Is there a better way to do dependency injection in Swift than this ?

Question

New to swift, I was trying to create a service registry:

class ServiceRegistry {

    static var instance = ServiceRegistry()

    private var registry = [String:AnyObject]()
    private init(){}

    func register<T>(key:T, value:AnyObject) {
        self.registry["\(T.self)"] = value
    }

    func get<T>(_:T) -> AnyObject? {
        return registry["\(T.self)"]
    }

}

but is not super friendly:

Register:

 ServiceRegistry.instance.register(CacheServiceProtocol.self, value:ImageCacheService())

Retrieve:

if let cache = ServiceRegistry.instance.get(CacheServiceProtocol) as? CacheServiceProtocol { ... }

Any better way ? It would be useful to get rid of the as? CacheServiceProtocol in the if let ...

Answer 1

Swinject is a dependency injection framework for Swift. In your case, you can use it without the cast with as?.

Register:

let container = Container()
container.register(CacheServiceProtocol.self) { _ in ImageCacheService() }

Retrieve:

let cache = container.resolve(CacheServiceProtocol.self)!

Here cache is inferred as CacheServiceProtocol type. The resolve method returns nil if the specified type is not registered. We know CacheServiceProtocol is already registered, so the force-unwrap with ! is used.

UPDATE

I didn't exactly answer to the question. An implementation to remove the cast is storing factory closures instead of values in the registry. Here is the example. I also modified the type of key.

class ServiceRegistry {
    static var instance = ServiceRegistry()

    private var registry = [String:Any]()
    private init(){}

    func register<T>(key:T.Type, factory: () -> T) {
        self.registry["\(T.self)"] = factory
    }

    func get<T>(_:T.Type) -> T? {
        let factory = registry["\(T.self)"] as? () -> T
        return factory.map { $0() }
    }
}

Register:

ServiceRegistry.instance.register(CacheServiceProtocol.self) {
    return ImageCacheService()
}

Retrieve:

// The type of cache is CacheServiceProtocol? without a cast.
let cache = ServiceRegistry.instance.get(CacheServiceProtocol.self)

Using @autoclosure might be also good.