AdaptiveConstraintGroup.swift

//
//  AdapativeConstraintGroup.swift
//  CartographyDemo
//
//  Created by Felix Jendrusch on 2/3/15.
//  Copyright (c) 2015 Felix Jendrusch. All rights reserved.
//

import Cartography

public class AdaptiveConstraintGroup: ConstraintGroup {
    public var traitCollection: UITraitCollection

    public init(traitCollection: UITraitCollection) {
        self.traitCollection = traitCollection
    }

    public class func activateConstraintsInGroups(groups: [AdaptiveConstraintGroup], ifTraitsContainedInCollection traitCollection: UITraitCollection) {
        // Deactivate all unwated constraints before activating any new ones to
        // avoid possible conflicts.
        for group in groups {
            if !traitCollection.containsTraitsInCollection(group.traitCollection) {
                group.active = false
            }
        }

        for group in groups {
            if traitCollection.containsTraitsInCollection(group.traitCollection) {
                group.active = true
            }
        }
    }

    public func activateConstraintsIfTraitsContainedInCollection(traitCollection: UITraitCollection) {
        self.active = traitCollection.containsTraitsInCollection(self.traitCollection)
    }
}

public func adapt(view: UIView, traitCollection: UITraitCollection, block: LayoutProxy -> ()) -> AdaptiveConstraintGroup {
    let group = AdaptiveConstraintGroup(traitCollection: traitCollection)
    constrain(view, replace: group, block)
    // Deactivate new constraints to avoid possible conflicts with existing ones.
    group.active = false
    return group
}

ViewController.swift

//
//  ViewController.swift
//  CartographyDemo
//
//  Created by Felix Jendrusch on 2/3/15.
//  Copyright (c) 2015 Felix Jendrusch. All rights reserved.
//

import UIKit
import Cartography

class ViewController: UIViewController {
    var box: UIView!

    var groups: [AdaptiveConstraintGroup] = []

    override func viewDidLoad() {
        super.viewDidLoad()
        
        box = UIView(frame: CGRectMake(0, 0, 100, 100))
        box.backgroundColor = UIColor.redColor()

        view.addSubview(box)
    }

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)

        layout(box) { box in
            box.width == 100
            box.height == 100
        }

        groups.append(adapt(box, UITraitCollection(verticalSizeClass: .Compact)) { box in
            box.top == box.superview!.top
            box.centerX == box.superview!.centerX
        })

        groups.append(adapt(box, UITraitCollection(verticalSizeClass: .Regular)) { box in
            box.center == box.superview!.center; return
        })

        AdaptiveConstraintGroup.activateConstraintsInGroups(groups, ifTraitsContainedInCollection: traitCollection)
    }

    override func willTransitionToTraitCollection(newCollection: UITraitCollection, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
        super.willTransitionToTraitCollection(newCollection, withTransitionCoordinator: coordinator)

        coordinator.animateAlongsideTransition({ context in
            AdaptiveConstraintGroup.activateConstraintsInGroups(self.groups, ifTraitsContainedInCollection: self.traitCollection)
        }, completion: nil)
    }
}