ScratchCard-DrawRect.swift

override func draw(_ rect: CGRect) {
    super.draw(rect)
}

override func draw(_ layer: CALayer, in context: CGContext) {
    super.draw(layer, in: context)
    
    // 1. Let the subviews to render into the context 
    foregroundView?.layer.render(in: context)

    // 2. Iterate through line collection and add lines into context
    for line in lineCollection {
        for (index, point) in line.enumerated() {
            if index == 0 {
                context.move(to: point)
            } else {
                context.addLine(to: point)
            }
        }
    }
  
    // 3. Specify the blend mode. This is the key for the scratch card UI. Otherwise, it will just draw path on top.
    context.setBlendMode(.clear)
    context.setLineWidth(lineWidth)
    context.setLineCap(.round)
    context.setLineJoin(.round)
    
    // 4. Stroke path. This makes the rendering to happen
    context.strokePath()
}