Resize+crop image to a smaller size (using scale aspect fill logic), if image is indeed larger than required size. In Swift.

DownscaleImage.swift

func downscaleImage(image: UIImage, toPixelSize targetSize: CGSize) -> UIImage {
	return downscaleImage(image, toSize: targetSize, scale: 1.0)
}

func downscaleImage(image: UIImage, toSize targetSize: CGSize, scale: CGFloat = 0.0) -> UIImage {
	let actualScaleFactor = (scale == 0.0) ? UIScreen.mainScreen().scale : scale
	let size = image.size
	let imageScaleFactor = image.scale
	let imagePixelSize = CGSizeMake(size.width * imageScaleFactor, size.height * imageScaleFactor)
	
	let requiredMinPixelSize = CGSizeMake(targetSize.width * actualScaleFactor, targetSize.height * actualScaleFactor)
	let canBeDownscaled = (requiredMinPixelSize.width < imagePixelSize.width) && (requiredMinPixelSize.height < imagePixelSize.height)
	
	if !canBeDownscaled {
		return image
	}
	
	let widthRatio  = targetSize.width  / size.width
	let heightRatio = targetSize.height / size.height
	
	// This part is based on https://gist.github.com/hcatlin/180e81cd961573e3c54d
	// but it fixes the bug with wrong ratio used
	let downscaledImageSize: CGSize
	if (widthRatio > heightRatio) {
		downscaledImageSize = CGSizeMake(size.width * widthRatio,  size.height * widthRatio)
	} else {
		downscaledImageSize = CGSizeMake(size.width * heightRatio, size.height * heightRatio)
	}
	
	
	let imageRect: CGRect
	if CGSizeEqualToSize(downscaledImageSize, targetSize) {
		imageRect = CGRectMake(0, 0, targetSize.width, targetSize.height)
	} else {
		let xDiff = (downscaledImageSize.width > targetSize.width) ? 0.5 * (downscaledImageSize.width - targetSize.width) : 0.0
		let yDiff = (downscaledImageSize.height > targetSize.height) ? 0.5 * (downscaledImageSize.height - targetSize.height) : 0.0
		imageRect = CGRectMake(-xDiff, -yDiff, downscaledImageSize.width, downscaledImageSize.height)
	}
	
	UIGraphicsBeginImageContextWithOptions(targetSize, false, scale)
	image.drawInRect(imageRect)
	let newImage = UIGraphicsGetImageFromCurrentImageContext()
	UIGraphicsEndImageContext()
	
	return newImage
}