strange bug with drawing in UISplitViewController - swift

Question

I'm trying to make screen with view in which user can draw something. I created custom view with such code:
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
swiped = false
if let touch = touches.first {
lastPoint = touch.location(in: imageView)
}
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
swiped = true
if let touch = touches.first {
let currentPoint = touch.location(in: imageView)
drawLine(fromPoint: lastPoint, toPoint: currentPoint)
lastPoint = currentPoint
}
}
override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
if !swiped {
// draw a single point
drawLine(fromPoint: lastPoint, toPoint: lastPoint)
}
and drawing function
func drawLine(fromPoint: CGPoint, toPoint: CGPoint) {
UIGraphicsBeginImageContext(imageView.frame.size)
let context = UIGraphicsGetCurrentContext()
imageView.image?.draw(in: CGRect(x: 0, y: 0, width: imageView.frame.size.width, height: imageView.frame.size.height))
context?.move(to: fromPoint)
context?.addLine(to: toPoint)
context?.setLineCap(.round)
context?.setLineWidth(lineWidth)
context?.setStrokeColor(lineColor.cgColor)
context?.strokePath()
imageView.image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
}
when I show that view in view controller all is normal:
but when I show it in detail view in UISplitViewController, while user continue drawing, part of already drew image moving and fading out:
I can't find anything about what bug in web and have no idea what is produced such behaviour
Is anybody have any ideas about that?
That is example project where you can reproduce that bug:
https://github.com/fizzy871/DrawingBug
btw, in real project not only master view of split view controller, but navigation bar affect drawing too

Answer 1

Turned out that it behaves such way because of imageView frame with fractional part in size.
I just multiplied drawing context by 2 and problem solved:
func drawLine(fromPoint fromPoint: CGPoint, toPoint: CGPoint) {
// multiply to avoid problems when imageView frame value is XX.5
let fixedFrameForDrawing = CGRect(x: 0, y: 0, width: imageView.frame.size.width*2, height: imageView.frame.size.height*2)
let point1 = CGPoint(x: fromPoint.x*2, y: fromPoint.y*2)
let point2 = CGPoint(x: toPoint.x*2, y: toPoint.y*2)
UIGraphicsBeginImageContext(fixedFrameForDrawing.size)
if let context = UIGraphicsGetCurrentContext() {
imageView.image?.draw(in: fixedFrameForDrawing)
context.move(to: point1)
context.addLine(to: point2)
context.setLineCap(.round)
context.setLineWidth(lineWidth*2)
context.setStrokeColor(lineColor.cgColor)
context.strokePath()
let imageFromContext = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
imageView.image = imageFromContext
}