ViewDecoratorUsingCombinatorSample.swift

import UIKit
import PlaygroundSupport

struct Decorator<View: UIView> {
    let decorate: (View) -> View
}

func decorate<View: UIView>(_ view: View, with decorator: Decorator<View>) -> View {
    return decorator.decorate(view)
}

func decorate<View: UIView>(_ view: View, with decorators: [Decorator<View>]) -> View {
    for decorator in decorators {
        decorator.decorate(view)
    }
    return view
}

func cornerRadious<View: UIView>(cornerRadius: CGFloat) -> Decorator<View> {
    return Decorator { view in
        view.layer.cornerRadius = cornerRadius
        return view
    }
}

func shadow<View: UIView>(radius: CGFloat) -> Decorator<View> {
    return Decorator { view in
        view.layer.shadowRadius = radius
        view.layer.shadowOpacity = 0.3
        view.layer.shadowColor = UIColor.black.cgColor
        view.layer.shadowOffset = CGSize(width: 0, height: radius / 2)
        view.layer.masksToBounds = false
        return view
    }
}

func background<View: UIView>(color: UIColor) -> Decorator<View> {
    return Decorator { view in
        view.backgroundColor = color
        return view
    }
}

func buttonTitle<View: UIButton>(_ title: String) -> Decorator<View> {
    return Decorator { button in
        button.setTitle(title, for: .normal)
        return button
    }
}

func buttonTitleColor<View: UIButton>(_ color: UIColor) -> Decorator<View> {
    return Decorator { button in
        button.setTitleColor(color, for: .normal)
        return button
    }
}

func okButton(frame: CGRect) -> UIView {
    let button = UIButton(frame: frame)
    return decorate(button, with:[
        buttonTitle("OK"),
        buttonTitleColor(.white),
        // background(color: .black) // ここをどうにかしたい
    ])
}

func cardView(frame: CGRect) -> UIView {
    let view = UIView(frame: frame)
    return decorate(view, with: [
        background(color: .red),
        cornerRadious(cornerRadius: 2),
        shadow(radius: 4)
        ])
}


class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.frame = CGRect(x: 0, y: 0, width: 320, height: 480)
        self.view.backgroundColor = .white
        let card = cardView(frame: CGRect(x: 100, y: 100, width: 80, height: 80))
        self.view.addSubview(card)
        
        let ok = okButton(frame: CGRect(x: 100, y: 200, width: 50, height: 50))
        self.view.addSubview(ok)

    }
}

let viewController = ViewController()
PlaygroundPage.current.liveView = viewController