ContainerView.swift

public class ContainerView<V: UIView>: UIView {

    private var topConstraint: NSLayoutConstraint!
    private var bottomConstraint: NSLayoutConstraint!
    private var leftConstraint: NSLayoutConstraint!
    private var rightConstraint: NSLayoutConstraint!

    /// The view inside the container.
    public let contentView: V

    /**
     * Wraps a view inside a container.
     * - parameter view: The subview to use as the content view
     */
    public init(_ view: V) {
        self.contentInset = .zero
        self.contentView = view
        
        super.init(frame: .zero)
        addSubview(contentView)
        
        contentView.translatesAutoresizingMaskIntoConstraints = false

        topConstraint = contentView.topAnchor.constraint(equalTo: topAnchor)
        bottomConstraint = contentView.bottomAnchor.constraint(equalTo: bottomAnchor)
        leftConstraint = contentView.leftAnchor.constraint(equalTo: leftAnchor)
        rightConstraint = contentView.rightAnchor.constraint(equalTo: rightAnchor)
        
        NSLayoutConstraint.activate([topConstraint, bottomConstraint, leftConstraint, rightConstraint])
    }

    public required init?(coder aDecoder: NSCoder) {
        fatalError("InsetView must be initialized with a view using the init(_:) initalizer.")
    }

    // MARK: - Insets

    /// The spacing between the edges of the container and those of the content view.
    public var contentInset: UIEdgeInsets {
        didSet {
            updateInsets()
        }
    }

    private func updateInsets() {
        topConstraint.constant = contentInset.top
        bottomConstraint.constant = -(contentInset.bottom)
        leftConstraint.constant = contentInset.left
        rightConstraint.constant = -(contentInset.right)
    }

}