KeyPathBasedConfiguration.swift

//
//  KeyPathBasedConfiguration.swift
//
//  Created by Maxim Kotliar on 13.02.2020.
//  Copyright © 2020 Wikrgroup. All rights reserved.
//

import Foundation

@dynamicMemberLookup
struct KeyPathBasedConfiguration<Base: AnyObject> {

    struct KeyPathConfig {
        fileprivate let keyPath: PartialKeyPath<Base>
        fileprivate let value: Any
        fileprivate let apply: ((Base) -> Void)

        init<T>(_ keyPath: ReferenceWritableKeyPath<Base, T>, _ value: T) {
            self.keyPath = keyPath
            self.value = value
            self.apply = { $0[keyPath: keyPath] = value }
        }
    }

    init(_ keyPathConfigurations: [KeyPathConfig]) {
        var configs: [AnyKeyPath: KeyPathConfig] = [:]
        for config in keyPathConfigurations {
            configs[config.keyPath] = config
        }
        self.configurations = configs
    }

    private var configurations: [AnyKeyPath: KeyPathConfig]

    subscript <T>(dynamicMember keyPath: ReferenceWritableKeyPath<Base, T>) -> T? {
        get {
            configurations[keyPath]?.value as? T
        }
        set {
            guard let newValue = newValue else {
                configurations[keyPath] = nil
                return
            }
            configurations[keyPath] = .init(keyPath, newValue)
        }
    }

    func apply(to base: Base, ignoring: Set<PartialKeyPath<Base>>) {
        for (_, config) in configurations {
            guard !ignoring.contains(config.keyPath) else { continue }
            config.apply(base)
        }
    }
}


// Example 
import UIKit

extension UINavigationItem {
    typealias NavigationBarConfiguration = KeyPathBasedConfiguration<UINavigationBar>
}

extension UINavigationItem.NavigationBarConfiguration {

    static let `default` = UINavigationItem.NavigationBarConfiguration([.init(\.isHidden, false),
                                                                        .init(\.isTranslucent, true),
                                                                        .init(\.backgroundColor, .clear),
                                                                        .init(\.tintColor, .white)])
}