AVFoundation框架解析（二十）—— AVAudioEngine之详细说明和一个简单示例源码（三）

版本记录

版本号	时间
V1.0	2018.08.19

前言

AVFoundation框架是ios中很重要的框架，所有与视频音频相关的软硬件控制都在这个框架里面，接下来这几篇就主要对这个框架进行介绍和讲解。感兴趣的可以看我上几篇。
1. AVFoundation框架解析（一）—— 基本概览
2. AVFoundation框架解析（二）—— 实现视频预览录制保存到相册
3. AVFoundation框架解析（三）—— 几个关键问题之关于框架的深度概括
4. AVFoundation框架解析（四）—— 几个关键问题之AVFoundation探索（一）
5. AVFoundation框架解析（五）—— 几个关键问题之AVFoundation探索（二）
6. AVFoundation框架解析（六）—— 视频音频的合成（一）
7. AVFoundation框架解析（七）—— 视频组合和音频混合调试
8. AVFoundation框架解析（八）—— 优化用户的播放体验
9. AVFoundation框架解析（九）—— AVFoundation的变化（一）
10. AVFoundation框架解析（十）—— AVFoundation的变化（二）
11. AVFoundation框架解析（十一）—— AVFoundation的变化（三）
12. AVFoundation框架解析（十二）—— AVFoundation的变化（四）
13. AVFoundation框架解析（十三）—— 构建基本播放应用程序
14. AVFoundation框架解析（十四）—— VAssetWriter和AVAssetReader的Timecode支持（一）
15. AVFoundation框架解析（十五）—— VAssetWriter和AVAssetReader的Timecode支持（二）
16. AVFoundation框架解析（十六）—— 一个简单示例之播放、录制以及混合视频（一）
17. AVFoundation框架解析（十七）—— 一个简单示例之播放、录制以及混合视频之源码及效果展示（二）
18. AVFoundation框架解析（十八）—— AVAudioEngine之基本概览（一）
19. AVFoundation框架解析（十九）—— AVAudioEngine之详细说明和一个简单示例（二）

源码

下面我们一起看一下源码。

1. ViewController.swift

import UIKit
import AVFoundation

class ViewController: UIViewController {

  // MARK: Outlets
  @IBOutlet weak var playPauseButton: UIButton!
  @IBOutlet weak var skipForwardButton: UIButton!
  @IBOutlet weak var skipBackwardButton: UIButton!
  @IBOutlet weak var progressBar: UIProgressView!
  @IBOutlet weak var meterView: UIView!
  @IBOutlet weak var volumeMeterHeight: NSLayoutConstraint!
  @IBOutlet weak var rateSlider: UISlider!
  @IBOutlet weak var rateLabel: UILabel!
  @IBOutlet weak var rateLabelLeading: NSLayoutConstraint!
  @IBOutlet weak var countUpLabel: UILabel!
  @IBOutlet weak var countDownLabel: UILabel!

  // MARK: AVAudio properties
  var engine = AVAudioEngine()
  var player = AVAudioPlayerNode()
  var rateEffect = AVAudioUnitTimePitch()

  var audioFile: AVAudioFile? {
    didSet {
      if let audioFile = audioFile {
        audioLengthSamples = audioFile.length
        audioFormat = audioFile.processingFormat
        audioSampleRate = Float(audioFormat?.sampleRate ?? 44100)
        audioLengthSeconds = Float(audioLengthSamples) / audioSampleRate
      }
    }
  }
  var audioFileURL: URL? {
    didSet {
      if let audioFileURL = audioFileURL {
        audioFile = try? AVAudioFile(forReading: audioFileURL)
      }
    }
  }
  var audioBuffer: AVAudioPCMBuffer?

  // MARK: other properties
  var audioFormat: AVAudioFormat?
  var audioSampleRate: Float = 0
  var audioLengthSeconds: Float = 0
  var audioLengthSamples: AVAudioFramePosition = 0
  var needsFileScheduled = true
  let rateSliderValues: [Float] = [0.5, 1.0, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0]
  var rateValue: Float = 1.0 {
    didSet {
      rateEffect.rate = rateValue
      updateRateLabel()
    }
  }
  var updater: CADisplayLink?
  var currentFrame: AVAudioFramePosition {
    guard let lastRenderTime = player.lastRenderTime,
      let playerTime = player.playerTime(forNodeTime: lastRenderTime) else {
        return 0
    }

    return playerTime.sampleTime
  }
  var seekFrame: AVAudioFramePosition = 0
  var currentPosition: AVAudioFramePosition = 0
  let pauseImageHeight: Float = 26.0
  let minDb: Float = -80.0

  enum TimeConstant {
    static let secsPerMin = 60
    static let secsPerHour = TimeConstant.secsPerMin * 60
  }

  // MARK: - ViewController lifecycle
  //
  override func viewDidLoad() {
    super.viewDidLoad()

    setupRateSlider()
    countUpLabel.text = formatted(time: 0)
    countDownLabel.text = formatted(time: audioLengthSeconds)
    setupAudio()

    updater = CADisplayLink(target: self, selector: #selector(updateUI))
    updater?.add(to: .current, forMode: .defaultRunLoopMode)
    updater?.isPaused = true
  }

  override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    updateRateLabel()
  }
}

// MARK: - Actions
//
extension ViewController {
  @IBAction func didChangeRateValue(_ sender: UISlider) {
    let index = round(sender.value)
    rateSlider.setValue(Float(index), animated: false)
    rateValue = rateSliderValues[Int(index)]
  }

  @IBAction func playTapped(_ sender: UIButton) {
    sender.isSelected = !sender.isSelected

    if currentPosition >= audioLengthSamples {
      updateUI()
    }

    if player.isPlaying {
      disconnectVolumeTap()
      updater?.isPaused = true
      player.pause()
    } else {
      updater?.isPaused = false
      connectVolumeTap()
      if needsFileScheduled {
        needsFileScheduled = false
        scheduleAudioFile()
      }
      player.play()
    }
  }

  @IBAction func plus10Tapped(_ sender: UIButton) {
    guard let _ = player.engine else { return }
    seek(to: 10.0)
  }

  @IBAction func minus10Tapped(_ sender: UIButton) {
    guard let _ = player.engine else { return }
    needsFileScheduled = false
    seek(to: -10.0)
  }

  @objc func updateUI() {
    currentPosition = currentFrame + seekFrame
    currentPosition = max(currentPosition, 0)
    currentPosition = min(currentPosition, audioLengthSamples)

    progressBar.progress = Float(currentPosition) / Float(audioLengthSamples)
    let time = Float(currentPosition) / audioSampleRate
    countUpLabel.text = formatted(time: time)
    countDownLabel.text = formatted(time: audioLengthSeconds - time)

    if currentPosition >= audioLengthSamples {
      player.stop()
      updater?.isPaused = true
      playPauseButton.isSelected = false
      disconnectVolumeTap()
    }
  }
}

// MARK: - Display related
//
extension ViewController {
  func setupRateSlider() {
    let numSteps = rateSliderValues.count-1
    rateSlider.minimumValue = 0
    rateSlider.maximumValue = Float(numSteps)
    rateSlider.isContinuous = true
    rateSlider.setValue(1.0, animated: false)
    rateValue = 1.0
    updateRateLabel()
  }

  func updateRateLabel() {
    rateLabel.text = "\(rateValue)x"
    let trackRect = rateSlider.trackRect(forBounds: rateSlider.bounds)
    let thumbRect = rateSlider.thumbRect(forBounds: rateSlider.bounds , trackRect: trackRect, value: rateSlider.value)
    let x = thumbRect.origin.x + thumbRect.width/2 - rateLabel.frame.width/2
    rateLabelLeading.constant = x
  }

  func formatted(time: Float) -> String {
    var secs = Int(ceil(time))
    var hours = 0
    var mins = 0

    if secs > TimeConstant.secsPerHour {
      hours = secs / TimeConstant.secsPerHour
      secs -= hours * TimeConstant.secsPerHour
    }

    if secs > TimeConstant.secsPerMin {
      mins = secs / TimeConstant.secsPerMin
      secs -= mins * TimeConstant.secsPerMin
    }

    var formattedString = ""
    if hours > 0 {
      formattedString = "\(String(format: "%02d", hours)):"
    }
    formattedString += "\(String(format: "%02d", mins)):\(String(format: "%02d", secs))"
    return formattedString
  }
}

// MARK: - Audio
//
extension ViewController {
  func setupAudio() {
    audioFileURL  = Bundle.main.url(forResource: "Intro", withExtension: "mp4")

    engine.attach(player)
    engine.attach(rateEffect)
    engine.connect(player, to: rateEffect, format: audioFormat)
    engine.connect(rateEffect, to: engine.mainMixerNode, format: audioFormat)

    engine.prepare()

    do {
      try engine.start()
    } catch let error {
      print(error.localizedDescription)
    }
  }

  func scheduleAudioFile() {
    guard let audioFile = audioFile else { return }

    seekFrame = 0
    player.scheduleFile(audioFile, at: nil) { [weak self] in
      self?.needsFileScheduled = true
    }
  }

  func connectVolumeTap() {
    let format = engine.mainMixerNode.outputFormat(forBus: 0)
    engine.mainMixerNode.installTap(onBus: 0, bufferSize: 1024, format: format) { buffer, when in

      guard let channelData = buffer.floatChannelData,
        let updater = self.updater else {
          return
      }

      let channelDataValue = channelData.pointee
      let channelDataValueArray = stride(from: 0,
                                         to: Int(buffer.frameLength),
                                         by: buffer.stride).map{ channelDataValue[$0] }
      let rms = sqrt(channelDataValueArray.map{ $0 * $0 }.reduce(0, +) / Float(buffer.frameLength))
      let avgPower = 20 * log10(rms)
      let meterLevel = self.scaledPower(power: avgPower)

      DispatchQueue.main.async {
        self.volumeMeterHeight.constant = !updater.isPaused ? CGFloat(min((meterLevel * self.pauseImageHeight),
                                                                          self.pauseImageHeight)) : 0.0
      }
    }
  }

  func scaledPower(power: Float) -> Float {
    guard power.isFinite else { return 0.0 }

    if power < minDb {
      return 0.0
    } else if power >= 1.0 {
      return 1.0
    } else {
      return (fabs(minDb) - fabs(power)) / fabs(minDb)
    }
  }

  func disconnectVolumeTap() {
    engine.mainMixerNode.removeTap(onBus: 0)
    volumeMeterHeight.constant = 0
  }

  func seek(to time: Float) {
    guard let audioFile = audioFile,
      let updater = updater else {
      return
    }

    seekFrame = currentPosition + AVAudioFramePosition(time * audioSampleRate)
    seekFrame = max(seekFrame, 0)
    seekFrame = min(seekFrame, audioLengthSamples)
    currentPosition = seekFrame

    player.stop()

    if currentPosition < audioLengthSamples {
      updateUI()
      needsFileScheduled = false

      player.scheduleSegment(audioFile, startingFrame: seekFrame, frameCount: AVAudioFrameCount(audioLengthSamples - seekFrame), at: nil) { [weak self] in
        self?.needsFileScheduled = true
      }

      if !updater.isPaused {
        player.play()
      }
    }
  }

}

2. sb文件

下面看一下sb文件。

---

效果展示

下面看一下实现效果。

可以看见，实现了快进、倒退、倍率调整等。

后记

本篇主要讲述了AVAudioEngine之详细说明和一个简单示例源码和效果展示，感兴趣的给个赞或者关注~~~