AudioKit, exporting AVAudioPCMBuffer array to audio file with fade in/out - swift

Question

I'm capturing audio from AKLazyTap and rendering the accumulated [AVAudioPCMBuffer] to an audio file, in the background, while my app's audio is running. This works great, but I want to add fade in/out to clean up the result. I see the convenience extension for adding fades to a single AVAudioPCMBuffer, but I'm not sure how I'd do it on an array. I'd thought to concatenate the buffers, but there doesn't appear to be support for that. Does anyone know if that's currently possible? Basically it would require something similar to copy(from:readOffset:frames), but would need to have a write offset as well...
Or maybe there's an easier way?
UPDATE
Okay, after studying some related AK code, I tried directly copying buffer data over to a single, long buffer, then applying the fade convenience function. But this gives me an empty (well, 4k) file. Is there some obvious error here that I'm just not seeing?
func renderBufferedAudioToFile(_ audioBuffers: [AVAudioPCMBuffer], withStartOffset startOffset: Int, endOffset: Int, fadeIn: Float64, fadeOut: Float64, atURL url: URL) {
// strip off the file name
let name = String(url.lastPathComponent.split(separator: ".")[0])
var url = self.module.stateManager.audioCacheDirectory
// UNCOMPRESSED
url = url.appendingPathComponent("\(name).caf")
let format = Conductor.sharedInstance.sourceMixer.avAudioNode.outputFormat(forBus: 0)
var settings = format.settings
settings["AVLinearPCMIsNonInterleaved"] = false
// temp buffer for fades
let totalFrameCapacity = audioBuffers.reduce(0) { $0 + $1.frameLength }
guard let tempAudioBufferForFades = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: totalFrameCapacity) else {
print("Failed to create fade buffer!")
return
}
// write ring buffer to file.
let file = try! AVAudioFile(forWriting: url, settings: settings)
var writeOffset: AVAudioFrameCount = 0
for i in 0 ..< audioBuffers.count {
var buffer = audioBuffers[i]
let channelCount = Int(buffer.format.channelCount)
if i == 0 && startOffset != 0 {
// copy a subset of samples in the buffer
if let subset = buffer.copyFrom(startSample: AVAudioFrameCount(startOffset)) {
buffer = subset
}
} else if i == audioBuffers.count - 1 && endOffset != 0 {
if let subset = buffer.copyTo(count: AVAudioFrameCount(endOffset)) {
buffer = subset
}
}
// write samples into single, long buffer
for i in 0 ..< buffer.frameLength {
for n in 0 ..< channelCount {
tempAudioBufferForFades.floatChannelData?[n][Int(i + writeOffset)] = (buffer.floatChannelData?[n][Int(i)])!
}
}
print("buffer \(i), writeOffset = \(writeOffset)")
writeOffset = writeOffset + buffer.frameLength
}
// update!
tempAudioBufferForFades.frameLength = totalFrameCapacity
if let bufferWithFades = tempAudioBufferForFades.fade(inTime: fadeIn, outTime: fadeOut) {
try! file.write(from: bufferWithFades)
}
}