AVAssetWriter: Momentary Lag When Starting Write - ios

Question

I am developing an app that records a video using AVAssetWriter (the source media are sample buffers output from captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection).
I've noticed that the very first time I instantiate my AVAssetWriter, then call startWriting() and startSession(atSourceTime: CMTime), there is a lag of about 30ms. This causes the video preview on the screen to momentarily hang, which also results in the first few frames of the video not write properly.
Curiously, if I then re-instantiate my AVAssetWriter and take all of the same steps again, everything works fine from that point onward. This only occurs the very first time.
While perhaps too lengthy to post, this is the function in which I am certain the error is occurring;
func insert(pixel buffer: CVPixelBuffer, with time: CMTime) {
// Check for unknown status
if fileWriter.status == .unknown {
guard startingVideoTime == nil else {
print("We've received an error setting the starting video time.")
return
}
startingVideoTime = time
if fileWriter.startWriting() {
fileWriter.startSession(atSourceTime: startingVideoTime!)
isRecording = true
}
}
// Append buffer
if videoInput.isReadyForMoreMediaData {
append(pixel: buffer, with: time)
isRecording = true
}
}
For posterity, I am calling the above function from my captureOutput, only doing so when my self.isRecording = true, which I set when the user taps a "Start Record" button.

Answer 1

let videoCompressionSettings: [String: Any] = [
AVVideoCodecKey: AVVideoCodecType.h264,
AVVideoWidthKey: NSNumber(value: 1080),
AVVideoHeightKey: NSNumber(value: 1920)
]
let audioCompressionSettings: [String: Any] = [
AVNumberOfChannelsKey: NSNumber(value: 1),
AVEncoderAudioQualityForVBRKey: NSNumber(value: 91),
AVEncoderBitRatePerChannelKey:NSNumber(value: 9600),
AVEncoderBitRateStrategyKey: AVAudioBitRateStrategy_Variable,
AVFormatIDKey: NSNumber(value: 1633772320),
AVSampleRateKey: NSNumber(value: 44100)
]
// My own wrapper for AVAssetWriter
movieWriterManager = MovieWriterManager(videoUrl: recordingVideoURL(), audioUrl: recordingAudioURL(), videoCompressionSettings: videoCompressionSettings, audioCompressionSettings: audioCompressionSettings)
movieWriterManager?.warmup()
You may try to buy time by running the same steps at AppDelegate's didFinishLaunchingWithOptions with a default videoCompressionSettings and audioCompressionSettings. It's actually not blocking UI thread but only the video output connection of AVCaptureVideoDataOutput.