scalecrop.swift

import UIKit

// Swift rewrite challenge
// Starting point: https://gist.github.com/jkereako/200342b66b5416fd715a#file-scale-and-crop-image-swift

func scaleAndCropImage(
    image: UIImage,
    toSize size: CGSize,
    fitImage: Bool = true
    ) -> UIImage {
    
    // Return original when cropping is not needed
    guard !CGSizeEqualToSize(image.size, size) else { return image }
    
    // Calculate scale factor for fit or fill
    let (widthFactor, heightFactor) = (size.width / image.size.width, size.height / image.size.height)
    let fitFillTest = fitImage ? widthFactor < heightFactor : widthFactor > heightFactor
    let scaleFactor = fitFillTest ? widthFactor : heightFactor
    
    // Establish drawing destination, which may start outside the drawing context bounds
    let (scaledWidth, scaledHeight) = (image.size.width * scaleFactor, image.size.height * scaleFactor)
    let drawingOrigin = CGPoint(
        x: (size.width - scaledWidth) / 2.0,
        y: (size.height - scaledHeight) / 2.0)
    
    // Perform drawing and return image
    UIGraphicsBeginImageContextWithOptions(size, false, 0.0)
    let scaledImage: UIImage
    do {
        // Fill background
        UIColor.blackColor().setFill(); UIRectFill(CGRect(origin: .zero, size: size))
        
        // Draw scaled image
        let drawingRect: CGRect = CGRect(
            origin: drawingOrigin,
            size: CGSize(width: scaledWidth, height: scaledHeight))
        image.drawInRect(drawingRect)
        
        // Fetch image
        scaledImage = UIGraphicsGetImageFromCurrentImageContext()!
    }
    UIGraphicsEndImageContext()
    return scaledImage
}

// Test with some basic placeholder data
guard let url = NSURL(string: "http://placehold.it/300x150") else { fatalError("Bad URL") }
guard let data = NSData(contentsOfURL: url) else { fatalError("Bad data") }
guard let img = UIImage(data: data) else { fatalError("Bad data") }

let outImageFit = scaleAndCropImage(img, toSize: CGSize(width: 200, height: 200))
let outImageFill = scaleAndCropImage(img, toSize: CGSize(width: 200, height: 200), fitImage: false)

Useful method for performing drawing in the resulting context
Using calculation of x, y in both branches instead of checking for if widthFactor > heightFactor

import UIKit

extension UIImage {

    /**
     Creates image of specified size and perfomrs drawing commands

     - parameter size:    result image size
     - parameter drawing: closure that contains deawing operations

     - returns: image with the drawing operation
     */
    func drawingWithSize(size:CGSize, @noescape drawing:()->()) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(size, false, 0.0)
        drawing()
        let image = UIGraphicsGetImageFromCurrentImageContext()!
        UIGraphicsEndImageContext()
        return image
    }

    /**
     Scales image to specified size, savin aspect ratio
     Crops parts of image that out of provided size 
     Works as UICntentModeAspectFill

     - parameter size: result image size

     - returns: scaled image
     */
    func scaleAndCrop(toSize size:CGSize) -> UIImage {

        // Skip unneeded scaling
        guard CGSizeEqualToSize(self.size, size) == false else {
            return self
        }

        let scaleFactor = max(size.width / self.size.width,
                              size.height / self.size.height)

        let (scaledWidth, scaledHeight) = (self.size.width * scaleFactor,
                                           self.size.height * scaleFactor)

        let drawingRect = CGRectMake(
            (size.width - scaledWidth) / 2.0,
            (size.height - scaledHeight) / 2.0,
            scaledWidth,
            scaledHeight)

        let scaledImage = drawingWithSize(size) {
            self.drawInRect(drawingRect)
        }
        return scaledImage
    }
}

@PaulTaykalo

In my real world code, I use code that passes a context. You can always get context from a valid drawing session and UIKit supports a context stack, so you can push the context, perform drawing, and then pull an image to return and pop the context.