store.swift

import Foundation

protocol StoreKey {
    associatedtype Value
    static func defaultValue() -> Value
}

struct MyKey: StoreKey {
    typealias Value = Int
    static func defaultValue() -> Int {
        return 1
    }
}

struct Store {
    
    private var storage: [String: Any] = [:]
    
    subscript<V: StoreKey>(member: V.Type) -> V.Value   {
        get {
            let keyStr = String(describing: member)
            return storage[keyStr] as? V.Value ?? V.defaultValue()
        }
        set {
            let keyStr = String(describing: member)
            storage[keyStr] = newValue
        }
    }
}

var store = Store()
let a = store[MyKey.self]
print(a)
// prints 1 (the default value)
store[MyKey.self] = 2
let b = store[MyKey.self]
print(b)
// prints 2 (the value we just set using subscript)

In SwiftUI, the environment allows setting and getting values where keys are types conforming to a protocol, and have default values. Here's a sketch of what the implementation might look like