ClipToBounds没有边框颜色问题-Xamarin.iOS，C＃

Question

Program:

• I have a UIImageView. The image view has an image, a border and a
  corner radius (rounded).

• The image view property "ClipsToBounds" is set to true so the image matches the rounded corners.

Code:

UIImageView imageView = new UIImageView(); 
imageView.Layer.BorderWidth = 5f; //set border thickness
imageView.Layer.CornerRadius = 25; //make corner's rounded
imageView.Layer.BorderColor = UIColor.Red.CGColor; //set border color red
imageView.Image = new UIImage(imgPath); //set img
imageView.ClipsToBounds = true; //required - so image is rounded

Problem:

If I change the border colour to white (transparent) you can see small black lines at every corner. How can I get rid of these?

Answer 1

I looked at the core animation layers that being rendered via Revel on the border radius and those are rendering artifacts on the composite (I assume those are rounding errors in the mini-map/anti-aliasing in the radius calculations of the original image color, but that is just a wild guess...)

So if you create a CAShapeLayer with the same radius you are using and apply it as the layer's mask, the artifact becomes smaller. (The Core Animation bezier path is more accurate but not perfect?)

If you create a CAShapeLayer that is one pixel larger on the corner's radius as a "hack", the artifacts are totally masked:

CAShapeLayer/Mask Fix-up Example:

UIImageView imageView = new UIImageView
{
    Image = UIImage.FromBundle("foo.jpg"),
    ClipsToBounds = true,
    Bounds = View.Bounds
};
imageView.Layer.BorderWidth = 5f; //set border thickness
imageView.Layer.CornerRadius = 25; //make corner's rounded
imageView.Layer.BorderColor = UIColor.White.CGColor; //set border color red
var maskingShapeLayer = new CAShapeLayer()
{
    Path = UIBezierPath.FromRoundedRect(imageView.Bounds, UIRectCorner.AllCorners, new CGSize(26, 26)).CGPath
};
imageView.Layer.Mask = maskingShapeLayer;

Note: Maybe someone that is a "hard-core" Core Animation expert can explain way the rendering artifacts exist on these border curves in the first place as I still use CAShapeLayers to do my corner masking which do not have the rendering artifacts even though in iOS 11+ you can just set your UIView's Layer.CornerRadius like the question does.