ViewStyle.swift

import Foundation
import UIKit

struct ViewStyle<T> {
    let style: (T) -> Void
}

let filled = ViewStyle<UIButton> {
    $0.setTitleColor(.white, for: .normal)
    $0.backgroundColor = .red
}

let rounded = ViewStyle<UIButton> {
    $0.layer.cornerRadius = 4.0
}

extension ViewStyle {

    func compose(with style: ViewStyle<T>) -> ViewStyle<T> {
        return ViewStyle<T> {
            self.style($0)
            style.style($0)
        }
    }
}

let roundedAndFilled = rounded.compose(with: filled)

extension ViewStyle where T: UIButton {

    static var filled: ViewStyle<UIButton> {
        return ViewStyle<UIButton> {
            $0.setTitleColor(.white, for: .normal)
            $0.backgroundColor = .red
        }
    }

    static var rounded: ViewStyle<UIButton> {
        return ViewStyle<UIButton> {
            $0.layer.cornerRadius = 4.0
        }
    }

    static var roundedAndFilled: ViewStyle<UIButton> {
        return rounded.compose(with: filled)
    }
}

func style<T>(_ object: T, with style: ViewStyle<T>) {
    style.style(object)
}

protocol Stylable {
    init()
}

extension UIView: Stylable {}

extension Stylable {

    init(style: ViewStyle<Self>) {
        self.init()
        apply(style)
    }


    func apply(_ style: ViewStyle<Self>) {
        style.style(self)
    }
}


let button = UIButton(style: .roundedAndFilled)
button.setTitle("My Button", for: .normal)
button.sizeToFit()
button

This is a very good idea @rbresjer, we would also have to return inside the static property, something like:

static var rounded: ViewStyle<UIButton> {
        return ViewStyle<UIButton> {
            $0.layer.cornerRadius = 4.0
            return $0
        }
    }

static var fullyRounded: ViewStyle<UIButton> {
        return ViewStyle<UIButton> {
            $0.layer.cornerRadius = $0.frame.height / 2
            return $0
        }
    }

I would also add a ready method without any return inside the Stylable extension, or something on these lines so it will stop complaining that the return is not being used:

extension Stylable {
    
    func apply(_ style: ViewStyle<Self>) -> Self {
        return style.style(self)
    }
    
    func ready() {}
}

So you can do:

class ViewController: UIViewController {

    @IBOutlet weak var button: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        button
            .apply(.filled)
            .apply(.fullyRounded)
            .ready()   
    }
}

@erickva I think you can use @discardableResult for that. In that way you wouldn't need to add a ready function, which doesn't add any value.

Also, there's no need to import both Foundation and UIKit. UIKit is enough. Read this for more info:
https://stackoverflow.com/a/30181218

What if I want to parameterize the ViewStyle<UIButton>.rounded style? Couldn't functions be used as well?

static func rounded(_ radius: Double) -> ViewStyle<UIButton> {
    return ViewStyle<UIButton> {
        $0.layer.cornerRadius = radius
        return $0
    }
}

Resulting in:

class ViewController: UIViewController {

    @IBOutlet weak var button: UIButton!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        button
            .apply(.filled)
            .apply(.rounded(10.0))
    }
}

I've built something similar in the past:
https://gist.github.com/IanKeen/ac051da0eeeaefcae9cbb8c1e4bb9c1b

You might like the styleAll variant which uses the appearance proxy

How could you allow composing ViewStyles for UIView subclasses with ViewStyles for UIView?

extension ViewStyle {
    static var buttonStyle: ViewStyle<UIButton> {
        return ViewStyle<UIButton> {
            $0.titleLabel?.font = UIFont.systemFont(ofSize: 15)
        }
    }
    
    static var roundedAndFilledButton: ViewStyle<UIButton> {
        // This doesn't work because 'roundedAndFilled' returns a 'ViewStyle<UIView>' and buttonStyle is expecting 'ViewStyle<UIButton>'
        return buttonStyle.compose(with: .roundedAndFilled)
    }
}

I like the idea of overloading the + operator

extension ViewStyle {

    static func +(left: ViewStyle<T>, right: ViewStyle<T>) -> ViewStyle<T> {
        return ViewStyle<T> {
            left.style($0)
            right.style($0)
            return $0
        }
    }
}

So then you can do

let button = UIButton(style: rounded + filled)