Programmatically implementing two different layouts using size classes

In the meantime I have found a good solution. Since this question has so many upvotes, I thought I would quickly describe it. I was inspired to this solution by a WWDC session.

I have moved on to Swift so please excuse that the code will be in swift – the concept is however the same for Obj-C.

You start out by declaring three constraint arrays:

 // Constraints
 private var compactConstraints: [NSLayoutConstraint] = []
 private var regularConstraints: [NSLayoutConstraint] = []
 private var sharedConstraints: [NSLayoutConstraint] = []

And then you fill the constraints accordingly. You can i.e. do this in a separate function that you call from viewDidLoad or you do it in viewDidLoad directly.

sharedConstraints.append(contentsOf: [
     btnStackView.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
    ...
])

compactConstraints.append(contentsOf: [
     btnStackView.widthAnchor.constraint(equalTo: self.view.widthAnchor, multiplier: 0.7),
    ...
])

regularConstraints.append(contentsOf: [
     btnStackView.widthAnchor.constraint(equalTo: self.view.widthAnchor, multiplier: 0.4),
    ...
])

The important part is switching between the size classes and activating/deactivating the appropriate constraints.

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {

    super.traitCollectionDidChange(previousTraitCollection)

    if (!sharedConstraints[0].isActive) {
       // activating shared constraints
       NSLayoutConstraint.activate(sharedConstraints)
    }


    if traitCollection.horizontalSizeClass == .compact && traitCollection.verticalSizeClass == .regular {
        if regularConstraints.count > 0 && regularConstraints[0].isActive {
            NSLayoutConstraint.deactivate(regularConstraints)
        }
        // activating compact constraints
        NSLayoutConstraint.activate(compactConstraints)
    } else {
        if compactConstraints.count > 0 && compactConstraints[0].isActive {
            NSLayoutConstraint.deactivate(compactConstraints)
        }
        // activating regular constraints
        NSLayoutConstraint.activate(regularConstraints)
    }
}

I know that the constraints don’t fit to the ones in the question. But the constraints themselves are irrelevant. The main thing is how one switches between two sets of constraints based on the size class.

Hope this helps.