ViewDecoratorSample.swift

// https://medium.com/halcyon-mobile/view-decoration-in-swift-4023a68be5d3

import UIKit
import PlaygroundSupport

protocol ViewDecorator {
    associatedtype View: UIView
    func decorate(view: View)
}

struct AnyViewDecorator<V> {
    
    let _decorate: (V) -> Void
    init<D: ViewDecorator>(decorator: D) where D.View == V {
        _decorate = decorator.decorate
    }
    
    func decorate(view: V) {
        _decorate(view)
    }
}

extension UIView {
    
    func decorate(with decorator: AnyViewDecorator<UIView>) {
        decorator.decorate(view: self)
    }
    
    func decorate(with decorators: [AnyViewDecorator<UIView>]) {
        decorators.forEach { $0.decorate(view: self) }
    }
}

struct CornerRadiusViewDecorator: ViewDecorator {
    let cornerRadius: CGFloat
    
    func decorate(view: UIView) {
        view.clipsToBounds = true
        view.layer.cornerRadius = cornerRadius
    }
}

struct ShadowViewDecorator: ViewDecorator {
    let radius: CGFloat
    
    func decorate(view: UIView) {
        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
    }
}

struct CardViewDecorator: ViewDecorator {
    private let decorators: [AnyViewDecorator<UIView>] = [
        AnyViewDecorator(decorator: CornerRadiusViewDecorator(cornerRadius: 4)),
        AnyViewDecorator(decorator: ShadowViewDecorator(radius: 2))
    ]
    
    func decorate(view: UIView) {
        view.decorate(with: decorators)
    }
}

struct ButtonViewDecorator: ViewDecorator {
    typealias View = UIButton
    
    func decorate(view: UIButton) {
        view.setTitleColor(.white, for: .normal)
        view.setTitleShadowColor(.black, for: .normal)
        view.titleEdgeInsets = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 50)
    }
}

struct LabelViewDecorator: ViewDecorator {
    typealias View = UILabel
    
    func decorate(view: UILabel) {
        view.tintColor = .red
    }
}

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 button = UIButton(frame: CGRect(x: 100, y: 100, width: 100, height: 100))
        button.setTitle("OK", for: .normal)
        button.backgroundColor = .blue

        let cardDecorator = CardViewDecorator()
        cardDecorator.decorate(view: button)
        self.view.addSubview(button)

        let button2 = UIButton(frame: CGRect(x: 100, y: 300, width: 100, height: 100))
        button2.setTitle("NG", for: .normal)
        button2.backgroundColor = .red

        button2.decorate(with: [
            AnyViewDecorator(decorator: CornerRadiusViewDecorator(cornerRadius: 10)),
            AnyViewDecorator(decorator: ShadowViewDecorator(radius: 4))
        ])

        self.view.addSubview(button2)

        let button3 = UIButton(frame: CGRect(x: 100, y: 500, width: 100, height: 100))
        button3.setTitle("Not", for: .normal)
        button3.backgroundColor = .green

        let buttonDecorator = ButtonViewDecorator()
        buttonDecorator.decorate(view: button3)
        self.view.addSubview(button3)
    }
}

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