AVAsset from AVAudioFile has 0 duration

Question

I am trying to record Audio from a microphone + then combine that with video from multiple locations to create a single video w/ audio from the microphone.

Using an AVCaptureDevice + AVCaptureSession is a bit choppy and I want to do speech recognition so I've shifted to doing video capture w/ AVCaptureSession and I'm trying to do the audio w/ AVAudioEngine (since my understanding is that it's easy to attach an SFSpeechRecognizer to an AudioTap)

Here's the code:

class AudioCaptureSession {

    var engine = AVAudioEngine()
    var outputFile : AVAudioFile!

    func initAudioEngine() {
        engine.stop()
        engine.reset()
        engine = AVAudioEngine()

        do {
            try AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord)
            let ioBufferDur = 128.0/44100.0
            try AVAudioSession.sharedInstance().setPreferredIOBufferDuration(ioBufferDur)
        } catch {
            print("AVAudioSession setup error: \(error)")
        }
        let outputPath = NSTemporaryDirectory().appending(kTempAudioFilename)
        let outputFile = URL(fileURLWithPath: outputPath, isDirectory: false)
        if FileManager.default.fileExists(atPath: outputPath) {
            do {
                try FileManager.default.removeItem(at: outputFile)
            } catch {
                print("Filemanager can't delete the audio file: \(error)")
            }
        }
        do {
            print("Settings: \(engine.mainMixerNode.outputFormat(forBus: 0).settings)")
            try self.outputFile = AVAudioFile(forWriting: outputFile, settings: engine.mainMixerNode.outputFormat(forBus: 0).settings)
        } catch {
            print("Can't make file for writing: \(error)")
        }
        let input = engine.inputNode!
        let format = input.inputFormat(forBus: 0)
        engine.connect(input, to: engine.mainMixerNode, format: format)
        engine.prepare()
        try! engine.start()

    }

    func startRecord() {
        let mixer = engine.mainMixerNode
        let format = mixer.outputFormat(forBus: 0)

        mixer.installTap(onBus: 0, bufferSize: 1024, format: format) { (buffer: AVAudioPCMBuffer!, time: AVAudioTime!) -> Void in
            do {
                try self.outputFile.write(from: buffer)
            } catch {
                print("Can't write audio to file: \(error)")
            }
        }
    }

    func stopRecord() {
        print("Audio len: \(self.outputFile.length)")
        engine.mainMixerNode.removeTap(onBus: 0)
        do {
            let f = try AVAudioFile(forReading: outputFile.url)
            print("Length of reading file: \(f.length)")
            print("Pos: \(f.framePosition)")
        } catch {
            print("Error getting file for reading: \(error)")
        }
        let asset = AVAsset(url: outputFile.url)
        print("Stop recording asset duration: \(asset.duration)")

    }
}

Example output after starting + stopping is:

Audio len: 105840
Length of reading file: 0
Stop recording asset: CMTime(value: 0, timescale: 44100, flags: __C.CMTimeFlags(rawValue: 1), epoch: 0)

So as you can see the outputFile knows that it is some N units long, but if I open a file for reading it thinks it is 0, and the AVAsset also thinks it has 0 duration. I'm wondering if the file isn't saving? Or something? I'm completely at a loss since everything I've seen just says "Make an AVAudioFile, file.write(buffer), use file" which clearly isn't working. Do I have to export it? Or something?

Answer 1

AVAudioFile doesn't have an explicit close, so you need to let it go out of scope by setting self.outputFile = nil when you've finished writing to it.