Implements @Environment, but for UIKit stuff.

UIKitEnvironment.swift

// Implementation
public protocol UIKitEnvironmentKey {
    associatedtype Value
    static var defaultValue: Value { get }
}

public struct UIKitEnvironmentValues: CustomStringConvertible {
    private class Wrapper {
        init() {}
        var value: Any?
    }
    
    public var description: String { .init(describing: wrapper.value) }
    private var wrapper: Wrapper = .init()
    
    public init() {} // Should not me intialized outside of an @UIKitEnvironment
    
    public subscript<K>(key: K.Type) -> K.Value where K: UIKitEnvironmentKey {
        get { wrapper.value as? K.Value ?? K.defaultValue }
        set { wrapper.value = newValue }
    }
    
    mutating func update<V>(_ keypath: WritableKeyPath<UIKitEnvironmentValues, V>, _ newValue: V) { // fileprivate?
        self[keyPath: keypath] = newValue
    }
}

@frozen @propertyWrapper public struct UIKitEnvironment<Value> {
    public var values: UIKitEnvironmentValues = .init()
    public let keyPath: KeyPath<UIKitEnvironmentValues, Value>
    
    @inlinable public init(_ keyPath: KeyPath<UIKitEnvironmentValues, Value>) {
        self.keyPath = keyPath
    }
    
    @inlinable public var wrappedValue: Value { values[keyPath: keyPath] }
}

public protocol UKitEnviromentSetter {}
extension UKitEnviromentSetter {
    public func uikitEnvironment<V>(_ keypath: WritableKeyPath<UIKitEnvironmentValues, V>, _ newValue: V) -> Self {
        let mirror = Mirror(reflecting: self)
        var uiKitEnvironment = mirror
            .children
            .first(where: { $0.value is UIKitEnvironment<V> })?
            .value as? UIKitEnvironment<V>
        uiKitEnvironment?.values.update(keypath, newValue)
        return self
    }
}
extension UIViewController: UKitEnviromentSetter {}
extension UIView: UKitEnviromentSetter {}

// Usage
private struct UIKitStringContainerEnvironmentKey: UIKitEnvironmentKey {
    static let defaultValue: String = "defaultValue"
}

public extension UIKitEnvironmentValues {
    var stringValue: String {
        get { self[UIKitStringContainerEnvironmentKey.self] }
        set { self[UIKitStringContainerEnvironmentKey.self] = newValue }
    }
}

import UIKit
import SwiftUI

class ViewController2: UIViewController {
    @UIKitEnvironment(\.stringValue) var stringValue: String
    override func viewDidLoad() {
        super.viewDidLoad()
        let button = UIButton(configuration: .gray(), primaryAction: .init(handler: { action in
            print(self.stringValue)
        }))
        button.frame = .init(x: view.center.x, y: view.center.y, width: 50, height: 50)
        self.view.addSubview(button)
        self.view.backgroundColor = .blue
    }
}

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let button = UIButton(configuration: .filled(), primaryAction: .init(handler: { action in
            let newController = ViewController2()
                .uikitEnvironment(\.stringValue, "Not Default!")
            self.present(newController, animated: true, completion: nil)
        }))
        button.frame = .init(x: view.center.x, y: view.center.y, width: 100, height: 100)
        self.view.addSubview(button)
        self.view.backgroundColor = .red
    }
}