问AudioKit的RenderToFile无法正常工作

EN

    let midiSampler = AKMIDISampler()
    let sequencer = AKSequencer()
    let midi = AKMIDI()

    do {
        try midiSampler.loadSoundFont("soundFontFile", preset: 0, bank: 0)
    } catch {
        AKLog("Error - Couldn't load Sample!!!")
    }

    AudioKit.output = midiSampler

    do {
        try AudioKit.start()
    } catch {
        AKLog("AudioKit didn't begin")
    }

    let drumTrack = sequencer.newTrack("Drum Track")
    midi.openInput()

    midiSampler.enableMIDI(midi.client, name: "MIDI Sampler MIDI In")
    drumTrack.setMIDIOutput(midiSampler.midiIn)

    sequencer.setLength(AKDuration(beats: 8))
    sequencer.setTempo(136)
    sequencer.setRate(40)

    midi = AudioKit.midi

    let path = "recordedMIDIAudio.caf"
    let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent(path)
    let format = AVAudioFormat(commonFormat: .pcmFormatFloat64, sampleRate: 44100, channels: 1, interleaved: true)!

    do {
        let audioFile = try AKAudioFile(forWriting: url, settings: format.settings, commonFormat: format.commonFormat, interleaved: format.isInterleaved)

        try AudioKit.renderToFile(audioFile, duration: 3.55, prerender: {
            self.sequencer.play()
        })
    } catch {
        AKLog("Error when converting")
    }