A concept of UI styling for iOS

StylingConcept2.swift

//: Playground - noun: a place where people can play

import UIKit

// MARK: - Skeleton

protocol StyleProtocol {
    func apply(to some: Any)
}

struct Style<Marker, Target>: StyleProtocol {
    private let body: (Target) -> ()
    func apply(to some: Any) {
        if let _ = some as? Marker, some = some as? Target {
            body(some)
        }
    }
}

struct StyleSheet: StyleProtocol {
    private let styles: [StyleProtocol]
    func apply(to some: Any) {
        styles.forEach { $0.apply(to: some) }
    }
}

// MARK: - Example

// Don't focus on this.
struct Palette {
    static let bodyFont = UIFont.systemFontOfSize(UIFont.systemFontSize())
    static let captionFont = UIFont.systemFontOfSize(UIFont.smallSystemFontSize())
}

protocol BodyFontStyle {}
protocol CaptionFontStyle {}
protocol MultilineLabelStyle {}

let style = StyleSheet(styles: [
    Style<BodyFontStyle, UILabel> {
        $0.font = Palette.bodyFont
    },
    Style<BodyFontStyle, UITextView> {
        $0.font = Palette.bodyFont
    },
    Style<CaptionFontStyle, UILabel> {
        $0.font = Palette.captionFont
    },
    Style<CaptionFontStyle, UITextView> {
        $0.font = Palette.captionFont
    },
    Style<MultilineLabelStyle, UILabel> {
        $0.numberOfLines = 0
    }
])

final class BodyLabel: UILabel, BodyFontStyle {
    override init(frame: CGRect) {
        super.init(frame: frame)
        style.apply(to: self)
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        style.apply(to: self)
    }
}

final class CaptionLabel: UILabel, CaptionFontStyle, MultilineLabelStyle {
    override init(frame: CGRect) {
        super.init(frame: frame)
        style.apply(to: self)
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        style.apply(to: self)
    }
}