Checks the constraints of this view's superview, as well as all of it's subviews, for the constraint with the identifier passed.

UIView+Constraint.swift

import UIKit

extension UIView {

    /// Checks the constraints of this view's superview, as well as all of it's subviews, for the constraint with the identifier passed.
    func constraint(withIdentifier identifier: String) -> NSLayoutConstraint? {
        if let constraint = superview?.constraintFiltered(identifier) {
            return constraint
        } else if let constraint = constraintFiltered(identifier) {
            return constraint
        } else {
            return checkSubviews(subviews, forIdentifier: identifier)
        }
    }

    private func checkSubviews(_ views: [UIView], forIdentifier identifier: String) -> NSLayoutConstraint? {

        if views.isEmpty { return nil }

        for view in views {
            if let constraint = view.constraintFiltered(identifier) {
                return constraint
            } else if let constraint = checkSubviews(view.subviews, forIdentifier: identifier) {
                return constraint
            }
        }
        return nil
    }

    private func constraintFiltered(_ identifier: String) -> NSLayoutConstraint? {
        return constraints.filter({$0.identifier == identifier}).first
    }
}