UIViewSafeAreaLayoutGuide+Extension.swift

// https://gist.github.com/ha1f/884ce4870dc395639e86c4395f5ea947 の案1のweak版

extension UIView {
    /// Returns UILayoutGuide compatible with safeAreaLayoutGuide.
    /// `.safeAreaLayoutGuide` is available only iOS 11+, but safeAreaLayoutGuide
    /// should be same as layoutguide of UIView under iOS 11.
    ///
    /// - returns safeAreaLayoutGuide or layoutGuide of self
    func safeLayoutGuideOrSelfLayoutGuide() -> UILayoutGuide {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide
        } else {
            return UIViewLayoutGuideProxy(self)
        }
    }
}

private final class UIViewLayoutGuideProxy: UILayoutGuide {

    private weak var _base: UIView?

    init(_ view: UIView) {
        self._base = view
        super.init()
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("UIViewLayoutGuideProxy cannot be instantiated from NSCoder. Use init(UIView) instead.")
    }

    override var leftAnchor: NSLayoutXAxisAnchor {
        return _base?.leftAnchor ?? NSLayoutXAxisAnchor()
    }

    override var topAnchor: NSLayoutYAxisAnchor {
        return _base?.topAnchor ?? NSLayoutYAxisAnchor()
    }

    override var bottomAnchor: NSLayoutYAxisAnchor {
        return _base?.bottomAnchor ?? NSLayoutYAxisAnchor()
    }

    override var rightAnchor: NSLayoutXAxisAnchor {
        return _base?.rightAnchor ?? NSLayoutXAxisAnchor()
    }

    override var widthAnchor: NSLayoutDimension {
        return _base?.widthAnchor ?? NSLayoutDimension()
    }

    override var heightAnchor: NSLayoutDimension {
        return _base?.heightAnchor ?? NSLayoutDimension()
    }

    override var leadingAnchor: NSLayoutXAxisAnchor {
        return _base?.leadingAnchor ?? NSLayoutXAxisAnchor()
    }

    override var trailingAnchor: NSLayoutXAxisAnchor {
        return _base?.trailingAnchor ?? NSLayoutXAxisAnchor()
    }

    override var centerXAnchor: NSLayoutXAxisAnchor {
        return _base?.centerXAnchor ?? NSLayoutXAxisAnchor()
    }

    override var centerYAnchor: NSLayoutYAxisAnchor {
        return _base?.centerYAnchor ?? NSLayoutYAxisAnchor()
    }
}