Change a CALayer's rotation transform to a value without changing the layer's scale transform

Question

Suppose you have a CALayer subclass as part of your view hierarchy, which has been created with a scale and rotation transform, where the angle and scale is only defined at runtime, i.e:

let layer = CALayer()
layer.frame = CGRect(x: 10, y: 10, width: 100, height: 100)

//where xx and yy are some precomputed values
layer.transform = CATransform3DConcat(CATransform3DMakeScale(xx, xx, 1), 
                                      CATransform3DMakeRotation(yy, 0, 0, 1)) 

let view = UIView() //your view hierarchy
view.addSublayer(layer)

Later in the program, I want to rotate this layer to a specific angle. How do I specify the transform so that

• A) The original scale applied to the layer is preserved
• B) The original rotation applied to the layer is discarded and reset to the new value?

let angle = CGFloat.pi/3
layer.transform = ???

Answer 1

You can reapply your transform again having xx value unchanged

    func rotateLayer(by angle: CGFloat) {
        layer.transform = CATransform3DConcat(CATransform3DMakeScale(xx, xx, 1),
        CATransform3DMakeRotation(angle, 0, 0, 1))
    }

Edit

Based on this answer you can get and reuse uniform scale factor like this

    func rotateLayer(by angle: CGFloat) {
        let scale = sqrt(pow(layer.transform.m11, 2) + pow(layer.transform.m12, 2))
        print(scale)
        layer.transform = CATransform3DConcat(CATransform3DMakeScale(scale, scale, 1), CATransform3DMakeRotation(angle, 0, 0, 1))
    }