What to input to Generate CGImages Asynchronously - ios

Question

In my app, I want to generate multiple thumbnails of a video, preferably good quality thumbnails. My old approach was to perform a loop 15 times, and copy a CGImage at different time. As shown below
func generateThumbnails(_ fileURL:URL) {
let asset = AVAsset(url: fileURL)
let imageGenerator = AVAssetImageGenerator(asset: asset)
imageGenerator.apertureMode = AVAssetImageGeneratorApertureMode.cleanAperture
imageGenerator.appliesPreferredTrackTransform = true
let duration = asset.duration
let seconds = CMTimeGetSeconds(duration)
let addition = seconds / 15
var number = 1.0
do {
while number < seconds {
let thumbnailCGImage = try imageGenerator.copyCGImage(at: CMTimeMake(Int64(number),1), actualTime: nil)
let image = UIImage(cgImage: thumbnailCGImage)
thumbnails.append(image)
number += addition
}
} catch let err {
return
}
}
However, after doing some more research, I found it was more logical to just generate thumbnails asynchronously using
let imageGenerator = AVAssetImageGenerator(asset: asset)
imageGenerator.generateCGImagesAsynchronously(forTimes: [NSValue], completionHandler: AVAssetImageGeneratorCompletionHandler)
However, I don't really know what the I am supposed to input into the [NSValue] and the completion handler.
I just need an explanation on how to generate thumbnails this way, and see if it's the better thing to do.

Answer 1

https://developer.apple.com/documentation/avfoundation/avassetimagegenerator/1388100-generatecgimagesasynchronously
requestedTimes An array of NSValue objects, each containing a CMTime,
specifying the asset times at which an image is requested.
Usage:
let duration = asset.duration
let seconds = CMTimeGetSeconds(duration)
let addition = seconds / 15
var number = 1.0
var times = [NSValue]()
times.append(NSValue(time: CMTimeMake(Int64(number), 1)))
while number < seconds {
number += addition
times.append(NSValue(time: CMTimeMake(Int64(number), 1)))
}
struct Formatter {
static let formatter: DateFormatter = {
let result = DateFormatter()
result.dateStyle = .short
return result
}()
}
imageGenerator.generateCGImagesAsynchronously(forTimes: times) { (requestedTime, cgImage, actualImageTime, status, error) in
let seconds = CMTimeGetSeconds(requestedTime)
let date = Date(timeIntervalSinceNow: seconds)
let time = Formatter.formatter.string(from: date)
switch status {
case .succeeded: do {
if let image = cgImage {
print("Generated image for approximate time: \(time)")
let img = UIImage(cgImage: image)
//do something with `img`
}
else {
print("Failed to generate a valid image for time: \(time)")
}
}
case .failed: do {
if let error = error {
print("Failed to generate image with Error: \(error) for time: \(time)")
}
else {
print("Failed to generate image for time: \(time)")
}
}
case .cancelled: do {
print("Image generation cancelled for time: \(time)")
}
}
}