How can I generate a barcode from a string in Swift?

You could use a CoreImage (import CoreImage) filter to do that!

    class Barcode {
        class func fromString(string : String) -> UIImage? {
             let data = string.data(using: .ascii)
             if let filter = CIFilter(name: "CICode128BarcodeGenerator") {
                  filter.setValue(data, forKey: "inputMessage")
                  if let outputCIImage = filter.outputImage {
                       return UIImage(ciImage: outputCIImage)
                  }
             }
             return nil
        }
    }

    let img = Barcode.fromString("whateva")

A newer version, with guard and failable initialiser:

extension UIImage {

    convenience init?(barcode: String) {
        let data = barcode.data(using: .ascii)
        guard let filter = CIFilter(name: "CICode128BarcodeGenerator") else {
            return nil
        }
        filter.setValue(data, forKey: "inputMessage")
        guard let ciImage = filter.outputImage else {
            return nil
        }
        self.init(ciImage: ciImage)
    }

}

Usage:

let barcode = UIImage(barcode: "some text") // yields UIImage?

According to the docs :

Generates an output image representing the input data according to the
ISO/IEC 15417:2007 standard. The width of each module (vertical line)
of the barcode in the output image is one pixel. The height of the
barcode is 32 pixels. To create a barcode from a string or URL,
convert it to an NSData object using the NSASCIIStringEncoding string
encoding.