Functional view style sample

FunctionalViewStyle.swift

import UIKit
import PlaygroundSupport

// 1. define style as function
typealias ViewStyle = (UIView) -> UIView

let cornerRadius10: ViewStyle = { view -> UIView in
    view.layer.cornerRadius = 10.0
    return view
}

let backgroundColorOrange: ViewStyle = { view -> UIView in
    view.backgroundColor = .orange
    return view
}

// cornerRadius10(backgroundColorOrange(someView))


// 2. make syntax readable (pipe operator)
precedencegroup ForwardApplication {
  associativity: left
}

infix operator |>: ForwardApplication
func |> (view: UIView, style: ViewStyle) -> UIView {
    return style(view)
}

/*
someView
  |> backgroundColorOrange
  |> cornerRadius10
*/


// 3. make parameter configurable (currying)
let cornerRadiusNotCurried: (CGFloat, UIView) -> UIView = { radius, view -> UIView in
    view.layer.cornerRadius = radius
    return view
}

// view |> cornerRadius doesn't work

// currying
let cornerRadius: (CGFloat) -> ViewStyle = { radius -> ViewStyle in
    return { view -> UIView in
        view.layer.cornerRadius = radius
        return view
    }
}

cornerRadius(5.0) // (UIView) -> UIView, ViewStyle
// view |> cornerRadius(5.0) works

let backgroundColor: (UIColor) -> ViewStyle = { color -> ViewStyle in
    return { view -> UIView in
        view.backgroundColor = color
        return view
    }
}


// 4. make same style reusable (function composition)
func compose(style1: @escaping ViewStyle, style2: @escaping ViewStyle) -> ViewStyle {
    return { view -> UIView in
        return style2(style1(view)) // view |> style1 |> style2
    }
}

// let cardStyle = compose(cornerRadius(5.0), backgroundColor(.orange))
// let someComplexStyle = compose(cornerRadius(5.0), compose(backgroundColor(.orange), ...))

infix operator >>>: ForwardApplication
func >>> (style1: @escaping ViewStyle, style2: @escaping ViewStyle) -> ViewStyle {
    return { view -> UIView in
        return view |> style1 |> style2
    }
}


// 5. generalize custom operators
func |> <A, B> (lhs: A, f: (A) -> B) -> B {
    return f(lhs)
}

func >>> <A, B, C> (f1: @escaping (A) -> B, f2: @escaping (B) -> C) -> (A) -> C {
    return { a -> C in
        return a |> f1 |> f2
    }
}

class ViewController: UIViewController {
    override func loadView() {
        self.view = UIView()
        self.view.backgroundColor = .darkGray

        let someView = UIView()
        someView.frame = CGRect(x: 37.5, y: 200, width: 300, height: 300)

        // 1
        cornerRadius10(backgroundColorOrange(someView))

        // 2
        let orangeView: UIView = someView |> backgroundColorOrange
        orangeView |> cornerRadius10

        someView
            |> backgroundColorOrange
            |> cornerRadius10

        /*
        someView
            . backgroundColorOrange
            . cornerRadius10
        */

        // 3
        someView
            |> cornerRadius(5.0)
            |> backgroundColor(.orange)

        view.addSubview(someView)

        // 4
        let cardStyle = cornerRadius(5) >>> backgroundColor(.orange)
        someView |> cardStyle
    }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = ViewController()