缘起

我做一个AI应用，需要实时播放音频，显示AI生成的文本，AI生成的文本是流式返回的，所以在UI上是滚动的，这个时候用户也可以滚动屏幕去查看内容。

我开始用的是Flutter_sound做的，这个玩意，只能在UI线程播放，也就是main线程播放。Flutter是单线程的，如果用Isolate隔离线程又播放不了，这里注意了，甚至你输出日志都不行，用得用原生的print才可以，如果用是用dev里的log是输出不了日志的，不要以为没有调用到。

那怎么办呢？

改成原生播放咯！

老本行

创建插件

shell

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flutter create --template=plugin --platforms=android,ios native_audio_player

创建完以后，项目结构里，会多出一个插件

图片描述

编辑以下文件

shell

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com.example.native_audio_player.NativeAudioPlayerPlugin.kt

代码如下

kotlin

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package com.example.native_audio_player

import android.media.AudioFormat
import android.media.AudioManager
import android.media.AudioTrack
import android.os.Handler
import android.os.HandlerThread
import androidx.annotation.NonNull
import io.flutter.embedding.engine.plugins.FlutterPlugin
import io.flutter.plugin.common.MethodCall
import io.flutter.plugin.common.MethodChannel
import java.util.concurrent.LinkedBlockingQueue

class NativeAudioPlayerPlugin : FlutterPlugin, MethodChannel.MethodCallHandler {
  private lateinit var channel: MethodChannel
  private var audioTrack: AudioTrack? = null
  private var audioThread: HandlerThread? = null
  private var audioHandler: Handler? = null
  private var pcmQueue: LinkedBlockingQueue<ByteArray>? = null
  @Volatile private var isPlaying = false

  override fun onAttachedToEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
    channel = MethodChannel(binding.binaryMessenger, "native_audio_player")
    channel.setMethodCallHandler(this)
  }

  override fun onDetachedFromEngine(@NonNull binding: FlutterPlugin.FlutterPluginBinding) {
    stopAudioTrack()
    channel.setMethodCallHandler(null)
  }

  override fun onMethodCall(call: MethodCall, result: MethodChannel.Result) {
    when (call.method) {
      "startPlayer" -> {
        val sampleRate = call.argument<Int>("sampleRate") ?: 16000
        val channels = call.argument<Int>("channels") ?: 1
        startAudioTrack(sampleRate, channels)
        result.success(null)
      }
      "writePcm" -> {
        val pcm = call.argument<ByteArray>("pcm")
        if (pcm != null && isPlaying) {
          pcmQueue?.offer(pcm)
        }
        result.success(null)
      }
      "stopPlayer" -> {
        stopAudioTrack()
        result.success(null)
      }
      else -> result.notImplemented()
    }
  }

  private fun startAudioTrack(sampleRate: Int, channels: Int) {
    val channelConfig = if (channels == 1) AudioFormat.CHANNEL_OUT_MONO else AudioFormat.CHANNEL_OUT_STEREO
    val bufferSize = AudioTrack.getMinBufferSize(
      sampleRate,
      channelConfig,
      AudioFormat.ENCODING_PCM_16BIT
    )

    audioTrack = AudioTrack(
      AudioManager.STREAM_MUSIC,
      sampleRate,
      channelConfig,
      AudioFormat.ENCODING_PCM_16BIT,
      bufferSize,
      AudioTrack.MODE_STREAM
    )

    pcmQueue = LinkedBlockingQueue()
    isPlaying = true

    audioThread = HandlerThread("AudioPlayerThread").apply { start() }
    audioHandler = Handler(audioThread!!.looper)
    audioHandler?.post { playLoop() }

    audioTrack?.play()
  }

  private fun playLoop() {
    try {
      while (isPlaying && !Thread.currentThread().isInterrupted) {
        val data = pcmQueue?.take()
        if (data != null && data.isNotEmpty()) {
          audioTrack?.write(data, 0, data.size)
        }
      }
    } catch (e: InterruptedException) {
      Thread.currentThread().interrupt()
    }
  }

  private fun stopAudioTrack() {
    isPlaying = false
    audioThread?.quitSafely()
    audioThread = null
    audioHandler = null

    audioTrack?.stop()
    audioTrack?.release()
    audioTrack = null

    pcmQueue?.clear()
    pcmQueue = null
  }
}

ios的编辑这个文件

shell

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ios/Classes/NativeAudioPlayerPlugin.swift

代码如下：

swift

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import Flutter
import UIKit
import AVFoundation

public class NativeAudioPlayerPlugin: NSObject, FlutterPlugin {
  var engine: AVAudioEngine?
  var playerNode: AVAudioPlayerNode?
  var sampleRate: Double = 16000
  var channels: AVAudioChannelCount = 1

  public static func register(with registrar: FlutterPluginRegistrar) {
    let channel = FlutterMethodChannel(name: "native_audio_player", binaryMessenger: registrar.messenger())
    let instance = NativeAudioPlayerPlugin()
    registrar.addMethodCallDelegate(instance, channel: channel)
  }

  public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
    switch call.method {
    case "startPlayer":
      if let args = call.arguments as? [String: Any] {
        sampleRate = Double(args["sampleRate"] as? Int ?? 16000)
        channels = AVAudioChannelCount(args["channels"] as? Int ?? 1)
        startEngine()
      }
      result(nil)
    case "writePcm":
      if let args = call.arguments as? [String: Any],
         let pcm = args["pcm"] as? FlutterStandardTypedData {
        playPcm(pcm.data)
      }
      result(nil)
    case "stopPlayer":
      stopEngine()
      result(nil)
    default:
      result(FlutterMethodNotImplemented)
    }
  }

  func startEngine() {
    engine = AVAudioEngine()
    playerNode = AVAudioPlayerNode()
    guard let engine = engine, let playerNode = playerNode else { return }

    let format = AVAudioFormat(
      commonFormat: .pcmFormatInt16,
      sampleRate: sampleRate,
      channels: channels,
      interleaved: true
    )

    engine.attach(playerNode)
    engine.connect(playerNode, to: engine.mainMixerNode, format: format)

    try? engine.start()
    playerNode.play()
  }

  func playPcm(_ data: Data) {
    guard let playerNode = playerNode else { return }

    let buffer = AVAudioPCMBuffer(
      pcmFormat: playerNode.outputFormat(forBus: 0),
      frameCapacity: AVAudioFrameCount(data.count / 2)
    )!
    buffer.frameLength = buffer.frameCapacity

    let ptr = buffer.int16ChannelData![0]
    data.copyBytes(to: ptr, count: data.count)

    playerNode.scheduleBuffer(buffer, completionHandler: nil)
  }

  func stopEngine() {
    playerNode?.stop()
    engine?.stop()
    engine = nil
    playerNode = nil
  }
}

插件代码

shell

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lib/native_audio_player.dart

代码如下：

dart

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import 'package:flutter/services.dart';

class NativeAudioPlayer {
  static const MethodChannel _channel = MethodChannel('native_audio_player');

  static Future<void> startPlayer({required int sampleRate, required int channels}) async {
    await _channel.invokeMethod('startPlayer', {
      'sampleRate': sampleRate,
      'channels': channels,
    });
  }

  static Future<void> writePcm(Uint8List pcmData) async {
    await _channel.invokeMethod('writePcm', {
      'pcm': pcmData,
    });
  }

  static Future<void> stopPlayer() async {
    await _channel.invokeMethod('stopPlayer');
  }
}

插件注册

yaml

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dependencies:
  native_audio_player:
    path: ./native_audio_player

然后pub get 添加依赖

使用

dart

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  void _startFeedingPCM() async {
    await NativeAudioPlayer.startPlayer(sampleRate: 16000, channels: 1);
    // 然后再监听 PCM 流并转发
    _pcmStreamController.stream.listen((msg) {
      NativeAudioPlayer.writePcm(msg);
    });
  }

业务思路：后台流式返回数据，喂给播放器，播放器去播放。