AVOperation.swift

import AVFoundation

let avSpeechSynthesizer = AVSpeechSynthesizer()
avSpeechSynthesizer.delegate = self

// Session 1 --- AVSpeechUtterance: 7 Parameters of synthesized speech 
let utterance = AVSpeechUtterance(string: "Hello world")
utterance.voice = ... 
utterance.rate = ...
utterance.pitchMultiplier = ...
utterance.volume = ...
utterance.preUtteranceDelay = ...
utterance.postUtteranceDelay = ...

// Session 2 --- AVSpeechSynthesizer: 4 Speech operations 
avSpeechSynthesizer.speak(utterance) // Start speech

avSpeechSynthesizer.pauseSpeaking(at: .immediate) // Pause even in the middle of a word
avSpeechSynthesizer.pauseSpeaking(at: .word) // Pause after the current word has been voiced out completely

avSpeechSynthesizer.stopSpeaking(at: .immediate) // Stop --- Similar to the `pauseSpeaking(at: .immediate)`
avSpeechSynthesizer.stopSpeaking(at: .word) // Stop --- Similar to the `pauseSpeaking(at: .word)`

avSpeechSynthesizer.continueSpeaking() // Resume a paused speech

// Session 3 --- AVSpeechSynthesizerDelegate: 6 Speech callbacks 
extension FooViewController: AVSpeechSynthesizerDelegate {
 func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didStart utterance: AVSpeechUtterance) { }
 func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didFinish utterance: AVSpeechUtterance) { }
 func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didPause utterance: AVSpeechUtterance) { }
 func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didContinue utterance: AVSpeechUtterance) {}
 func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didCancel utterance: AVSpeechUtterance) { }
 func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, willSpeakRangeOfSpeechString characterRange: NSRange, utterance: AVSpeechUtterance) { }
}