DependencyInjection

A microframework for dependency injection based on the service locator pattern utilizing Swift’s property wrappers.

Usage

On App start register dependencies by overriding AppDelegate’s initializer:

import DependencyInjection
import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    @LazyInject private var config: AppConfiguration
    @LazyInject private var router: Router

    override init() {
        super.init()

        DIContainer.register {
            Shared(AppConfigurationImpl() as AppConfiguration)
            Shared(RouterImpl() as Router)
        }
    }

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        // use `config` and `router`

        return true
    }
}

Lazy vs eager evaluation

As the initializers of the injected properties the example above would be called before the AppDelegate’s initializer, it’s necessary to separate usage and initialization of the injected property. @LazyInject will only resolve when its property is first accessed. To resolve properties eagerly use @Inject instead.

Mutable vs immutable injected properties

Properties injected via @Inject and @LazyInject are immutable which is enforced by the compiler. To resolve mutable properties use @MutableInject and @MutableLazyInject.

Optional dependencies

Dependencies that under certain circumstances cannot be resolved can simply be marked as Optionals.

@Inject var player: MediaPlayer?

If no MediaPlayer instance is registered, player resolves to nil.
Note: If an Optional would be registered player also resolves to nil as the registered type is not the expected MediaPlayer but Optional<MediaPlayer> in this case.

Shared vs new instances

Registering a dependency as Shared will always resolve to the same (identical) instance. To get a new instance in each property use New:

DIContainer.register(New(MockRouter() as Router))

By doing so registrations made in production code could for example be overridden by mock objects in tests that are not shared across objects.

Aliases

Instances can also be registered with multiple alias protocols that each only expose certain parts of their functionality:

DIContainer.register(Shared(RouterImpl.init, as: Router.self, DeeplinkHandler.self))

Registering dependencies that have dependencies themselves

In case the registered dependencies depend on other dependencies themselves that should be passed via initializer injection there are overloads for registering Shared and New instances that pass a Resolver object in a closure:

DIContainer.register {
    Shared { resolve in RouterImpl(config: resolve()) }
}

Modules

To group dependencies or to avoid exposing concrete types outside a Swift module it’s an option to use DI Modules. Those are convenience wrappers around registrations and can either be defined in different parts of the code base and then registered themselves e.g. in AppDelegate:

// Feature 1

struct FeatureOneDependencyInjection {
    static let module = Module {
        Shared(FeatureOneImplementation() as FeatureOne)
        New(FeatureOneViewModelImplementation() as FeatureOneViewModel)
    }
}

// Feature 2

struct FeatureTwoDependencyInjection {
    static let module = Module(Shared(FeatureTwoImplementation() as FeatureTwo))
}

// AppDelegate

override init() {
    super.init()

    DIContainer.register {
        FeatureOneDependencyInjection.module
        FeatureTwoDependencyInjection.module
    }
}

Alternatively Modules could also be used inline in a central location:

DIContainer.register {
    Module {
        Shared(FeatureOneImplementation() as FeatureOne)
        New(FeatureOneViewModelImplementation() as FeatureOneViewModel)
    }
    Module {
        Shared(FeatureTwoImplementation() as FeatureTwo)
    }
}

Alternative registering outside AppDelegate

Dependencies can be registered by conforming DIContainer to the DependencyRegistering protocol and implementing the registerDependencies method. Dependencies will then be registered once the first dependency is resolved.

extension DIContainer: DependencyRegistering {
    public static func registerDependencies() {
        register(Shared(RouterImpl() as Router))
    }
}

Usage without property wrappers

As property wrappers can currently not be used inside function bodies, dependencies can be resolved “manually”:

func foo() {
    DIContainer.resolve(Router.self)
}

or if the compiler can infer the type to resolve:

func foo() {
    bar(router: DIContainer.resolve())
}

func bar(router: Router) {
    // …
}

Parameterized resolution

If inversion of control should also be applied to types where some or all arguments are provided at a later point it’s possible to register a closure that receives arguments and returns the desired object. In this case it makes most sense to register a New instance, meaning every time the dependency is resolved a new object is created. If a Shared instance would be registered the resolved instances would always be the one that was first resolved for the respective type and arguments would be ignored.

func register() {
    DIContainer.register {
        New({ resolver, id in ConcreteViewModel(id: id) }, as: ViewModelProtocol.self)

        // alternatively:
        New { _, id in ConcreteViewModel(id: id) as ViewModelProtocol }
    }
}

The id argument needs to be provided to use an instance implementing ViewModelProtocol. The first argument resolver can be used or ignored to resolve further arguments via dependency injection. Providing the argument is done in a closure:

func resolve() -> ViewModelProtocol {
    DIContainer.resolve(ViewModelProtocol.self, arguments: { "id_goes_here" })
}

func resolve() -> PresenterProtocol {
    // multiple arguments are provided as tuple:
    DIContainer.resolve(PresenterProtocol.self) { ("argument 1", 23, "argument 3") }
}

As initializers of properties (and Property Wrappers for that matter) are called before self is available in Swift only hard coded arguments could be provided in initalizers of @Inject and @LazyInject. Parameterized resolution is therefore currently limited to the resolve method of DIContainer as shown above.

Thread safety

Registering as well as resolving dependencies is handled on a dedicated synchronous and reentrant queue.