ConstraintStateMachineType.swift

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()
        }
    }
}