Swift/Cocoa：如何将 AVMutableComposition 导出为 wav 或 aiff 音频文件

Question

我有一个 AVMutableComposition 只包含我想导出到 .wav 音频文件的音频。

我发现的最简单的音频导出解决方案是使用AVAssetExportSession，就像在这个简化的示例中一样：

let composition = AVMutableComposition()
// add tracks...

let exportSession = AVAssetExportSession(asset: composition,
                                         presetName: AVAssetExportPresetAppleM4A)!
exportSession.outputFileType = .m4a
exportSession.outputURL = someOutUrl
exportSession.exportAsynchronously {
    // done
}

但它只适用于 .m4a

这个post 提到要导出到其他格式，必须使用AVAssetReader 和AVAssetWriter，但遗憾的是它没有详细说明。

我曾尝试实现它，但被卡在了这个过程中。
这是我到目前为止所拥有的（再次简化）：

let composition = AVMutableComposition()
let outputSettings: [String : Any] = [
    AVFormatIDKey: kAudioFormatLinearPCM,
    AVLinearPCMIsBigEndianKey: false,
    AVLinearPCMIsFloatKey: false,
    AVLinearPCMBitDepthKey: 32,
    AVLinearPCMIsNonInterleaved: false,
    AVSampleRateKey: 44100.0,
    AVChannelLayoutKey: NSData(),
]

let assetWriter = try! AVAssetWriter(outputURL: someOutUrl, fileType: .wav)
let input = AVAssetWriterInput(mediaType: .audio, outputSettings: outputSettings)
assetWriter.add(input)
assetWriter.startWriting()
assetWriter.startSession(atSourceTime: CMTime.zero)
input.requestMediaDataWhenReady(on: .main) {

    // as I understand, I need to bring in data from my
    // AVMutableComposition here...

    let sampleBuffer: CMSampleBuffer = ???
    input.append(sampleBuffer)
}

assetWriter.finishWriting {
    // done
}

归结为我的问题：

您能否提供一个将音频从 AVMutableComposition 导出到 wav 文件的工作示例？

Answer 1

经过更多研究，我想出了以下解决方案。
缺少的部分是 AVAssetReader 的使用。

（简化代码）

// composition
let composition = AVMutableComposition()
// add stuff to composition

// reader
guard let assetReader = try? AVAssetReader(asset: composition) else { return }
assetReader.timeRange = CMTimeRange(start: .zero, duration: CMTime(value: composition.duration.value, timescale: composition.duration.timescale))
let assetReaderAudioMixOutput = AVAssetReaderAudioMixOutput(audioTracks: composition.tracks(withMediaType: .audio), audioSettings: nil)
assetReader.add(assetReaderAudioMixOutput)
guard assetReader.startReading() else { return }

// writer
let outputSettings: [String : Any] = [
    AVFormatIDKey: kAudioFormatLinearPCM,
    AVLinearPCMIsBigEndianKey: false,
    AVLinearPCMIsFloatKey: false,
    AVLinearPCMBitDepthKey: 32,
    AVLinearPCMIsNonInterleaved: false,
    AVSampleRateKey: 44100.0,
    AVChannelLayoutKey: NSData(),
]
guard let assetWriter = try? AVAssetWriter(outputURL: someOutUrl, fileType: .wav) else { return }
let writerInput = AVAssetWriterInput(mediaType: .audio, outputSettings: outputSettings)
assetWriter.add(writerInput)
guard assetWriter.startWriting() else { return }
assetWriter.startSession(atSourceTime: CMTime.zero)

let queue = DispatchQueue(label: "my.queue.id")
input.requestMediaDataWhenReady(on: queue) {

    // capture assetReader in my block to prevent it being released
    let readerOutput = assetReader.outputs.first!

    while writerInput.isReadyForMoreMediaData {
        if let nextSampleBuffer = readerOutput.copyNextSampleBuffer() {
            writerInput.append(nextSampleBuffer)
        } else {
            writerInput.markAsFinished()
            assetWriter.endSession(atSourceTime: composition.duration)
            assetWriter.finishWriting() {
                DispatchQueue.main.async {

                    // done, call my completion
                }
            }
            break;
        }
    }
}

Answer 2

正如您所说，您正在成功创建 .m4a 为什么不将 .m4a 文件转换为 .wav 文件，只需输入几行代码

我只是将文件的扩展名更改为 .wav 并删除了 .m4a 文件，它就可以工作了。

func getDirectory() -> URL {
    let path = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    let documentDirectory = path[0]
    return documentDirectory
}

let date = Date().timeIntervalSince1970

fileName = getDirectory().appendingPathComponent("\(date).m4a")
wavFileName = getDirectory().appendingPathComponent("\(date).wav")

try! FileManager.default.copyItem(at: fileName, to: wavFileName)
try! FileManager.default.removeItem(at: fileName)

我什至播放了 .wav 文件，它运行良好。

audioPlayer = try! AVAudioPlayer(contentsOf: wavFileName)
audioPlayer.play()

查看上面的示例，此扩展替换是否会导致您的应用中的任何负载或时间