to add our own creativity.
func speechSynthesizer(_ sender: NSSpeechSynthesizer, didFinishSpeaking finishedSpeaking: Bool) {
//by setting this variable to FALSE, it will fire off the didSet computed property which this variable has both storage and behavior.
isSpeaking = false
}
// MARK: - NSWindowDelegate Methods
func windowShouldClose(_ sender: NSWindow) -> Bool {
return !isSpeaking
}
}

Answer 1

Your windowDidLoad method contains this line:
speechSynth?.delegate = self
This means the speech synthesizer object has a reference back to your MainWindowController, so the speech synthesizer object can send messages to your MainWindowController.
A simplified implementation inside NSSpeechSynthesizer could look something like this in Swift:
class NSSpeechSynthesizer: NSSoundDelegate {
weak var delegate: NSSpeechSynthesizerDelegate?
func startSpeaking(_ string: String) {
guard
let audioData = audioData(for: string),
let sound = NSSound(data: audioData)
else { return }
sound.delegate = self
sound.play()
}
// Part of NSSoundDelegate
func sound(_ sound: NSSound, didFinishPlaying finished: Bool) {
// The first ? means Swift only sends the message if
// delegate is not nil.
// The second ? means Swift only sends the message if delegate
// implements speechSynthesizer(_:didFinishSpeaking:).
delegate?.speechSynthesizer?(self, didFinishSpeaking: finished)
}
}
But it's actually implemented in Objective-C, where you have to be more verbose about checking whether the delegate handles the message:
- (void)sound:(NSSound *)sound didFinishPlaying:(BOOL)finished {
if ([delegate respondsToSelector:#selector(speechSynthesizer:didFinishSpeaking:)]) {
[delegate speechSynthesizer:self didFinishSpeaking:finished];
}
}