Adding the same layer in a subview changes its visual position - ios

Question

I added a subview (with a black border) in a view and centered it.
Then I generate 2 identical triangles with CAShapeLayer and add one to the subview and the other to the main view.
Here is the visual result in Playground where we can see that the green triangle is totally off and should have been centered.
And here is the code:
let view = UIView()
let borderedView = UIView()
var containedFrame = CGRect(x: 0, y: 0, width: 100, height: 100)
func setupUI() {
view.frame = CGRect(x: 0, y: 0, width: 300, height: 600)
view.backgroundColor = .white
borderedView.frame = containedFrame
borderedView.center = view.center
borderedView.backgroundColor = .clear
borderedView.layer.borderColor = UIColor.black.cgColor
borderedView.layer.borderWidth = 1
view.addSubview(borderedView)
setupTriangles()
}
private func setupTriangles() {
view.layer.addSublayer(createTriangle(color: .red)) // RED triangle
borderedView.layer.addSublayer(createTriangle(color: .green)) // GREEN triangle
}
private func createTriangle(color: UIColor) -> CAShapeLayer {
let layer = CAShapeLayer()
let bezierPath = UIBezierPath()
bezierPath.move(to: CGPoint(x: 0, y: 0))
bezierPath.addLine(to: CGPoint(x: -containedFrame.width, y: 0))
bezierPath.addLine(to: CGPoint(x: 0, y: -containedFrame.height))
bezierPath.close()
layer.position = borderedView.center
layer.path = bezierPath.cgPath
layer.fillColor = color.cgColor
return layer
}
Note: All position (of view, the borderedView and both triangles) are the same (150.0, 300.0)
Question: Why is the green layer not in the right position?

Answer 1

#DuncanC is right that each view has its own coordinate system. Your problem is this line:
layer.position = borderedView.center
That sets the layer's position to the center of the frame for the borderedView which is in the coordinate system of view. When you create the green triangle, it needs to use the coordinate system of borderedView.
You can fix this by passing the view to your createTriangle function, and then use the center of the bounds of that view as the layer position:
private func setupTriangles() {
view.layer.addSublayer(createTriangle(color: .red, for: view)) // RED triangle
borderedView.layer.addSublayer(createTriangle(color: .green, for: borderedView)) // GREEN triangle
}
private func createTriangle(color: UIColor, for view: UIView) -> CAShapeLayer {
let layer = CAShapeLayer()
let bezierPath = UIBezierPath()
bezierPath.move(to: CGPoint(x: 0, y: 0))
bezierPath.addLine(to: CGPoint(x: -containedFrame.width, y: 0))
bezierPath.addLine(to: CGPoint(x: 0, y: -containedFrame.height))
bezierPath.close()
layer.position = CGPoint(x: view.bounds.midX, y: view.bounds.midY)
layer.path = bezierPath.cgPath
layer.fillColor = color.cgColor
return layer
}
Note: When you do this, the green triangle appears directly below the red one, so it isn't visible.

Answer 2

Every view/layer uses the coordinate system of it's superview/superlayer. If you add a layer to self.view.layer, it will be positioned in self.view.layer's coordinate system. If you add a layer to borderedView.layer, it will be in borderedView.layer's coordinate system.
Think of the view/layer hierarchy as stacks of pieces of graph paper. You place a new piece of paper on the current piece (the superview/layer) in the current piece's coordinates system, but then if you draw on the new view/layer, or add new views/layer inside that one, you use the new view/layer's coordinate system.