Add HairlineView

[jvisenti]

final class HairlineView: UIView {

    var axis: UILayoutConstraintAxis {
        didSet {
            invalidateIntrinsicContentSize()
            setNeedsUpdateConstraints()
        }
    }

    var thickness: CGFloat {
        didSet {
            invalidateIntrinsicContentSize()
            setNeedsUpdateConstraints()
        }
    }

    private var thicknessConstraint: NSLayoutConstraint?

    init(axis: UILayoutConstraintAxis, thickness: CGFloat = CGFloat(1.0 / UIScreen.mainScreen().scale), color: UIColor? = UIColor(named: .Black).colorWithAlphaComponent(0.3)) {
        self.axis = axis
        self.thickness = thickness
        super.init(frame: .zero)
        self.backgroundColor = color

        setNeedsUpdateConstraints()
    }

    @available(*, unavailable) required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func updateConstraints() {
        thicknessConstraint?.active = false
        thicknessConstraint = ((axis == .Horizontal ? heightAnchor : widthAnchor) == thickness)

        super.updateConstraints()
    }

    override func intrinsicContentSize() -> CGSize {
        var size = CGSize(width: UIViewNoIntrinsicMetric, height: UIViewNoIntrinsicMetric)

        switch axis {
        case .Horizontal:
            size.height = thickness
        case .Vertical:
            size.width = thickness
        }

        return size
    }

    override func contentHuggingPriorityForAxis(axis: UILayoutConstraintAxis) -> UILayoutPriority {
        return (self.axis == axis ? UILayoutPriorityRequired : UILayoutPriorityDefaultLow)
    }

    override func contentCompressionResistancePriorityForAxis(axis: UILayoutConstraintAxis) -> UILayoutPriority {
        return contentHuggingPriorityForAxis(axis)
    }

}

[KingOfBrian]

Just a note that this will require a dependency on Anchorage

[jvisenti]

@KingOfBrian Only for 1 line. We should just update this to use a regular NSConstraint init

[KingOfBrian]

Yea, probably makes sense here.

I have some code that depends on Anchorage too. It may be inevitable that we want to include it eventually if we want to include custom views in the repository.