flattenImage.swift

var flattenedImage: UIImage?
var line = [CGPoint]() {
    didSet {
        checkIfTooManyPointsIn(&line)
    }
}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    super.touchesBegan(touches, with: event)
    flattenImage()
}

// called when adding new points
func checkIfTooManyPointsIn(_ line: inout [CGPoint]) {
    let maxPoints = 200
    if line.count > maxPoints {
        flattenedImage = self.getImageRepresentation()

        // we leave one point to ensure no gaps in drawing
        _ = line.removeFirst(maxPoints - 1)
    }
}

// called from touches ended
func flattenImage() {
    flattenedImage = self.getImageRepresentation()
    line.removeAll()
}

// convert view to bitmap
func getImageRepresentation() -> UIImage? {
    UIGraphicsBeginImageContextWithOptions(self.bounds.size, self.isOpaque, 0.0)
    defer { UIGraphicsEndImageContext() }
    if let context = UIGraphicsGetCurrentContext() {
        self.layer.render(in: context)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        return image
    }
    return nil
}