For me the simplest way was to use the .withHorizontallyFlippedOrientation() instance method of UIImage as follows:

let flippedImage = straightImage.withHorizontallyFlippedOrientation()

Simple one-liners always make me happy :)

In Swift.... (6.3.1)

YourUIImage.transform = CGAffineTransformMakeScale(-1, 1)

This also works with a UIView

Changing image orientation parameter is not actually flipping the image in all cases. Image must be redrawn somehow... For example like this:

Swift 3

func flipImageLeftRight(_ image: UIImage) -> UIImage? {

    UIGraphicsBeginImageContextWithOptions(image.size, false, image.scale)
    let context = UIGraphicsGetCurrentContext()!
    context.translateBy(x: image.size.width, y: image.size.height)
    context.scaleBy(x: -image.scale, y: -image.scale)

    context.draw(image.cgImage!, in: CGRect(origin:CGPoint.zero, size: image.size))

    let newImage = UIGraphicsGetImageFromCurrentImageContext()

    UIGraphicsEndImageContext()

    return newImage
}

Swift 5 Solution

Here is the simplest solution for actually flipping the image

extension UIImage {
    func flipHorizontally() -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(self.size, false, self.scale)
        let context = UIGraphicsGetCurrentContext()!
        
        context.translateBy(x: self.size.width/2, y: self.size.height/2)
        context.scaleBy(x: -1.0, y: 1.0)
        context.translateBy(x: -self.size.width/2, y: -self.size.height/2)
        
        self.draw(in: CGRect(x: 0, y: 0, width: self.size.width, height: self.size.height))
        
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return newImage
    }
}

and to use this solution you can do the following

let image = UIImage(named: "image.png")!
let newImage = image.flipHorizontally()

Swift 4

YOURIMAGEVIEW.transform = CGAffineTransform(scaleX: -1, y: 1)

This might also be handy:

YOURIMAGE.imageFlippedForRightToLeftLayoutDirection()

To flip image in swift 4

class ViewController: UIViewController {
var first = 1
var second = 1

@IBOutlet weak var img: UIImageView!
override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
}

@IBAction func flip1(_ sender: Any) {

    if first == 1 {
        img.transform = CGAffineTransform(scaleX: -1, y: 1)
        first = 2
    }
    else if first == 2
    {
        img.transform = CGAffineTransform(scaleX: +1, y: 1)
        first = 1
    }

}


}

Swift 5

If your use case requires saving the UIImage after transforming, it will need to be drawn into a new CGContext.

extension UIImage {
  
  enum Axis {
    case horizontal, vertical
  }
  
  func flipped(_ axis: Axis) -> UIImage {
    let renderer = UIGraphicsImageRenderer(size: size)
    
    return renderer.image {
      let context = $0.cgContext
      context.translateBy(x: size.width / 2, y: size.height / 2)
      
      switch axis {
      case .horizontal:
        context.scaleBy(x: -1, y: 1)
      case .vertical:
        context.scaleBy(x: 1, y: -1)
      }
      
      context.translateBy(x: -size.width / 2, y: -size.height / 2)
      
      draw(at: .zero)
    }
  }
}

There is couple of similar questions on SO. And I believe it's worth to post solution using CoreImage here too.

Please note: when getting final UIImage, it's necessary to convert to CGImage first to respect extent of CIImage

extension UIImage {
    func imageRotated(by degrees: CGFloat) -> UIImage {

        let orientation = CGImagePropertyOrientation(imageOrientation)
        // Create CIImage respecting image's orientation 
        guard let inputImage = CIImage(image: self)?.oriented(orientation) 
            else { return self }

        // Flip the image itself
        let flip = CGAffineTransform(scaleX: 1, y: -1)
        let outputImage = inputImage.transformed(by: flip)

        // Create CGImage first
        guard let cgImage = CIContext().createCGImage(outputImage, from: outputImage.extent) 
            else { return self }

        // Create output UIImage from CGImage
        return UIImage(cgImage: cgImage)
    }
}