Create thumbnail image from URL. https://developer.apple.com/videos/play/wwdc2018/219/

ImageDownSample.swift

import UIKit

func downsample(imageAt imageURL: URL, to pointSize: CGSize, scale: CGFloat = UIScreen.main.nativeScale) -> UIImage {
    let imageSourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
    let imageSource = CGImageSourceCreateWithURL(imageURL as CFURL, imageSourceOptions)!
    let maxDimensionInPixels = max(pointSize.width, pointSize.height) * scale
    let downsampleOptions = [
        kCGImageSourceCreateThumbnailFromImageAlways: true,
        kCGImageSourceShouldCacheImmediately: true,
        kCGImageSourceCreateThumbnailWithTransform: true,
        kCGImageSourceThumbnailMaxPixelSize: maxDimensionInPixels
    ] as CFDictionary
    let downsampledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, downsampleOptions)!
    let thumbnail = UIImage(cgImage: downsampledImage)
    return thumbnail
}