Guise 11

Guise is a flexible, minimal dependency resolution framework for Swift.

• Flexible dependency resolution, with optional caching
• Elegant, straightforward registration
• Thread-safe
• Supports throwing, async and MainActor-isolated initializers and resolution
• Simplifies unit testing
• Pass arbitrary state when resolving
• Lazy resolution
• Nested containers
• Swift 6+
• Support for iOS 13+, macOS 10.15+, watchOS 6+, tvOS 13+

What's New?

Support for Swift 6 concurrency, especially MainActor-isolated registrations and resolutions in a synchronous MainActor-isolated context. Read the section below on concurrency to understand the whys and wherefores of this.

Basic Documentation

Registration

To register a dependency with the container, four facts are needed: The type to be registered, the tags under which it will be registered, the arguments needed in order to resolve the registration, and the lifetime of the registration. The first three uniquely identify a registration. We'll discuss the fourth fact, lifetime, below.

Let's start with these one by one.

Types

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

The first argument to register tells Guise (and Swift) which type we're registering, and the block passed at the end tells Guise how to construct that type. (Ignore the _ parameter for the moment.)

Since the type we're registering is the same as the return type of the block, we can omit the first argument of register:

container.register { _ in Service() }

One of the main purposes of dependency injection is to locate implementations of abstract interfaces, so that they can be substituted in unit tests or for some other purpose. In fact, that's why this framework is called Guise.

Guise, n. An external form, appearance, or manner of presentation, typically concealing the true nature of something.

protocol Service {
  func performService() async
}

class ConcreteService: Service {
  // Perhaps this one talks to a real database
}

class TestService: Service {
  // This is the one we want in unit tests
}

let container = Container()
container.register(Service.self) { _ in
  ConcreteService()
}

Notice that in this example, the block returns ConcreteService but the registered type is Service. Another way to express this is

container.register { _ in ConcreteService() as Service }

Later, when we resolve this registration, we ask for Service, not ConcreteService.

For simple registrations, Guise has a convenient overload which uses an @autoclosure:

container.register(instance: ConcreteService() as Service)

Tags

Tags can help locate, describe, and disambiguate registrations. A tag can be any type which is both Hashable and Sendable, such as String, UUID, Int or a custom type you create yourself.

enum Types: Hashable, Sendable {
  case plugin
}

container.register(Plugin.self, tags: Types.plugin, 1, instance: PluginImpl1())

The order of tags is not important and any number may be used. Tags are collected into a Set<AnySendableHashable>, so repetition has no effect.

Because the tags used in a registration are part of the unique Key that identifies that registration, the same tags must be used when resolving:

let plugin: Plugin = try container.resolve(Plugin.self, tags: 1, Types.plugin) 

The tags weren't given in the same order as when the registration was made, but that makes no difference.

Tags become really powerful when resolving arrays of dependencies. In that case, only a subset of the tags needs to be specified. See the discussion on arrays in the resolution section of this README.

Arguments

It's very common to need to pass some state to a dependency when resolving. Guise supports any number of arguments.

class Service {
  let id: Int
  let state: String
}

container.register { _, id, state in
  Service(id: id, state: state)
}

When resolving, these arguments must be given or Guise will throw an error:

let service: Service = try container.resolve(args: 1, "foo")

Lifetimes

Guise supports two lifetimes: transient and singleton. Transient is the default.

In a transient registration, a new instance of the dependency is created and returned each time. In other words, the resolution factory that is passed as the last argument to register is called, the arguments are passed, and its result is returned to the caller.

In a singleton registration, the factory is invoked the first time, but every subsequent request for that registration returns the same instance.

container.register(lifetime: .singleton, instance: Service())

Transitive dependencies

One of the primary functions of dependency injection is to locate and resolve complex hierarchies of dependencies. Guise can do this as well. The first argument of every resolution block is an instance of Resolver, which allows registrations to be located and resolved.

class Database {}
class Service {
  let database: Database
  init(database: Database) {
    self.database = database
  }
}

container.register(lifetime: .singleton, instance: Database())
container.register(lifetime: .singleton) { r in
  Service(database: try r.resolve())
}

Whenever we resolve Service, the Database parameter in its constructor will be located and resolved.

This pattern is so common that Guise has a higher-order function, auto, that can handle any number of dependencies:

container.register(lifetime: .singleton, factory: auto(Service.init))

In order to use auto, the sub-dependencies must not have any tags or factory arguments.

Resolution

Resolution looks up and instantiates a dependency given its type, tags, and factory arguments, and taking into account its lifetime.

Resolution is simpler than registration, so a few examples will suffice:

class Service {}
container.register(instance: Service())

// Two alternate ways to resolve the above registration
let service: Service = try container.resolve()
let service = try container.resolve(Service.self)

container.register(tags: 2, instance: Service())
let service: Service = try container.resolve(tags: 2)

class Something {
  let id: Int

  init(id: Int) { self.id = id }
}

container.register { _, id in
  Something(id: id)
}
let something = try container.resolve(Something.self, args: 7)

Optional Resolution

Guise can resolve optionals as the wrapped type:

class Service {}
container.register(instance: Service())
let service: Service? = try container.resolve()

What happens behind the scenes is that Guise first looks for the exact registration, i.e., a registration of the type Service?. If it doesn't find that, then it attempts to resolve Service.

When resolving an optional, Guise returns nil instead of throwing an error if the registration cannot be found.

Array Resolution

Imagine a plugin architecture in which we want to locate and resolve many instances of the same type.

protocol Plugin {}

container.register(Plugin.self, tags: UUID(), instance: Plugin1())
container.register(Plugin.self, tags: UUID(), instance: Plugin2())
container.register(Plugin.self, tags: UUID(), instance: Plugin3())

We can get all of these plugins very easily:

let plugins: [Plugin] = try container.resolve()

Just as with optional resolution, Guise first looks for an exact match for this registration. Not finding one, it notices that this is trying to resolve an array. It then locates all registrations of type Plugin and attempts to resolve them all. If any fail, all fail.

When resolving an array, tags are processed differently. Guise looks for all registrations containing all of the given tags.

container.register(Plugin.self, tags: "type1", UUID(), instance: Plugin1())
container.register(Plugin.self, tags: "type1", UUID(), instance: Plugin2())
container.register(Plugin.self, tags: "type2", UUID(), instance: Plugin3())
container.register(Plugin.self, tags: "type2", UUID(), instance: Plugin4())

Here we have four plugins registered. Each is disambiguated with an anonymous UUID and divided into two types: type 1 and type 2. To get all of the type 1 registrations…

let plugins: [Plugin] = try container.resolve(tags: "type1")

This gets Plugin1 and Plugin2 but not Plugin3 and Plugin4. Of course, we can still get all four of them with this incantation:

let plugins: [Plugin] = try container.resolve()

If no registrations are found, Guise returns an empty array by default instead of throwing an error.

Lazy Resolution

Occasionally there's a need to depend on a service that isn't ready yet. Or we wish to prevent a cycle because two services depend on each other.

One way to solve this problem is to pass an instance of the Resolver itself:

class Service {
  weak var resolver: Resolver! 

  init(resolver: Resolver) {
    self.resolver = resolver
  }

  func performService() throws {
    let database = try resolver.resolve(Database.self)
    database.doSomething()
  }
}

The problem with the pattern above is that it breaks one of the fundamental rules of dependency injection: make dependencies explicit. A user of Service must read the source code in order to know what other dependencies it has. This makes the class harder to use and harder to test.

Guise solves this problem with lazy resolvers.

class Service {}
container.register(tags: "s", instance: Service())

let lr: LazyResolver<Service> = try container.resolve()

Lazy resolvers don't have to be registered. Guise automatically constructs them as needed. A lazy resolver resolves dependencies of the type Service. Tags and arguments are specified when resolving:

let service = lr.resolve(tags: "s")

Async

Guise supports async registrations and resolution.

class Service {
  let database: Database

  init(database: Database) async {
    self.database = database
    await database.setup()
  }
}

container.register { r in
  try await Service(database: r.resolve())
}
let service = try await container.resolve(Service.self)

Any synchronous registration may be resolved asynchronously, but the reverse is not true. By default, if an attempt is made to resolve an async registration in a synchronous context, Guise throws .requiresAsync.

Concurrency

Guise has special support for MainActor-isolated registrations and resolutions. This uses the isolation parameter to distinguish between other overloads of register and resolve:

registrar.register(isolation: MainActor.shared) { _ in
  MyViewController()
}
let myViewController = try resolver.resolve(MyViewController.self, isolation: MainActor.shared)

The obvious question is: Why only support MainActor? Why not support any global actor? Well, actually Guise does support any global actor, it's just that both registration and resolution must be async:

registrar.register { @MyActor _ async in
  MyService()
}
let myService: MyService = try await resolver.resolve()

The special support for MainActor is because, in general, we want MainActor-isolated resolutions not to be async within the MainActor context, such as a view controller:

override func awakeFromNib() {
  super.awakeFromNib()
  MainActor.assumeIsolated {
    // Note that this is synchronous.
    let otherViewController = try resolver.resolve(OtherViewController.self, isolation: MainActor.shared)
  }
}

Currently, Apple does not provide a mechanism to "flow" the isolation context all the way through the internal dance of a DI framework, including type erasure, etc. while maintaining synchronous call semantics. I'm aware of isolated parameters and they looked promising. You can flow the actor all the way through using that, but the problem is that when you try to resolve, everything is async, which kills ergonomics, particularly for MainActor. To call an actor-isolated method without using await, the Swift compiler must be able to prove statically at compile time that the actor-isolated method is being called within the very same isolation context.

I made the pragmatic choice to make MainActor a special case. And it really should be, because its semantics are a bit different from other actors. It's the actor within which all UI code executes, including a mountain of synchronous legacy code. As a result, we want resolution of MainActor-isolated types to be synchronous when possible. This greatly improves ergonomics.

As a matter of fact, you can resolve MainActor-isolated registrations in an async context if you wish. For example:

registrar.register(isolation: MainActor.shared) { _ in
  MyViewController()
}
let myViewController: MyViewController = try await resolver.resolve()

Why the isolation parameter instead of using a different name, like registerMain?

There are two imperfect reasons for this:

1. I wanted to stay consistent with using overloads of register and resolve for everything, whether synchronous, asynchronous, or MainActor-isolated.
2. If Swift ever does make it possible to flow the actor context through and preserve synchronous call semantics, this is the obvious syntax to use, so it's a bit of future-proofing.

If you think register(isolation: MainActor.shared) is long-winded, it's easy enough to create an extension method:

public extension Registrar {
  func registerMain<T, each Tag: Hashable & Sendable, each Arg: Sendable>(
    _ type: T.Type = T.self,
    tags: repeat each Tag,
    lifetime: Lifetime = .transient,
    factory: @escaping @MainActor @Sendable (any Resolver, repeat each Arg) throws -> T
  ) -> Key<T> {
    register(type, isolation: MainActor.shared, tags: repeat each tags, lifetime: lifetime, factory: factory)
  }
}

mainauto

Analogous to auto, Guise has a helper function called mainauto which can be used to simplify registration of MainActor-isolated types with constructor dependencies:

class MyViewController: UIViewController {
  let myService: MyService
  init(myService: MyService) {
    self.myService = myService
  }
}
await MainActor.run {
  registrar.register(isolation: MainActor.shared, factory: mainauto(MyViewController.init))
}

Why is MainActor.run necessary here? It really shouldn't be. Unfortunately, there's a bug in the Swift compiler's static concurrency analysis. It thinks that MyViewController.init — which is a MainActor-isolated initializer — is being called here instead of just passed as a higher-order function parameter. So unfortunately we must use MainActor.run. Hopefully this bug will be fixed in an upcoming version of Swift. If you don't use mainauto, you don't have to do this:

class MyViewController: UIViewController {
  let myService: MyService
  init(myService: MyService) {
    self.myService = myService
  }
}
registrar.register(isolation: MainActor.shared) { r in
  try MyViewController(myService: r.resolve())
}

The Resolution Matrix

Dependencies often depend on other dependencies. These dependencies may have differences in isolation, which can cause problems when resolving. For example:

// This type is MainActor-isolated
class ViewController: UIViewController {}
registrar.register(
  isolation: MainActor.shared,
  instance: ViewController()
)

// This type is non-isolated, which we call "sync" in Guise.
class Presenter {
  let vc: ViewController
  init(vc: ViewController) {
    self.vc = vc
  }
  @MainActor
  func present(on other: UIViewController) {
    other.present(vc, animated: true)
  }
}
// This won't even compile
registrar.register(factory: auto(Presenter.init))

The reason Presenter's registration won't compile is because its dependency, ViewController, has a MainActor-isolated initializer, which must be called within a MainActor-isolated context. There are several ways we can solve this.

First, we can use async registration:

registrar.register { r in
  try await Presenter(vc: r.resolve())
}

This works but kills ergonomics in many cases because now we must also resolve using await:

let presenter: Presenter = try await resolver.resolve()

The second solution is to make Presenter's registration MainActor-isolated as well:

registrar.register(isolation: MainActor.shared, mainauto(Presenter.init))

But now we must always resolve in a MainActor-isolated context.

The best solution for this, in my opinion, is to use a LazyResolver. For that, we have to rewrite our Presenter a bit:

class Presenter {
  let lvc: LazyViewController
  init(lvc: LazyResolver<ViewController>) {
    self.lvc = lvc
  }
  @MainActor
  func present(on other: UIViewController) throws {
    let vc = try lvc.resolve(isolation: MainActor.shared)
    other.present(vc, animated: true)
  }
}

So what's the matrix? The matrix tells us what kind of resolution can resolve what kind of registration.

RSLV 👇 RGSTR 👉	Sync	Async	MainActor
Sync	✅	❌	❌
Async	✅	✅	✅
MainActor	✅	❌	✅

Across the top are the different kinds of registrations and along the side are the different kinds of resolutions. This tells us, for example, that a synchronous non-isolated ("sync") registration can be resolved by any kind of resolution. Asynchronous non-isolated ("async") resolution can resolve any kind of registration. And so on.

My recommendation is to use a LazyResolver everywhere you see an ❌ in the table above.

Assemblies

In a complex application with many modules, it can be helpful to organization registrations exported from the module. Guise provides assemblies for this purpose.

class AwesomeAssembly: Assembly {
  func register(in registrar: any Registrar) {
    registrar.register(assemblies: CoolAssembly())
    registrar.register(lifetime: .singleton, instance: Service())
  }

  // This method is optional. A default implementation
  // is provided, which does nothing.
  func registered(to resolver: any Resolver) {
    do {
      let service = try resolver.resolve(Service.self)
      service.configure()
    } catch {
      // Handle error
    }
  }
}

Assemblies are organized in a hierarchy. An assembly should register its dependent assemblies using register(assemblies:).

Assemblies are keyed by their type, so adding the same assembly twice will not result in double registration.

The act of assembling first creates an ordered set of assemblies, i.e., a list without duplicates in order of first registration. It then iterates through this list and calls register(in:) on each one. After which it iterates through the list and calls registered(to:) on each one.

The purpose of registered(to:) is to perform additional initialization after dependencies have been registered without exposing the dependencies outside of the assembly.

Once all dependencies have been registered, call assemble on the container to make everything work. Additional assemblies can be passed as arguments to assemble or it can be called without arguments if they've already been registered.

container.assemble(AwesomeAssembly(), CoolAssembly())

or

container.register(assemblies: AwesomeAssembly(), CoolAssembly())
container.assemble()

Nested Containers

Guise supports nested Containers. When constructing a Container, simply pass its parent in the constructor:

let parent = Container()
let child = Container(parent: parent)

When resolving, if an entry can't be found in the child, the parent is searched. Child entries always override parent entries for matching Keys. The reverse is not true: A parent has no knowledge of its child containers. Searching the parent directly will not discover any child entries.

Nested Containers & Assemblies

Assemblies are simply a way to make many registrations en masse. If a container cannot find a registration, it will search its parent. This is not true of assemblies. Assemblies are specific to a container.

let parent = Container()
parent.assemble(CoolAssembly())
let child = Container(parent: parent)
child.assemble(AwesomeAssembly())