Swift API for theming on iOS

Theming.swift

//  Created by Indragie on 5/3/15.
//  Copyright (c) 2015 Indragie Karunaratne. All rights reserved.

import UIKit

public protocol ThemeType {
    typealias Target
    
    func apply(target: Target)
}

// This wouldn't work as a Swift-only protocol for some reason.
// EXC_BAD_ACCESS{code=EXC_i386_GPFLT} when trying to call Themable.activeThemeGroupChanged()
//
// For the same reason, `ThemeGroup` is a class instead of a struct.
@objc public protocol Themable {
    func activeThemeGroupChanged(themeGroup: ThemeGroup)
}

public final class Weak<T: AnyObject>: Equatable {
    private weak var value: T?
    
    public init(_ value: T) {
        self.value = value
    }
}

public func ==<T: AnyObject>(lhs: Weak<T>, rhs: Weak<T>) -> Bool {
    return lhs.value === rhs.value
}

public final class ThemeManager {
    static let sharedInstance = ThemeManager(defaultThemeGroup: ThemeGroup())
    
    public var activeThemeGroup: ThemeGroup {
        didSet {
            for observer in observers {
                if let observer = observer.value {
                    observer.activeThemeGroupChanged(activeThemeGroup)
                }
            }
        }
    }
    
    private var observers = [Weak<Themable>]()
    
    public init(defaultThemeGroup: ThemeGroup) {
        activeThemeGroup = defaultThemeGroup
    }
    
    public func addObserver(observer: Themable) {
        observers.append(Weak(observer))
        observer.activeThemeGroupChanged(activeThemeGroup)
    }
    
    public func removeObserver(observer: Themable) {
        if let index = find(observers, Weak(observer)) {
            observers.removeAtIndex(index)
        }
    }
}

public typealias ThemeKey = String

@objc public final class ThemeGroup {
    public var viewThemes: [ThemeKey: ViewTheme]
    public var labelThemes: [ThemeKey: LabelTheme]
    
    public init(viewThemes: [ThemeKey: ViewTheme] = [:], labelThemes: [ThemeKey: LabelTheme] = [:]) {
        self.viewThemes = viewThemes
        self.labelThemes = labelThemes
    }
}

public struct ViewTheme: ThemeType {
    private let backgroundColor: UIColor?
    private let tintColor: UIColor?
    private let borderColor: UIColor?
    
    public init(backgroundColor: UIColor? = nil, tintColor: UIColor? = nil, borderColor: UIColor? = nil) {
        self.backgroundColor = backgroundColor
        self.borderColor = borderColor
        self.tintColor = tintColor
    }
    
    // MARK: ThemeType
    
    public func apply(target: UIView) {
        if let backgroundColor = backgroundColor {
            target.backgroundColor = backgroundColor
        }
        if let tintColor = tintColor {
            target.tintColor = tintColor
        }
        if let borderColor = borderColor?.CGColor {
            target.layer.borderColor = borderColor
        }
    }
}

public struct LabelTheme: ThemeType {
    private let textColor: UIColor
    private let viewTheme: ViewTheme?
    
    public init(textColor: UIColor, viewTheme: ViewTheme? = nil) {
        self.textColor = textColor
        self.viewTheme = viewTheme
    }
    
    // MARK: ThemeType
    
    public func apply(target: UILabel) {
        target.textColor = textColor
        if let viewTheme = viewTheme {
            viewTheme.apply(target)
        }
    }
}

@IBDesignable public class ThemableView: UIView, Themable {
    @IBInspectable var themeKey: String?
    
    private func commonInit() {
        ThemeManager.sharedInstance.addObserver(self)
    }
    
    init(frame: CGRect, themeKey: String? = nil) {
        self.themeKey = themeKey
        super.init(frame: frame)
        commonInit()
    }

    required public init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }
    
    // MARK: Themable
    
    public func activeThemeGroupChanged(themeGroup: ThemeGroup) {
        if let themeKey = themeKey, theme = themeGroup.viewThemes[themeKey] {
            theme.apply(self)
        }
    }
}

@IBDesignable public class ThemableLabel: UILabel, Themable {
    @IBInspectable var themeKey: String?
    
    private func commonInit() {
        ThemeManager.sharedInstance.addObserver(self)
    }
    
    init(frame: CGRect, themeKey: String? = nil) {
        self.themeKey = themeKey
        super.init(frame: frame)
        commonInit()
    }
    
    required public init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }
    
    // MARK: Themable
    
    public func activeThemeGroupChanged(themeGroup: ThemeGroup) {
        if let themeKey = themeKey, theme = themeGroup.labelThemes[themeKey] {
            theme.apply(self)
        }
    }
}

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