Adaptive Font styles

StyleProxy.swift

public protocol Stylish: class {
    func updateStyle()
}

public class StyleProxy<S: Stylish>: NSObject {
    fileprivate override init() { }
}

private class StyleProxyView<S: Stylish>: UIView {
    
    var instance: S? { return superview as? S }
    var style: StyleProxy<S> = StyleProxy()
    
    override public func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        instance?.updateStyle()
    }
    
}

extension Stylish where Self: UIView {
    
    private(set) var style: StyleProxy<Self> {
        get {
            if let proxy = subviews.first(where: { $0 is StyleProxyView<Self> }) as? StyleProxyView<Self> {
                return proxy.style
            }
            
            let proxy = StyleProxyView<Self>()
            addSubview(proxy)
            return proxy.style
        }
        set {
            guard let proxy = subviews.first(where: { $0 is StyleProxyView<Self> }) as? StyleProxyView<Self> else { return }
            styleView.style = newValue
            updateStyle()
        }
    }
    
}

TextStyle.swift

public typealias TextStyle = (UITraitCollection) -> UIFont?

UIButton+TextStyle.swift

private var _textStyleKey: Void?

public extension StyleProxy where S: UIButton {
    
    var font: TextStyle? {
        get {
            return associatedValue(forKey: &_textStyleKey)
        }
        set {
            retain(newValue, forKey: &_textStyleKey)
        }
    }
    
}

extension UIButton: Stylish {
    
    public var textStyle: TextStyle? {
        get { return style.font }
        set { style.font = newValue }
    }
    
    public func updateStyle() {
        self.titleLabel?.font = textStyle?(traitCollection) ?? titleLabel?.font
    }
    
}

UILabel+TextStyle.swift

private var _textStyleKey: Void?

public extension StyleProxy where S: UILabel {
    
    var font: TextStyle? {
        get {
            return associatedValue(forKey: &_textStyleKey)
        }
        set {
            retain(newValue, forKey: &_textStyleKey)
        }
    }
    
}

extension UILabel: Stylish {
    
    public var textStyle: TextStyle? {
        get { return style.font }
        set { style.font = newValue }
    }

    
    public func updateStyle() {
        self.font = textStyle?(traitCollection) ?? font
        invalidateIntrinsicContentSize()
    }
    
}