SimpleDrawing.swift

import UIKit

class ViewController: UIViewController {
    
    var bezierPath:UIBezierPath!
    
    var canvas:UIImageView!
    
    var lastDrawImage:UIImage?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let width = self.view.frame.width
        let height = self.view.frame.height
        
        //キャンバスを設置
        canvas = UIImageView()
        canvas.frame = CGRect(x:0,y:0,width:width,height:height)
        canvas.backgroundColor = UIColor.clear
        self.view.addSubview(canvas)
    }
    
    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
        let touchEvent = touches.first!
        let currentPoint:CGPoint = touchEvent.location(in: self.canvas)
        bezierPath = UIBezierPath()
        bezierPath.lineWidth = 4.0
        bezierPath.lineCapStyle = .butt
        bezierPath.move(to:currentPoint)
    }
    
    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
        if bezierPath == nil {
            return
        }
        let touchEvent = touches.first!
        let currentPoint:CGPoint = touchEvent.location(in: self.canvas)
        bezierPath.addLine(to: currentPoint)
        drawLine(path: bezierPath)
    }
    
    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
        if bezierPath == nil {
            return
        }
        let touchEvent = touches.first!
        let currentPoint:CGPoint = touchEvent.location(in: canvas)
        bezierPath.addLine(to: currentPoint)
        drawLine(path: bezierPath)
        self.lastDrawImage = canvas.image
    }

    //描画処理
    func drawLine(path:UIBezierPath){
        UIGraphicsBeginImageContext(canvas.frame.size)
        if let image = self.lastDrawImage {
            image.draw(at: CGPoint.zero)
        }
        let lineColor = UIColor.black
        lineColor.setStroke()
        path.stroke()
        self.canvas.image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
    }
}