Theme controller to apply a color set over all UI views in an iOS app

Theme.swift

//
//  Theme.swift
//
//  Created by Rigoberto Sáenz Imbacuán (https://www.linkedin.com/in/rsaenzi/)
//  Copyright © 2019. All rights reserved.
//

import UIKit

class Theme {
    
    // Singleton
    static let shared = Theme()
    
    // Theme Colors
    var primaryColor = #colorLiteral(red: 0.992156862745098, green: 0.427450980392157, blue: 0.152941176470588, alpha: 1)
    var subTextColor = #colorLiteral(red: 0.494117647058824, green: 0.501960784313725, blue: 0.509803921568627, alpha: 1)
    
    
    // MARK: Life Cycle
    private init() {
        
        defer {
            UIView.appearance().tintColor = primaryColor
            UIApplication.shared.delegate?.window??.tintColor = primaryColor
        }
        
        // Load the file
        guard let path = Bundle.main.path(forResource: "Theme", ofType: "plist"),
              let xml = FileManager.default.contents(atPath: path),
              let file = try? PropertyListDecoder().decode(ThemeFile.self, from: xml) else {
            return
        }
        
        // Detect which theme must be used
        let selectedTheme = file.allThemes.first { item -> Bool in
            return item.name.lowercased() == file.activeThemeName.lowercased()
        }

        guard let validTheme = selectedTheme else {
            return
        }

        // Create colors using rgb values read from selected theme
        primaryColor = getColor(from: validTheme.primaryColor)
        subTextColor = getColor(from: validTheme.subTextColor)
    }

    
    // MARK: Internal
    private func getColor(from colorValues: ThemeColor) -> UIColor {
        return UIColor.init(
            red: CGFloat(colorValues.red) / 256,
            green: CGFloat(colorValues.green) / 256,
            blue: CGFloat(colorValues.blue) / 256,
            alpha: 1)
    }
}


private struct ThemeFile: Codable {
    var activeThemeName: String
    var allThemes: [ThemeItem]
}

private struct ThemeItem: Codable {
    var name: String
    var primaryColor: ThemeColor
    var subTextColor: ThemeColor
}

private struct ThemeColor: Codable {
    var red: Int
    var green: Int
    var blue: Int
}



// Goes in a Theme.plist file

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>activeThemeName</key>
	<string>ThemeA</string>
	<key>allThemes</key>
	<array>
		<dict>
			<key>name</key>
			<string>ThemeA</string>
			<key>primaryColor</key>
			<dict>
				<key>red</key>
				<integer>255</integer>
				<key>green</key>
				<integer>0</integer>
				<key>blue</key>
				<integer>0</integer>
			</dict>
			<key>secondaryColor</key>
			<dict>
				<key>red</key>
				<integer>0</integer>
				<key>green</key>
				<integer>255</integer>
				<key>blue</key>
				<integer>0</integer>
			</dict>
		</dict>
		<dict>
			<key>name</key>
			<string>ThemeB</string>
			<key>primaryColor</key>
			<dict>
				<key>red</key>
				<integer>0</integer>
				<key>green</key>
				<integer>255</integer>
				<key>blue</key>
				<integer>255</integer>
			</dict>
			<key>secondaryColor</key>
			<dict>
				<key>red</key>
				<integer>0</integer>
				<key>green</key>
				<integer>0</integer>
				<key>blue</key>
				<integer>255</integer>
			</dict>
		</dict>
	</array>
</dict>
</plist>