【swift 5】 AVSpeechSynthesizer で英語→日本語→英語→日本語と交互に話すアプリ

AVSpeechSynthesizer で英語→日本語→英語→日本語と交互に話すアプリを作ってみました。AVSpeechSynthesizer は一個の String を喋ると終了してしまいます。複数の String や複数の言語を喋らせるには工夫が必要です。

以下のコードをコピペして IBAction func speechButtonPushed() を画面のスピーチボタンに紐付けます。

import UIKit
import AVFoundation

class ViewController: UIViewController, AVSpeechSynthesizerDelegate {

    let synthesizer = AVSpeechSynthesizer()
    var speechCounter = 0
    var speechText = [String]()

    override func viewDidLoad() {
        super.viewDidLoad()
        synthesizer.delegate = self
        let english = "alternately"
        let japanese = "交互に"
        let sentence = "This app speaks English and Japanese alternately."
        let yaku = "このアプリは英語と日本語を交互に話します。"
        speechText = [english,japanese,sentence,yaku]
    }

    @IBAction func speechButtonPushed(){
        speechCounter = 0
        speech(text: speechText[0])
    }

    func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didFinish utterance: AVSpeechUtterance) {
        speechCounter += 1
        guard speechCounter < speechText.count else {return}
        speech(text: speechText[speechCounter])
    }

    func speech(text: String) {
        let utterance = AVSpeechUtterance(string: text)
        switch speechCounter {
        case 0, 2:
            utterance.voice = AVSpeechSynthesisVoice(language: "en-US")
        case 1, 3:
            utterance.voice = AVSpeechSynthesisVoice(language: "ja-JP")
        default:
            break
        }
        synthesizer.speak(utterance)
    }
}