var tempImageView : UIImageView!
var patternImage = UIImage(named: "xxx.png")!

var brushWidth : CGFloat = 50.0
var opacity : CGFloat = 1.0
var lastPoint = CGPoint.zero

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {

    super.touchesBegan(touches, with: event)

    if let touch = touches.first {
        lastPoint = touch.location(in: self)
    }
}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {

    super.touchesMoved(touches, with: event)

    if let touch = touches.first {
        let currentPoint = touch.location(in: self)
        drawLine(fromPoint: lastPoint, toPoint: currentPoint)

        lastPoint = currentPoint
    }
}

func drawLine(fromPoint:CGPoint, toPoint:CGPoint) {

    UIGraphicsBeginImageContext(self.frame.size)
    let context = UIGraphicsGetCurrentContext()
    tempImageView.image?.draw(in: self.bounds)

    context?.draw(patternImage.cgImage!, in: CGRect(x: fromPoint.x , y: fromPoint.y, width:brushWidth , height:brushWidth))
    tempImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    tempImageView.alpha = opacity
    UIGraphicsEndImageContext()
}