Generate a thumbnail from UIImage in Swift - swift

Question

I would like to take a UIImage and output a 100 by 100 version of it to use as a thumbnail. I found some answers on SO for how to do this in objective-C but not swift and wasn't sure where to start. I also found the link (https://nshipster.com/image-resizing/#technique-3-creating-a-thumbnail-with-image-io) which suggests it isn't as straight forward as I would have hoped. That link had me hopeful that one of the approaches may work, but each references a URL argument which confused me since I am starting with a UIImage as the input.
In a a similar situation (user uploading a picture from phone) I use the code below to create a thumbnail from the asset, I am looking for help doing the same thing when the input is a UIImage instead of a PHAsset.
func getAssetThumbnail(asset: PHAsset) -> UIImage {
let manager = PHImageManager.default()
let option = PHImageRequestOptions()
var thumbnail = UIImage()
option.isSynchronous = true
manager.requestImage(for: asset, targetSize: CGSize(width: 100, height: 100), contentMode: .aspectFit, options: option, resultHandler: {(result, info)->Void in
thumbnail = result!
})
return thumbnail
}

Answer 1

iOS 15 added the following beta APIs to UIImage.
func prepareThumbnail(of: CGSize, completionHandler: (UIImage?) -> Void)
func preparingThumbnail(of: CGSize) -> UIImage?
https://developer.apple.com/documentation/uikit/uiimage/

Answer 2

Tested the code, and this works fine for me:- (Swift 5.0)
let yourImage = UIImage()
if let imageData = yourImage.pngData(){
let options = [
kCGImageSourceCreateThumbnailWithTransform: true,
kCGImageSourceCreateThumbnailFromImageAlways: true,
kCGImageSourceThumbnailMaxPixelSize: 100] as CFDictionary // Specify your desired size at kCGImageSourceThumbnailMaxPixelSize. I've specified 100 as per your question
imageData.withUnsafeBytes { ptr in
guard let bytes = ptr.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
return
}
if let cfData = CFDataCreate(kCFAllocatorDefault, bytes, imageData.count){
let source = CGImageSourceCreateWithData(cfData, nil)!
let imageReference = CGImageSourceCreateThumbnailAtIndex(source, 0, options)!
let thumbnail = UIImage(cgImage: imageReference) // You get your thumbail here
}
}
}

Answer 3

For future reference, I've just come across the same issue and this thread has some nice solutions: Creating a thumbnail from UIImage using CGImageSourceCreateThumbnailAtIndex
I went with this, which is working nicely in Swift 5.3:
let uiImage = someUIImage
let options = [
kCGImageSourceCreateThumbnailWithTransform: true,
kCGImageSourceCreateThumbnailFromImageAlways: true,
kCGImageSourceThumbnailMaxPixelSize: 100] as CFDictionary
guard let imageData = uiImage.pngData(),
let imageSource = CGImageSourceCreateWithData(imageData as NSData, nil),
let image = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options)
else {
return nil
}
return UIImage(cgImage: image)