问当AVAudioUnitTimePitch改变播放速率时，如何使用AVAudioEngine进行离线音频处理？

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// engine: AVAudioEngine
// playerNode: AVAudioPlayerNode
// audioFile: AVAudioFile

open func render(to destinationFile: AVAudioFile) throws {
    
    playerNode.scheduleFile(audioFile, at: nil)
    
    do {
        let buffCapacity: AVAudioFrameCount = 4096
        try engine.enableManualRenderingMode(.offline, format: audioFile.processingFormat, maximumFrameCount: buffCapacity)
    }
    catch {
        print("Failed to enable manual rendering mode: \(error)")
        throw error
    }
    
    do {
        try engine.start()
    }
    catch {
        print("Failed to start the engine: \(error)")
    }
    
    playerNode.play()
    
    let outputBuff = AVAudioPCMBuffer(pcmFormat: engine.manualRenderingFormat,
                                      frameCapacity: engine.manualRenderingMaximumFrameCount)!
    
    while engine.manualRenderingSampleTime < audioFile.length {
        let remainingSamples = audioFile.length - engine.manualRenderingSampleTime
        let framesToRender = min(outputBuff.frameCapacity, AVAudioFrameCount(remainingSamples))
        
        do {
            let renderingStatus = try engine.renderOffline(framesToRender, to: outputBuff)
            
            switch renderingStatus {
            
            case .success:
                do {
                    try destinationFile.write(from: outputBuff)
                }
                catch {
                    print("Failed to write from file to buffer: \(error)")
                    throw error
                }
                
            case .insufficientDataFromInputNode:
                break
            
            case.cannotDoInCurrentContext:
                break
            
            case .error:
                print("An error occured during rendering.")
                throw AudioPlayer.ExportError.renderingError
            
            @unknown default:
                fatalError("engine.renderOffline() returned an unknown value.")
            }
        }
        catch {
            print("Failed to render offline manually: \(error)")
            throw error
        }
    }
    
    playerNode.stop()
    engine.stop()
    engine.disableManualRenderingMode()
}