UIViewSafeAreaLayoutGuide+Extension.swift

// 案1

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 unowned 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
    }
    
    override var topAnchor: NSLayoutYAxisAnchor {
        return _base.topAnchor
    }
    
    override var bottomAnchor: NSLayoutYAxisAnchor {
        return _base.bottomAnchor
    }
    
    override var rightAnchor: NSLayoutXAxisAnchor {
        return _base.rightAnchor
    }
    
    override var widthAnchor: NSLayoutDimension {
        return _base.widthAnchor
    }
    
    override var heightAnchor: NSLayoutDimension {
        return _base.heightAnchor
    }
    
    override var leadingAnchor: NSLayoutXAxisAnchor {
        return _base.leadingAnchor
    }
    
    override var trailingAnchor: NSLayoutXAxisAnchor {
        return _base.trailingAnchor
    }
    
    override var centerXAnchor: NSLayoutXAxisAnchor {
        return _base.centerXAnchor
    }
    
    override var centerYAnchor: NSLayoutYAxisAnchor {
        return _base.centerYAnchor
    }
}




// 案2

extension UIView {
    var safeLeftAnchor: NSLayoutXAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.leftAnchor
        } else {
            return leftAnchor
        }
    }
    
    var safeRightAnchor: NSLayoutXAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.rightAnchor
        } else {
            return rightAnchor
        }
    }
    
    var safeBottomAnchor: NSLayoutYAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.bottomAnchor
        } else {
            return bottomAnchor
        }
    }
    
    var safeTopAnchor: NSLayoutYAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.topAnchor
        } else {
            return topAnchor
        }
    }
    
    var safeWidthAnchor: NSLayoutDimension {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.widthAnchor
        } else {
            return widthAnchor
        }
    }
    
    var safeHeightAnchor: NSLayoutDimension {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.heightAnchor
        } else {
            return heightAnchor
        }
    }
    
    var safeCenterYAnchor: NSLayoutYAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.centerYAnchor
        } else {
            return centerYAnchor
        }
    }
    
    var safeCenterXAnchor: NSLayoutXAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.centerXAnchor
        } else {
            return centerXAnchor
        }
    }
    
    var safeLeadingAnchor: NSLayoutXAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.leadingAnchor
        } else {
            return leadingAnchor
        }
    }
    
    var safeTrailingAnchor: NSLayoutXAxisAnchor {
        if #available(iOS 11.0, *) {
            return self.safeAreaLayoutGuide.trailingAnchor
        } else {
            return trailingAnchor
        }
    }
}