在Macos Mojave上,在Cocoa应用程序中,如何使用AvSpeechSynther?

AVSpeechSynthesizer is marked as available on macOS Mojave beta.

以前它只在iOS、tvos和watchos上提供。但是,如果我在Xcode10中准备一个小型MacOS测试项目,它会给我一个错误“使用未解析的标识符‘avspeech合成器’”。在顶部,我有:

import Cocoa
import NaturalLanguage
import AVFoundation

我的代码是:

let string = "Mickey mouse went to town"

let recognizer = NLLanguageRecognizer()
recognizer.processString(string)
let language = recognizer.dominantLanguage!.rawValue

let speechSynthesizer = AVSpeechSynthesizer()
let utterance = AVSpeechUtterance(string: string)
utterance.voice = AVSpeechSynthesisVoice(language: language)
speechSynthesizer.speak(utterance)

它与iOS上的代码完全相同,但在iOS上它起作用,在MacOS上它给出了错误。任何帮助都非常感谢。谢谢