Swift - Stylesheet

Style.swift

struct Style<View: UIView> {

    private let style: (View) -> Void
    init(style: @escaping (View) -> Void) {
        self.style = style
    }

    func apply(to view: View) {
        style(view)
    }
}

extension UIView {

    convenience init<V>(style: Style<V>) {
        self.init(frame: .zero)
        apply(style)
    }

    func apply<V>(_ style: Style<V>) {
        guard let view = self as? V else {
            print("💥 Could not apply style for \(V.self) to \(type(of: self))")
            return
        }
        style.apply(to: view)
    }

}

let style = Style<UIView> {
    $0.backgroundColor = .red
    $0.translatesAutoresizingMaskIntoConstraints = false
}

let frame = CGRect(x: 0, y: 0, width: 500, height: 500)
let view = UIView(style: style)
view.frame = frame
let label = UILabel(style: style)
label.text = "Test"
label.sizeToFit()
view.addSubview(label)