Swift Dependency Injection via Property Wrapper

Inject.swift

//
//  Inject.swift
//
//  Created by Iurii Iaremenko on 11.01.2020.
//

import Foundation

@propertyWrapper
public class Inject<Value> {
    private let name: String?
    private var storage: Value?

    public var wrappedValue: Value {
        storage ?? {
            let value: Value = Dependencies.root.resolve(for: name)
            storage = value
            return value
        }()
    }

    public init() {
        self.name = nil
    }

    public init(_ name: String) {
        self.name = name
    }
}

final class Dependencies {
    static let root = Dependencies()
    private var factories = [String: () -> Any]()
    private var inited = [Any]()

    private init() {
        setupDependencies()
    }

    func add<T>(_ factory: @autoclosure @escaping () -> T) {
        let key = String(describing: T.self)
        factories[key] = factory
    }

    func resolve<T>(for name: String? = nil) -> T {
        if let component: T = inited.first(where: { $0 is T }) as? T {
            return component
        }

        let name = name ?? String(describing: T.self)

        guard let component: T = factories[name]?() as? T else {
            fatalError("Dependency '\(T.self)' not resolved!")
        }
        inited.append(component)
        return component
    }
}

extension Dependencies {
    /// Important: all dependencies must be registered here
    func setupDependencies() {
        add(Service())
    }
}