Functional View Styling using `callAsFunction`

FunctionalViewStyle2.swift

import UIKit

struct Style<View: UIView> {
    let apply: (View) -> View
    
    // from swift 5.2
    func callAsFunction(_ view: View) -> View {
        apply(view)
    }
}

extension Style {
    // general style composition (functional composition)
    func with(_ another: Style<View>) -> Style<View> {
        return .init { view -> View in
            another(self(view))
        }
    }
}

// styles for UIView (generic)
extension Style {
    static func cornerRadius(_ radius: CGFloat) -> Self {
        .init { view -> View in
            view.layer.cornerRadius = radius
            return view
        }
    }
    
    static func backgroundColor(_ color: UIColor) -> Self {
        .init { view -> View in
            view.backgroundColor = color
            return view
        }
    }
}

// styles only for UILabel
extension Style where View == UILabel {
    static func font(_ font: UIFont) -> Self {
        .init { label -> UILabel in
            label.font = font
            return label
        }
    }
}

// styles only for UIButton
extension Style where View == UIButton {
    static func titleColor(_ color: UIColor, for state: UIControl.State) -> Self {
        .init { button -> UIButton in
            button.setTitleColor(color, for: state)
            return button
        }
    }
}

// composed style
let labelStyle: Style<UILabel> = Style.cornerRadius(5)
    .with(.backgroundColor(.white))
    .with(.font(UIFont.systemFont(ofSize: 15)))

let buttonStyle: Style<UIButton> = Style.cornerRadius(10)
    .with(.backgroundColor(.black))
    .with(.titleColor(.white, for: .normal))

// apply style to view
let label = UILabel()
labelStyle(label)

let button = UIButton()
buttonStyle(button)

// confirm
print(label.layer.cornerRadius == 5) // true
print(label.backgroundColor == UIColor.white) // true
print(label.font == UIFont.systemFont(ofSize: 15)) // true

print(button.layer.cornerRadius == 10) // true
print(button.backgroundColor == UIColor.black) // true
print(button.titleColor(for: .normal) == UIColor.white) // true