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How to change a view’s anchor point without moving it

Paul Hudson       @twostraws

Every UIView has an anchor point, which is the point around which animations take place. Usually this is the center of the view – X:0.5 Y:0.5 – which means if you rotate a view it will spin around its center.

If you wanted the view to rotate around its top corner, as if someone had driven a nail into that point and you were spinning the view around that corner rather than the center, you can change the anchor point using the layer.anchorPoint property.

However, there’s a problem: changing the anchor point also changes the point where the view’s position is calculated, which means changing the anchor point also moves the view’s position.

So, if you want to change a view’s anchor point without moving it, here’s a little extension to do just that:

extension UIView {
    func setAnchorPoint(_ point: CGPoint) {
        var newPoint = CGPoint(x: bounds.size.width * point.x, y: bounds.size.height * point.y)
        var oldPoint = CGPoint(x: bounds.size.width * layer.anchorPoint.x, y: bounds.size.height * layer.anchorPoint.y);

        newPoint = newPoint.applying(transform)
        oldPoint = oldPoint.applying(transform)

        var position = layer.position

        position.x -= oldPoint.x
        position.x += newPoint.x

        position.y -= oldPoint.y
        position.y += newPoint.y

        layer.position = position
        layer.anchorPoint = point
    }
}

If you want to see that in action, here’s some code to create a blue UIView then animate it rotating around its top-left corner:

let box = UIView(frame: CGRect(x: 50, y: 50, width: 256, height: 256))
box.backgroundColor = .blue
view.addSubview(box)

box.setAnchorPoint(CGPoint(x: 0, y: 0))

UIView.animate(withDuration: 3) {
    box.transform = CGAffineTransform(rotationAngle: .pi)
}

Available from iOS 2.0

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