code:

    func drawLineFrom(fromPoint: CGPoint, toPoint: CGPoint) {
        
        // 1
        UIGraphicsBeginImageContext(view.frame.size)
        let context = UIGraphicsGetCurrentContext()
        tempImageView.image?.drawInRect(CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height))
        
        // 2
        CGContextMoveToPoint(context, fromPoint.x, fromPoint.y)
        CGContextAddLineToPoint(context, toPoint.x, toPoint.y)
        
        // 3
        CGContextSetLineCap(context, kCGLineCapRound)
        CGContextSetLineWidth(context, brushWidth)
        CGContextSetRGBStrokeColor(context, red, green, blue, 1.0)
        CGContextSetBlendMode(context, kCGBlendModeNormal)
        
        // 4
        CGContextStrokePath(context)
        
        // 5
        tempImageView.image = UIGraphicsGetImageFromCurrentImageContext()
        tempImageView.alpha = opacity
        UIGraphicsEndImageContext()
        
    }

    
    override func touchesBegan(touches: Set<NSObject>, withEvent event: UIEvent) {
        swiped = false
        if let touch = touches.first as? UITouch {
            lastPoint = touch.locationInView(self.view)
        }
    }
    
    
    override func touchesMoved(touches: Set<NSObject>, withEvent event: UIEvent) {
        // 6
        swiped = true
        if let touch = touches.first as? UITouch {
            let currentPoint = touch.locationInView(view)
            drawLineFrom(lastPoint, toPoint: currentPoint)
            
            // 7
            lastPoint = currentPoint
        }
    }
    
    
    override func touchesEnded(touches: Set<NSObject>, withEvent event: UIEvent) {
        
        if !swiped {
            // draw a single point
            drawLineFrom(lastPoint, toPoint: lastPoint)
        }
        
        // Merge tempImageView into mainImageView
        UIGraphicsBeginImageContext(mainImageView.frame.size)
        mainImageView.image?.drawInRect(CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: kCGBlendModeNormal, alpha: 1.0)
        tempImageView.image?.drawInRect(CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: kCGBlendModeNormal, alpha: opacity)
        mainImageView.image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        tempImageView.image = nil
    }