Fast CPU-based drawing

FastCPU-basedDrawing.swift

class FreedrawingView: UIView {
    
    // we use a multi-dimensional array to store separate lines, otherwise 
    // all your lines will be connected
    var lines = [[CGPoint]]()
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        // create a new line
        lines.append([CGPoint]())
    }
    
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        guard let newTouchPoint = touches.first?.location(in: self) else { return }
        
        // use `indices` to safetly modify the last element
        if let lastIndex = lines.indices.last {
            lines[lastIndex].append(newTouchPoint)
        }
        
        setNeedsDisplay()
    }
    
    override func draw(_ rect: CGRect) {
        guard let context = UIGraphicsGetCurrentContext() else { return }
        
        context.setStrokeColor(UIColor.red.cgColor)
        context.setLineWidth(5)
        context.setLineCap(.round)
        
        lines.forEach { (line) in
            for (index, point) in line.enumerated() {
                if index == 0 {
                    context.move(to: point)
                } else {
                    context.addLine(to: point)
                }
            }
        }
        context.strokePath()
    }
}