Exporting Core Animations to Videos - ios

Question

I have been trying to get AVAssetExportSession to save a video with animations overlaid, but am running into a problem where the exported video has a runtime of 0:00.
As I understand it, the AVAssetExportSession APIs need a base video, which can be a black video that runs for a few seconds. You can then write over and cover up as much of that base video as you'd like using CALayers. If the animations run over the runtime of the base video, the exported video will extend itself to contain the runtime of the animations.
The base video is 5 sec long, but still yet the exported video is 0:00. Interestingly the exported video does contain the black background from the source video and the very first frame of the animation (the layers).
Has anyone run into this before and know of a good solution/what I'm missing?
Code for context...
#objc func saveMovie() {
print("save movie")
self.selectedFrame = CGRect(x: 0.0, y: 0.0, width: 1080.0, height: 1920.0)
self.selectedBounds = CGRect(x: 0.0, y: 0.0, width: 1080.0, height: 1920.0)
let mainLayer = CALayer()
mainLayer.frame = CGRect(x: 0.0, y: 0.0, width: 1080.0, height: 1920.0)
let videoLayer = CALayer()
videoLayer.frame = CGRect(x: 0.0, y: 0.0, width: 1080.0, height: 1920.0)
let animationLayer = CALayer()
animationLayer.frame = CGRect(x: 0.0, y: 0.0, width: 1080.0, height: 1920.0)
animationLayer.addSublayer(makeBackground())
animationLayer.addSublayer(makeHeadingTextLayer())
mainLayer.addSublayer(videoLayer)
mainLayer.addSublayer(animationLayer)
if let sourceVideoUrl = Bundle.main.url(
forResource: "SourceVideo",
withExtension: "mp4"
) {
// Load Video Asset to Use As Base
print(sourceVideoUrl.absoluteString)
let baseVideoAsset = AVURLAsset(url: sourceVideoUrl)
// Create Composition for the video to live in
let composition = AVMutableComposition()
composition.naturalSize = CGSize(width: 1080.0, height: 1920.0)
guard
let compositionTrack = composition.addMutableTrack(
withMediaType: AVMediaType.video,
preferredTrackID: kCMPersistentTrackID_Invalid
),
let assetTrack = baseVideoAsset.tracks(
withMediaType: .video
).first
else {
print("something is wrong with the asset")
return
}
do {
// this crashes, so just hard coding 5 seconds right now
//let baseVideoDuration = try await baseVideoAsset.load(.duration)
//print("\(baseVideoDuration)")
let timeRange = CMTimeRangeMake(
start: .zero,
duration: CMTime(value: 5, timescale: 30)
)
try compositionTrack.insertTimeRange(
timeRange,
of: assetTrack,
at: .zero
)
} catch {
print("issue with video track insert time range")
}
compositionTrack.preferredTransform = assetTrack.preferredTransform
let videoSize = CGSize(width: 1080.0, height: 1920.0)
let videoComposition = AVMutableVideoComposition()
videoComposition.renderSize = videoSize
videoComposition.frameDuration = CMTimeMake(value: 1, timescale: 30)
videoComposition.animationTool = AVVideoCompositionCoreAnimationTool(
postProcessingAsVideoLayer: videoLayer,
in: mainLayer
)
let videoCompositionInstruction = AVMutableVideoCompositionInstruction()
videoCompositionInstruction.timeRange = CMTimeRangeMake(
start: CMTime.zero,
duration: CMTimeMake(value: 10, timescale: 30)
)
videoComposition.instructions = [videoCompositionInstruction]
let layerInstruction = AVMutableVideoCompositionLayerInstruction(assetTrack: assetTrack)
let transform = assetTrack.preferredTransform
layerInstruction.setTransform(transform, at: .zero)
videoCompositionInstruction.layerInstructions = [layerInstruction]
guard let exporter = AVAssetExportSession(
asset: composition,
presetName: AVAssetExportPreset1920x1080
) else {
print("failed to create exporter")
return
}
let videoName = UUID().uuidString
let exportUrl = URL(fileURLWithPath: NSTemporaryDirectory())
.appendingPathComponent(videoName)
.appendingPathExtension("mov")
exporter.videoComposition = videoComposition
exporter.outputFileType = .mov
exporter.outputURL = exportUrl
exporter.timeRange = CMTimeRangeMake(start: .zero, duration: CMTimeMake(value: 10, timescale: 30))
NSLog("Composition Duration: %ld seconds", lround(CMTimeGetSeconds(composition.duration)));
exporter.exportAsynchronously {
DispatchQueue.main.async {
switch exporter.status {
case .failed:
print("failed to export")
print(exporter.error ?? "no error")
case .cancelled:
print("canceled")
case .completed:
print("completed")
UISaveVideoAtPathToSavedPhotosAlbum(
exportUrl.relativePath,
self,
nil,
nil
)
case .unknown:
print("unknown status")
default:
break
}
}
}
}
}