ios swift thumbnail generation - ios

Question

I am developing an ios video trimmer with swift 4. I am trying to render a horizontal list of video thumbnails spread out over various durations both from local video files and and remote urls. When I test it in the simulator the thumbnails get generated in less than a second which is ok. However, when I test this code on an actual device the thumbnail generation is really slow and sometimes crashes. I tried to add the actual image generation to a background thread and then update the UI on the main thread when it is completed but that doesnt seem to work very well and the app crashes after rendering the screen a few times. I am not sure if that is because I am navigating away from the screen while tasks are still trying to complete. I am trying to resolve this problem and have the app generate the thumbnails quicker and not crash. Here is the code that I am using below. I would really appreciate any assistance for this issue.
func renderThumbnails(view: UIView, videoURL: URL, duration: Float64) {
var offset: Float64 = 0
for i in 0..<self.IMAGE_COUNT{
DispatchQueue.global(qos: .userInitiated).async {
offset = Float64(i) * (duration / Float64(self.IMAGE_COUNT))
let thumbnail = thumbnailFromVideo(videoUrl: videoURL,
time: CMTimeMake(Int64(offset), 1))
DispatchQueue.main.async {
self.addImageToView(image: thumbnail, view: view, index: i)
}
}
}
}
static func thumbnailFromVideo(videoUrl: URL, time: CMTime) -> UIImage{
let asset: AVAsset = AVAsset(url: videoUrl) as AVAsset
let imgGenerator = AVAssetImageGenerator(asset: asset)
imgGenerator.appliesPreferredTrackTransform = true
do{
let cgImage = try imgGenerator.copyCGImage(at: time, actualTime: nil)
let uiImage = UIImage(cgImage: cgImage)
return uiImage
}catch{
}
return UIImage()
}

Answer 1

The first sentence of the documentation says not to do what you’re doing! And it even tells you what to do instead.
Generating a single image in isolation can require the decoding of a large number of video frames with complex interdependencies. If you require a series of images, you can achieve far greater efficiency using the asynchronous method, generateCGImagesAsynchronously(forTimes:completionHandler:), which employs decoding efficiencies similar to those used during playback.
(Italics mine.)