A small DSL to group and compose styles for UIKit components.

ComposableStyles.swift

//
//  Style.swift
//  ComposableStyles
//
//  Created by Daniel Cardona Rojas on 20/11/19.
//  Copyright © 2019 Daniel Cardona Rojas. All rights reserved.
//

import UIKit

struct GenericStyle {
    private let styling: (UIView) -> Void

    init(_ styling: @escaping (UIView) -> Void) {
        self.styling = styling
    }

    func apply(to view: UIView) {
        styling(view)
    }
}

struct Style<T: UIView> {

    private let styling: (T) -> Void

    init(_ styling: @escaping (T) -> Void) {
        self.styling = styling
    }

    func apply(to view: T) {
        styling(view)
    }
}

// MARK: Shorthand helpers
extension Style {
    static func property<P>(_ keyPath: ReferenceWritableKeyPath<T,P>, value: P) -> Style {
        return Style { v in
            v[keyPath: keyPath] = value
        }
    }

    static func adjustingProperty<P>(_ keyPath: ReferenceWritableKeyPath<T,P>) -> (P) -> Style {
        return { (value: P) in
            Style { v in v[keyPath: keyPath] = value }
        }
    }

}

extension GenericStyle {
    static func property<P>(_ keyPath: ReferenceWritableKeyPath<UIView,P>, value: P) -> GenericStyle {
        return GenericStyle { v in
            v[keyPath: keyPath] = value
        }
    }

    static func adjustingProperty<P>(_ keyPath: ReferenceWritableKeyPath<UIView,P>) -> (P) -> GenericStyle {
        return { (value: P) in
            GenericStyle { v in v[keyPath: keyPath] = value }
        }
    }
}

// MARK: Combining styles
func +<V>(_ lhs: Style<V>, rhs: Style<V>) -> Style<V> {
    return Style { (v: V) -> Void in
        lhs.apply(to: v)
        rhs.apply(to: v)
    }
}

func +<V>(_ lhs: Style<V>, rhs: GenericStyle) -> Style<V> {
    return Style { (v: V) -> Void in
        lhs.apply(to: v)
        rhs.apply(to: v)
    }
}

func +<V>(_ lhs: GenericStyle, rhs: Style<V>) -> Style<V> {
    return Style { (v: V) -> Void in
        lhs.apply(to: v)
        rhs.apply(to: v)
    }
}

func +(_ lhs: GenericStyle, rhs: GenericStyle) -> GenericStyle {
    return GenericStyle { (v: UIView) -> Void in
        lhs.apply(to: v)
        rhs.apply(to: v)
    }
}

// MARK: Component Specific helpers
extension UIView {
    func applying(_ style: GenericStyle) {
        style.apply(to: self)
    }

}
extension UILabel {
    func applying(_ style: Style<UILabel>) {
        style.apply(to: self)
    }
}

extension UIButton {
    func applying(_ style: Style<UIButton>) {
        style.apply(to: self)
    }
}

extension UICollectionViewCell {
    func applying(_ style: Style<UICollectionViewCell>) {
        style.apply(to: self)
    }
}