How to draw a transparent stroke (or anyway clear part of an image) on the iPhone

Question

I have a small app that allows the user to draw on the screen with the finger. I have a UIImageView where the user draws, by creating a CGContextRef

Answer 1

If you want to erase Image with touch this is my code .... the unerase part not recommended it is working but slightly slow.

func addNewPathToImage(){
    print("Start erasing or not erasing")

    UIGraphicsBeginImageContextWithOptions(TopImageUIImageView.bounds.size, false, UIScreen.main.scale) // or 0 for the scale

//My image is aspect fit so I need to get rect

    let aspect = TopImageUIImageView.image!.size.width / TopImageUIImageView.image!.size.height
    let viewRatio = TopImageUIImageView.bounds.size.width / TopImageUIImageView.bounds.size.height


    if aspect < viewRatio {
        let scale = TopImageUIImageView.bounds.size.height / TopImageUIImageView.image!.size.height
        let width = scale * TopImageUIImageView.image!.size.width
        let topLeftX = (TopImageUIImageView.bounds.size.width - width) * 0.5
        rect = CGRect(x: topLeftX, y: 0, width: width, height: TopImageUIImageView.bounds.size.height)
    }
    else {
        let scale = TopImageUIImageView.bounds.size.width / TopImageUIImageView.image!.size.width
        let height = scale * TopImageUIImageView.image!.size.height
        let topLeftY = (TopImageUIImageView.bounds.size.height - height) * 0.5
        rect = CGRect(x: 0.0, y: topLeftY, width: TopImageUIImageView.bounds.size.width, height: height)
    }
    ////
    context = UIGraphicsGetCurrentContext()
    TopImageUIImageView.image?.draw(in: rect)
    context?.setLineCap(.round)
    context?.setLineWidth(brushSize)



    if isErasing == true {

        context?.setShadow(offset: CGSize(width: 0, height: 0), blur: blurNumber)
        context?.setStrokeColor(UIColor.white.cgColor)
        context?.setBlendMode(.clear)


    }else{

        print("test the unerase image .... ?")
        context?.setStrokeColor(UIColor.init(patternImage: topImage.af_imageAspectScaled(toFit: CGSize(width: TopImageUIImageView.bounds.size.width, height: TopImageUIImageView.bounds.size.height))).cgColor)

    }


   // I am using these because I am using touch to define what to erase 
        context?.move(to: CGPoint(x: lastTouch.x, y: lastTouch.y))
        context?.addLine(to: CGPoint(x: currentTouch.x, y: currentTouch.y))
        context?.strokePath()
        context?.closePath() // when add this line or the "context?.beginPath" get ERROR CGContextClosePath: no current point.
        print("close the path")




        //get your image 
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        TopImageUIImageView.image = image

}