ThemeManager-Themer.swift

import Foundation

final class Themer<Theme: ThemeProtocol> {
    typealias TargetActionStorageType = TargetActionStorage<Theme>

    private var targetActionStorages = NSHashTable<TargetActionStorageType>
        .weakObjects()

    var theme: Theme {
        didSet {
            guard theme != oldValue else { return; }
            apply()
        }
    }

    init(defaultTheme: Theme) {
        self._theme = defaultTheme
    }

    func register<Target: AnyObject>(
        target: Target,
        action: @escaping (Target) -> (Theme) -> ()
    ) {
        var storage: TargetActionStorageType
        if let s = TargetActionStorageType.get(for: target) {
            storage = s
        } else {
            storage = TargetActionStorageType.setup(for: target)
            targetActionStorages.add(storage)
        }

        storage.register(target: target, action: action, initialTheme: theme)
    }

    private func apply() {
        targetActionStorages.allObjects.forEach {
            $0.applyTheme(theme)
        }
    }
}