MacOS下SFSpeechRecognizer批量音频转文本循环处理问题求助

我在MacOS系统上尝试批量转录文件夹中的音频文件为文本，单个文件处理正常，但批量时只有一个文件成功，其余都报错。试过用DispatchGroup，结果所有任务还是一次性提交，没等前一个完成就开下一个；换成串行DispatchQueue后，只能转录一个文件，程序还会挂起。下面是两段相关代码：

第一段代码 #

let recognizer = SFSpeechRecognizer()
recognizer?.supportsOnDeviceRecognition = true
let group = DispatchGroup()
let fd = FileManager.default
fd.enumerator(at: url, includingPropertiesForKeys: nil)?.forEach({ (e) in
    if let url = e as? URL, url.pathExtension == "wav" || url.pathExtension == "aiff" {
        let request = SFSpeechURLRecognitionRequest(url: url)
        group.enter()
        let task =  recognizer?.recognitionTask(with: request) { (result, error) in
            print("Transcribing \(url.lastPathComponent)")
            guard let result = result else {
                print("\(url.lastPathComponent): No message")
                group.leave()
                return
            }
            while  result.isFinal == false {
                sleep(1)
            }
            print("\(url.lastPathComponent): \(result.bestTranscription.formattedString)")
            group.leave()
        }
        group.wait()
    }
})
group.notify(queue: .main) {
    print("Done")
}

更新后的代码 #

let recognizer = SFSpeechRecognizer()
recognizer?.supportsOnDeviceRecognition = true
let fd = FileManager.default
let q = DispatchQueue(label: "serial q")
fd.enumerator(at: url, includingPropertiesForKeys: nil)?.forEach({ (e) in
    if let url = e as? URL, url.pathExtension == "wav" {
        let request = SFSpeechURLRecognitionRequest(url: url)
        q.sync {
            let task =  recognizer?.recognitionTask(with: request) { (result, error) in
                guard let result = result else {
                    print("\(url.lastPathComponent): No message")
                    return
                }
                if result.isFinal {
                    print("\(url.lastPathComponent): \(result.bestTranscription.formattedString)")
                }
            }
        }
    }
})
print("Done")

内容的提问来源于stack exchange，提问作者jl303