如何在CALayer上居中CGImage？

Question

I was trying to set a UIImage 's CGImage as a layer 's content and then add the layer to a view 's layer .
It's should be five stars at the center of the yellow view. That's what I want it to be.
But it seems the center of the stars is aligned with the origin of the view . What should I do to rectify it?

func putOnStars() {
    let rect = UIView(frame: CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: UIScreen.main.bounds.height))
    rect.backgroundColor = .yellow
    view.addSubview(rect)

    let baseLayer = CALayer()
    baseLayer.contents = UIImage(named: "stars")?.cgImage
    baseLayer.contentsGravity = kCAGravityCenter
    rect.layer.addSublayer(baseLayer)

}

Here is the stars image for you in case of you want to test.

Answer 1

baseLayer doesn't have a defined frame so baseLayer.contentsGravity = kCAGravityCenter will work fine but it'll still be on the potision (0, 0).

There are two possible solutions:

1 : Make a frame of baseLayer that is identical to rect . Implement this code:

baseLayer.frame = rect.frame

2 : Set the position of baseLayer to the center of rect . Implement this code:

baseLayer.position = rect.center

Answer 2

To place the stars image in the centre of the CALayer , give the frame of the layer, ie

    let baseLayer = CALayer()
    baseLayer.frame = rect.bounds //This Line
    baseLayer.contents = UIImage(named: "stars")?.cgImage
    baseLayer.contentsGravity = kCAGravityCenter
    rect.layer.addSublayer(baseLayer)

For any kind of CALayer , you need to define its frame explicitly.