LazyOnce implementation (for post: https://medium.com/@tinkoffhere/b22f51422345).

LazyOnce.swift

@propertyWrapper final class LazyOnce<T> {
    var wrappedValue: T {
        if let existStorage = storage { return existStorage }
        let newStorage = lazyBlock()
        self.storage = newStorage
        return newStorage
    }
    private var storage: T?
    private let lazyBlock: () -> T
    init(wrappedValue: @escaping @autoclosure () -> T) {
        self.lazyBlock = wrappedValue
    }
}