open class AVAudioEngine : NSObject {
...
On AVAudioSession supported platforms, this method may cause the audio session to be implicitly activated. It is recommended to configure and activate the app's audio session before starting the engine. For more information, see the `prepare` method above.
    */
    open func start() throws

func render(to audioFile: AVAudioFile,
                maximumFrameCount: AVAudioFrameCount = 4096,
                duration: Double,
                renderUntilSilent: Bool = false,
                silenceThreshold: Float = 0.00005,
                prerender: (() -> Void)? = nil,
                progress progressHandler: ((Double) -> Void)? = nil) throws
    {
        guard duration >= 0 else {
            throw NSError(domain: "AVAudioEngine ext", code: 1,
                          userInfo: [NSLocalizedDescriptionKey: "Seconds needs to be a positive value"])
        }

        // Engine can't be running when switching to offline render mode.
        if isRunning { stop() }
        try enableManualRenderingMode(.offline,
                                      format: audioFile.processingFormat,
                                      maximumFrameCount: maximumFrameCount)

        // This resets the sampleTime of offline rendering to 0.
        reset()
        try start()

        guard let buffer = AVAudioPCMBuffer(pcmFormat: manualRenderingFormat,
                                            frameCapacity: manualRenderingMaximumFrameCount)
        else {
            throw NSError(domain: "AVAudioEngine ext", code: 1,
                          userInfo: [NSLocalizedDescriptionKey: "Couldn't create buffer in renderToFile"])
        }

        // This is for users to prepare the nodes for playing, i.e player.play()
        prerender?()

        // Render until file contains >= target samples
        let targetSamples = AVAudioFramePosition(duration * manualRenderingFormat.sampleRate)
        let channelCount = Int(buffer.format.channelCount)
        var zeroCount = 0
        var isRendering = true

        while isRendering {
            if !renderUntilSilent, audioFile.framePosition >= targetSamples {
                break
            }
            let framesToRender = renderUntilSilent ? manualRenderingMaximumFrameCount
                : min(buffer.frameCapacity, AVAudioFrameCount(targetSamples - audioFile.framePosition))

            let status = try renderOffline(framesToRender, to: buffer)

            // Progress in the range of starting (0) - finished (1)
            var progress: Double = 0

            switch status {
            case .success:
                try audioFile.write(from: buffer)
                progress = min(Double(audioFile.framePosition) / Double(targetSamples), 1.0)
                progressHandler?(progress)
            case .cannotDoInCurrentContext:
                Log("renderToFile cannotDoInCurrentContext", type: .error)
                continue
            case .error, .insufficientDataFromInputNode:
                throw NSError(domain: "AVAudioEngine ext", code: 1,
                              userInfo: [NSLocalizedDescriptionKey: "render error"])
            @unknown default:
                Log("Unknown render result:", status, type: .error)
                isRendering = false
            }

            if renderUntilSilent, progress == 1, let data = buffer.floatChannelData {
                var rms: Float = 0.0
                for i in 0 ..< channelCount {
                    var channelRms: Float = 0.0
                    vDSP_rmsqv(data[i], 1, &channelRms, vDSP_Length(buffer.frameLength))
                    rms += abs(channelRms)
                }
                let value = (rms / Float(channelCount))

                if value < silenceThreshold {
                    zeroCount += 1
                    // check for consecutive buffers of below threshold, then assume it's silent
                    if zeroCount > 2 {
                        isRendering = false
                    }
                } else {
                    // Resetting consecutive threshold check due to positive value
                    zeroCount = 0
                }
            }
        }

        stop()
        disableManualRenderingMode()
    }