Swift - scale background while retaining top anchor - ios

Question

How can I scale a UIImageView while retaining the top anchor?
I tried:
UIView.animate(withDuration: 0.3, animations: {
self.backGroundImage.contentMode = .scaleToFill
self.backGroundImage.transform = CGAffineTransform(scaleX: 3, y: 3)
self.backGroundImage.topAnchor.constraint(equalTo: self.view.topAnchor).isActive = true
})
That scales the image but the anchor point is the middle of the image. It seems like setting the topAnchor doesn't do anything. Is there a way to set some sort of "scaleAnchor" or some other solution?
In the end I would like to achieve this:
Update
This is how I constrain the view:
NSLayoutConstraint.activate([
// constrain background image
backGroundImage.topAnchor.constraint(equalTo: view.topAnchor),
backGroundImage.bottomAnchor.constraint(equalTo: view.bottomAnchor),
backGroundImage.leadingAnchor.constraint(equalTo: view.leadingAnchor),
backGroundImage.trailingAnchor.constraint(equalTo: view.trailingAnchor), )]

Answer 1

This works for me in Swift 5.1:
public extension UIView {
func applyTransform(withScale scale: CGFloat, anchorPoint: CGPoint) {
let xPadding = (scale - 1) * (anchorPoint.x - 0.5) * bounds.width
let yPadding = (scale - 1) * (anchorPoint.y - 0.5) * bounds.height
transform = CGAffineTransform(scaleX: scale, y: scale).translatedBy(x: xPadding, y: yPadding)
}
}
Example:
yourView.applyTransform(withScale: 3, anchorPoint: CGPoint(x: 0.5, y: 0))

Answer 2

The transform matrix is applied after the frame has been calculated from the constraints. If you want to scale things up consistently I suggest just setting a multiple on the height and width constraints instead. Alternatively you can move the anchor point by using CALayers.anchorPoint and then apply you transform so that it scales from the top middle instead of the center (which is the default).
EDIT -- example:
import PlaygroundSupport
class V: UIViewController {
let square = UIView()
override func viewDidLoad() {
super.viewDidLoad()
square.translatesAutoresizingMaskIntoConstraints = false
square.backgroundColor = .red
view.addSubview(square)
NSLayoutConstraint.activate([
square.centerXAnchor.constraint(equalTo: view.centerXAnchor),
square.centerYAnchor.constraint(equalTo: view.centerYAnchor),
square.widthAnchor.constraint(equalToConstant: 100),
square.heightAnchor.constraint(equalToConstant: 100),
])
square.layer.anchorPoint = .init(x: 0.5, y: 0)
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
UIView.animate(withDuration: 10) {
self.square.transform = .init(scaleX: 3, y: 3)
}
}
}
PlaygroundPage.current.liveView = V()