imageByScalingProportionallyToSize

ImageScaling.swift

import UIKit

extension UIImage {
    /**
     Returns a new UIImage instance by applying the new size. 
     
     - discussion: http://stackoverflow.com/a/2025413/1926015
     
     - parameter targetSize: the new size
     
     - returns: a UIImage with the new size
     */
    func imageByScalingProportionallyToSize(_ targetSize: CGSize) -> UIImage {
        let sourceImage = self
        var newImage: UIImage?
        let imageSize = sourceImage.size
        let width = imageSize.width
        let height = imageSize.height
        let targetWidth = targetSize.width
        let targetHeight = targetSize.height
        var scaleFactor = CGFloat(0)
        var scaledWidth = targetWidth
        var scaledHeight = targetHeight
        var thumbnailPoint = CGPoint(x: 0.0, y: 0.0)
        
        if !imageSize.equalTo(targetSize) {
            let widthFactor = targetWidth / width
            let heightFactor = targetHeight / height
            if widthFactor < heightFactor {
                scaleFactor = widthFactor
            }
            else {
                scaleFactor = heightFactor
            }
            scaledWidth = width * scaleFactor
            scaledHeight = height * scaleFactor
            
            // center the image
            if widthFactor < heightFactor {
                thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5
            }
            else if widthFactor > heightFactor {
                thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5
            }
        }
        
        // this is actually the interesting part:
        UIGraphicsBeginImageContextWithOptions(targetSize, false, 0)
        var thumbnailRect: CGRect = .zero
        thumbnailRect.origin = thumbnailPoint
        thumbnailRect.size.width = scaledWidth
        thumbnailRect.size.height = scaledHeight
        sourceImage.draw(in: thumbnailRect)
        newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        if newImage == nil {
            print("Could not scale image to size: \(targetSize.debugDescription)")
        }
        
        return newImage!
    }
}