Keep autolayout constraints active status on device rotation

Question

I noticed that when I update my autolayout constraints programmatically, all changes are reverted when I rotate the screen.

Reproduce the issue:

• Basic S

Answer 1

Size Classes

Installed refers to Size Classes installation, not to active/inactive.

You must create another constraint programmatically, and activate/deactivate that one. This is because you cannot change the multiplier of a constraint (Can i change multiplier property for NSLayoutConstraint?), nor can you tinker with Size Classes (activateConstraints: and deactivateConstraints: not persisting after rotation for constraints created in IB).

There are a few ways to do so. In the example below, I create a copy of your x1 constraint, with a multiplier or 1/2. I then toggle between the two:

@IBOutlet var fullWidthConstraint: NSLayoutConstraint!
var halfWidthConstraint: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    halfWidthConstraint = NSLayoutConstraint(item: fullWidthConstraint.firstItem,
        attribute: fullWidthConstraint.firstAttribute,
        relatedBy: fullWidthConstraint.relation,
        toItem: fullWidthConstraint.secondItem,
        attribute: fullWidthConstraint.secondAttribute,
        multiplier: 0.5,
        constant: fullWidthConstraint.constant)
    halfWidthConstraint.priority = fullWidthConstraint.priority
}

@IBAction func changeConstraintAction(sender: UISwitch) {
    if sender.on {
        NSLayoutConstraint.deactivateConstraints([fullWidthConstraint])
        NSLayoutConstraint.activateConstraints([halfWidthConstraint])
    } else {
        NSLayoutConstraint.deactivateConstraints([halfWidthConstraint])
        NSLayoutConstraint.activateConstraints([fullWidthConstraint])
    }
}

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Tested on iOS 9+, Xcode 7+.