ConstraintStateMachine.swift

import UIKit

public protocol ConstraintStateMachineType: class {
    // MARK: Types
    typealias State: RawRepresentable
    
    // MARK: Properties
    var state: State? { get set }
    var allConstraints: [NSLayoutConstraint] { get }
    
    // MARK: Mutators
    func setNeedsUpdateConstraints()
}

public extension ConstraintStateMachineType where State.RawValue == String {
    // MARK: Properties
    var state: State? {
        get {
            let constraints = self.allConstraints.flatMap { $0 as? StatefulConstraint }
            let rawValue = constraints.filter { $0.priority == UILayoutPriorityDefaultHigh }
                .first?.state ?? ""
            return State(rawValue: rawValue)
        }
        
        set {
            guard let state = newValue else { return }

            let constraints = self.allConstraints.flatMap { $0 as? StatefulConstraint }
            constraints.filter { $0.state != state.rawValue }
                .forEach { $0.priority = UILayoutPriorityDefaultLow }
            constraints.filter { $0.state == state.rawValue }
                .forEach { $0.priority = UILayoutPriorityDefaultHigh }
            
            self.setNeedsUpdateConstraints()
        }
    }
}

public extension ConstraintStateMachineType where Self: UIViewController {
    // MARK: Properties
    var allConstraints: [NSLayoutConstraint] {
        return self.view.flatConstraints // helper function, recursively collects constraints
        // https://gist.github.com/pcperini/5a8ea37e8cbbe904a031
    }
    
    // MARK: Mutators
    func setNeedsUpdateConstraints() {
        self.view.setNeedsUpdateConstraints()
    }
}

@IBDesignable
public class StatefulConstraint: NSLayoutConstraint {
    // MARK: Properties
    @IBInspectable var state: String = ""
}

Helpers.swift

 public extension CollectionType {
    // MARK: Flatteners
    public func flatten(path: (Self.Generator.Element) -> [Self.Generator.Element]) -> [Self.Generator.Element] {
        guard !self.isEmpty else { return [] }
        return self + self.flatMap { path($0).flatten(path) }
    }
}

public extension UIView {
    // MARK: Properties
    public var flatSubviews: [UIView] {
        return self.subviews.flatten { $0.subviews }
    }
    
    public var flatConstraints: [NSLayoutConstraint] {
        return (self.flatSubviews + [self]).flatMap { $0.constraints }
    }
}

ViewController.swift

class ViewController: UIViewController, ConstraintStateMachineType {
    typealias State = Alignment
    enum Alignment: String {
        case Left = "Left"
        case Right = "Right"
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(UInt64(2) * NSEC_PER_SEC)), dispatch_get_main_queue()) { () -> Void in
            self.state = .Right
        }
    }
}