UIImage Calculate On-Screen Size (UIImageView: AspectFit)

UIImage+CalculateOnScreenSize.swift

extension UIImage {
     /**
     Calculates the resulting size (rendered on screen) of the receiver when it
     is displayed within a `UIImageView` instance of bounds size `containerSize`
     that has its `contentMode` property set to `.ScaleAspectFit`.
     
     - parameter contanerSize: The size of the hypothetical UIImageView instance
     that is to display the receiver on screen.
     - returns: the effective size at which the receiver would be rendered on 
     screen if displayed in said view.
     */
    func scaleAspectFitSizeWhenEmbeddedInSize(containerSize: CGSize) -> CGSize {
        let imageSize = self.size
        
        let imageRatio = imageSize.width / imageSize.height
        let containerRatio = containerSize.width / containerSize.height
        
        if imageRatio < containerRatio {
            let scale = containerSize.height / imageSize.height
            let width = scale * imageSize.width 
            return CGSizeMake(width, containerSize.height)

        } else{
            let scale = containerSize.width / imageSize.width
            let height = scale * imageSize.height
            return CGSizeMake(containerSize.width, height)
        }
    }
}