How to Crop UIImage considering screen scale - ios

Question

I'm trying to crop a UIImage using a crop box UIView that a user can drag around anywhere in the image view to crop. The logic I'm using to compute the crop rect is as follows:
extension UIImageView {
public func computeCropRect(for sourceFrame : CGRect) -> CGRect {
let widthScale = bounds.size.width / image!.size.width
let heightScale = bounds.size.height / image!.size.height
var x : CGFloat = 0
var y : CGFloat = 0
var width : CGFloat = 0
var height : CGFloat = 0
var offSet : CGFloat = 0
if widthScale < heightScale {
offSet = (bounds.size.height - (image!.size.height * widthScale))/2
x = sourceFrame.origin.x / widthScale
y = (sourceFrame.origin.y - offSet) / widthScale
width = sourceFrame.size.width / widthScale
height = sourceFrame.size.height / widthScale
} else {
offSet = (bounds.size.width - (image!.size.width * heightScale))/2
x = (sourceFrame.origin.x - offSet) / heightScale
y = sourceFrame.origin.y / heightScale
width = sourceFrame.size.width / heightScale
height = sourceFrame.size.height / heightScale
}
return CGRect(x: x, y: y, width: width, height: height)
}
}
The crop box frame looks like this and is positionable anywhere in the frame of the image view by dragging it:
This crop code works just fine until I combine this with another feature I'm trying to support which is the ability to let the user draw using their finger inside of the UIImageView. The code for that looks like this:
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
if let touchPoint = touches.first {
let currentPoint = touchPoint.location(in: self)
UIGraphicsBeginImageContextWithOptions(frame.size, false, UIScreen.main.scale)
if let context = UIGraphicsGetCurrentContext() {
image?.draw(in: imageEffectsService.computeAspectFitFrameFor(containerSize: frame.size, imageSize: image!.size), blendMode: .normal, alpha: CGFloat(imageOpacity))
drawLineAt(startPoint: lastTouchPoint, endPoint: currentPoint, currentContext: context, strokeColor: drawColor)
UIGraphicsEndImageContext()
}
}
}
private func drawLineAt(startPoint : CGPoint, endPoint : CGPoint, currentContext : CGContext, strokeColor : UIColor) {
currentContext.beginPath()
currentContext.setLineCap(CGLineCap.round)
currentContext.setLineWidth(brushSize)
currentContext.setStrokeColor(strokeColor.cgColor)
currentContext.move(to: startPoint)
currentContext.addLine(to: endPoint)
currentContext.strokePath()
image = UIGraphicsGetImageFromCurrentImageContext()
}
The crop method loses accuracy once I apply a drawing particularly because of this line:
UIGraphicsBeginImageContextWithOptions(frame.size, false, UIScreen.main.scale)
If instead I use:
UIGraphicsBeginImageContext(frame.size)
My crop code will be accurate but the drawing fidelity will look grainy and low quality because I am not accounting for retina screen devices. My question is how would I modify my crop function to account for the UIScreen.main.scale?