Adding a mask to CAGradientLayer makes UIBezierPath disappear - ios

Question

I want to add an inner border to a view with a gradient. The following code works and gives me this result
import UIKit
class InnerGradientBorderView: UIView {
override init(frame: CGRect) {
super.init(frame: frame)
backgroundColor = UIColor.clear
}
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
backgroundColor = UIColor.clear
}
override func draw(_ rect: CGRect) {
super.draw(rect)
addGradientInnerBorder(width: 8, color: FlatWhite())
}
func addGradientInnerBorder(width: CGFloat, color: UIColor) {
// Setup
let topLeftO = CGPoint(x: 0, y: 0)
let topLeftI = CGPoint(x: width, y: width)
let topRightO = CGPoint(x: frame.width, y: 0)
let topRightI = CGPoint(x: frame.width - width, y: width)
let bottomLeftO = CGPoint(x: 0, y: frame.height)
let bottomLeftI = CGPoint(x: width, y: frame.height - width)
let bottomRightO = CGPoint(x: frame.width, y: frame.height)
let bottomRightI = CGPoint(x: frame.width - width, y: frame.height - width)
// Top
let topPoints = [topLeftO, topLeftI, topRightI, topRightO, topLeftO]
let topGradientPoints = [CGPoint(x: 0, y: 0), CGPoint(x: 0, y: 1)]
addGradientToBeizerPath(path: addClosedPathForPoints(points: topPoints), color: color, gradientPoints: topGradientPoints)
// Left
let leftPoints = [topLeftO, topLeftI, bottomLeftI, bottomLeftO, topLeftO]
let leftGradientPoints = [CGPoint(x: 0, y: 0), CGPoint(x: 1, y: 0)]
addGradientToBeizerPath(path: addClosedPathForPoints(points: leftPoints), color: color, gradientPoints: leftGradientPoints)
// Right
let rightPoints = [topRightO, topRightI, bottomRightI, bottomRightO, topRightO]
let rightGradientPoints = [CGPoint(x: 1, y: 0), CGPoint(x: 0, y: 0)]
addGradientToBeizerPath(path: addClosedPathForPoints(points: rightPoints), color: color, gradientPoints: rightGradientPoints)
// Bottom
let bottomPoints = [bottomLeftO, bottomLeftI, bottomRightI, bottomRightO, bottomLeftO]
let bottomGradientPoints = [CGPoint(x: 0, y: 1), CGPoint(x: 0, y: 0)]
addGradientToBeizerPath(path: addClosedPathForPoints(points: bottomPoints), color: color, gradientPoints: bottomGradientPoints)
}
func addClosedPathForPoints(points: [CGPoint]) -> UIBezierPath? {
guard points.count == 5 else { return nil }
let path = UIBezierPath()
path.move(to: points[0])
path.addLine(to: points[1])
path.addLine(to: points[2])
path.addLine(to: points[3])
path.addLine(to: points[4])
path.close()
return path
}
func addGradientToBeizerPath(path: UIBezierPath?, color: UIColor, gradientPoints: [CGPoint]) {
guard let path = path, gradientPoints.count == 2 else { return }
let gradient = CAGradientLayer()
gradient.frame = path.bounds
gradient.colors = [color.cgColor, UIColor.clear.cgColor]
gradient.startPoint = gradientPoints[0]
gradient.endPoint = gradientPoints[1]
// let shapeMask = CAShapeLayer()
// shapeMask.path = path.cgPath
// gradient.mask = shapeMask
self.layer.insertSublayer(gradient, at: 0)
}
}
You will notice that the edges do not look that great.To fix that, I am giving the edges an angle. When I apply a mask to this gradient with this angle, the right and bottom paths disappear like this:
All I am doing here is using some closed bezierPaths and applying a gradient to them. If the gradient has a mask (the commented code is uncommented), two of the paths disappear. I have a feeling that I am not understanding something so hopefully someone here can tell me how to use CAShapeLayer properly.

Answer 1

This comment to CALayer mask property
explains it perfectly:
The mask layer lives in the masked layer's coordinate system just as if it were a sublayer.
In your case, the origin of the right and bottom gradient layer is not
at (0, 0) of the enclosing view, but at (frame.width - width, 0)
and (frame.height - width, 0) respectively.
On the other hand, the coordinates of the points in
oshapeMask.path are relative to (0, 0) of the enclosing view.
A possible simple fix is to transform the coordinate system of
the shape layer so that it uses the same coordinates as the points
of the given path:
let gradient = CAGradientLayer()
gradient.frame = path.bounds
gradient.bounds = path.bounds // <<--- ADDED HERE!
gradient.colors = [color.cgColor, UIColor.clear.cgColor]
gradient.startPoint = gradientPoints[0]
gradient.endPoint = gradientPoints[1]
let shapeMask = CAShapeLayer()
shapeMask.path = path.cgPath
gradient.mask = shapeMask
self.layer.addSublayer(gradient)