extension UIView {
    @discardableResult
    func constrain(attribute: NSLayoutAttribute, constant: CGFloat = 0) -> NSLayoutConstraint {
        return constrain {[
            NSLayoutConstraint(item: $0, attribute: attribute, relatedBy: .equal, toItem: $0.superview!, attribute: attribute, multiplier: 1, constant: constant)
        ]}.first!
    }

    @discardableResult
    func constrainToEdges(_ inset: UIEdgeInsets = UIEdgeInsets()) -> [NSLayoutConstraint] {
        if #available(iOS 9.0, *) {
            return constrain {[
                $0.topAnchor.constraint(equalTo: $0.superview!.topAnchor, constant: inset.top),
                $0.leadingAnchor.constraint(equalTo: $0.superview!.leadingAnchor, constant: inset.left),
                $0.bottomAnchor.constraint(equalTo: $0.superview!.bottomAnchor, constant: inset.bottom),
                $0.trailingAnchor.constraint(equalTo: $0.superview!.trailingAnchor, constant: inset.right)
            ]}
        } else {
            return constrain {
                NSLayoutConstraint.constraints(withVisualFormat: "H:|-\(inset.left)-\($0)-\(inset.right)-|", options: [], metrics: nil, views: ["view": $0]) +
                NSLayoutConstraint.constraints(withVisualFormat: "V:|-\(inset.top)-\($0)-\(inset.bottom)-|", options: [], metrics: nil, views: ["view": $0])
            }
        }
    }

    @discardableResult
    func constrainToCenter() -> [NSLayoutConstraint] {
        if #available(iOS 9.0, *) {
            return constrain {[
                $0.centerXAnchor.constraint(equalTo: $0.superview!.centerXAnchor),
                $0.centerYAnchor.constraint(equalTo: $0.superview!.centerYAnchor)
            ]}
        } else {
            return constrain {[
                $0.constrain(attribute: .centerX),
                $0.constrain(attribute: .centerY)
            ]}
        }
    }
}