Semi-transparent Emoji in NSAttributedString Workaround

TransparentEmojiAttributedString.swift

import UIKit

extension NSAttributedString {
  var image: UIImage {
    let options: NSStringDrawingOptions = [ .usesLineFragmentOrigin, .usesFontLeading, .usesDeviceMetrics ]
    let rect = boundingRect(with: self.size(), options: options, context: nil)
    return UIGraphicsImageRenderer(bounds: rect).image { _ in
      draw(at: .zero)
    }
  }
}

extension UIImage {
  func with(alpha: CGFloat = 1.0) -> UIImage {
    let format = UIGraphicsImageRendererFormat()
    format.opaque = false
    return UIGraphicsImageRenderer(size: self.size, format: format).image() { (context) in
      draw(in: context.format.bounds, blendMode: .normal, alpha: alpha)
    }
  }
}

func renderStars(selectedStars: String, notYetSelectedStars: String) -> UIImage {
  let selectedAttributes: [NSAttributedStringKey : Any] = [
    .foregroundColor : UIColor.blue.withAlphaComponent(1.0),
    .font : UIFont.systemFont(ofSize: 50)
  ]
  
  let selectedImage = NSAttributedString(string: selectedStars, attributes: selectedAttributes).image
  let selectedAttachment = NSTextAttachment()
  selectedAttachment.image = selectedImage
  
  let notYetSelectedImage = NSAttributedString(string: notYetSelectedStars, attributes: selectedAttributes).image.with(alpha: 0.4)
  let notYetSelectedAttachment = NSTextAttachment()
  notYetSelectedAttachment.image = notYetSelectedImage
  
  let string = NSMutableAttributedString()
  string.append(NSAttributedString(attachment: selectedAttachment))
  string.append(NSAttributedString(attachment: notYetSelectedAttachment))
  
  let image = UIGraphicsImageRenderer(size: string.size()).image() { context in
    string.draw(at: .zero)
  }
  return image
}

renderStars(selectedStars: "✪✪", notYetSelectedStars: "✪✪✪")

renderStars(selectedStars: "😖😖😖", notYetSelectedStars: "😖😖")