[BUG] macOS: synthesizeToFile Ignores Selected Voice

[YofarDev]

🐛 Bug Report

On macOS, the synthesizeToFile method does not use the voice set via setVoice. It always synthesizes the audio file using the default system voice. In contrast, the speak method works correctly and uses the selected voice.

Proposed Solution

This fix works for me. Code change in macos/Classes/FlutterTtsPlugin.swift:

// macos/Classes/FlutterTtsPlugin.swift

private func synthesizeToFile(text: String, fileName: String, result: @escaping FlutterResult) {
    var output: AVAudioFile?
    var failed = false
    let utterance = AVSpeechUtterance(string: text)

    // Add this block to apply the selected voice
    if self.voice != nil {
      utterance.voice = self.voice!
    } else {
      utterance.voice = AVSpeechSynthesisVoice(language: self.language)
    }

    if #available(iOS 13.0, *) { // This check should likely be macOS specific, e.g., macOS 10.15
      self.synthesizer.write(utterance) { (buffer: AVAudioBuffer) in
        guard let pcmBuffer = buffer as? AVAudioPCMBuffer else {
            NSLog("unknown buffer type: \(buffer)")
            failed = true
            return
        }
        if pcmBuffer.frameLength == 0 {
            // finished
        } else {
          // append buffer to file
          let fileURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent(fileName)
          NSLog("Saving utterance to file: \(fileURL.absoluteString)")

          if output == nil {
            do {
              output = try AVAudioFile(
              forWriting: fileURL,
              settings: pcmBuffer.format.settings,
              commonFormat: .pcmFormatFloat32,
              interleaved: false)
            } catch {
                NSLog(error.localizedDescription)
                failed = true
                return
            }
          }

          try! output!.write(from: pcmBuffer)
        }
      }
    } else {
        result("Unsupported macOS version")
    }
    if failed {
        result(0)
    }
    if self.awaitSynthCompletion {
      self.synthResult = result
    } else {
      result(1)
    }
}