var avSpeechSynthesizer = AVSpeechSynthesizer()
let utterance = AVSpeechUtterance(string: "Hello world")
utterance.rate = ...
utterance.pitchMultiplier = ...
utterance.volume = ...
utterance.preUtteranceDelay = ...
utterance.postUtteranceDelay = ...

// Common 4 operations
avSpeechSynthesizer.speak(utterance) // Start speech

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

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

avSpeechSynthesizer.continueSpeaking() // Resume a paused speech