UIView with auto layout constraints "jumps" before transform animation

Question

I have a UIView constrained with Auto Layout. It's centered and has a constraint for Width and Height. When it appears, I'm applying a rotation transform to it.

When I hit that animation button, I want it to animate to a point higher on the screen, while rotating back to an "upright" position (ie with no rotation applied). So, I set a new translation transform:

let translation = CGAffineTransform(translationX: 1, y: -100)
UIView.animate(withDuration: 0.5, animations: {
   self.blueView.transform = translation
})

What I expect to see is the view rotate back to an upright position while translating upwards.

What I get instead is the view "jumping" to a point off to the right, and then animating upwards while rotating.

How can I fix this so that it doesn't "jump" before animating?

Answer 1

You are seeing the jump because the blueView already has the rotation transform set when you animate the translation transform. That leads to unexpected results.

To make it work you combine the rotation and translation transform before the animation and then reset the transform animated:

To translate the blue view 100pt up and rotate it back to normal you do this:

1. Add a transform to the blueView that translates it 100pt down and rotates it 45°
2. Set the centerYAnchor constant to -100 to have the blueView in the correct position before the animation.
3. Animate the blueView.transform = .identity to remove the transform animated

This is a working example:

class ViewController: UIViewController {

    let blueView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()

        blueView.backgroundColor = .blue
        view.addSubview(blueView)

        blueView.transform = CGAffineTransform(translationX: 0, y: 100).rotated(by: -CGFloat.pi / 4)

        let button = UIButton(type: .custom)
        button.setTitle("Animate", for: .normal)
        button.setTitleColor(.blue, for: .normal)
        button.addTarget(self, action: #selector(didPress), for: .touchUpInside)
        view.addSubview(button)

        blueView.translatesAutoresizingMaskIntoConstraints = false
        button.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            blueView.widthAnchor.constraint(equalToConstant: 20),
            blueView.heightAnchor.constraint(equalToConstant: 20),
            blueView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            blueView.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: -100),

            button.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            button.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -40)
            ])
    }

    @objc func didPress(sender: UIButton) {
        UIView.animate(withDuration: 0.5, animations: {
            self.blueView.transform = .identity
        })
    }
}