Skein for Swift

Skein is a small, type-safe dependency-injection container for Swift. Its default API is MainActor-isolated, with explicit Sendable nonisolated and custom-global-actor alternatives.

Read the Skein website or browse the source repository.

Why Skein?

A skein is a length of thread gathered into a connected bundle. The name reflects how this library brings the individual threads of an application's dependency graph together into one explicit, inspectable composition.

Install

Add https://github.com/modern-swift-dev/skein-swift as a package dependency in Xcode, or add it to a Swift Package Manager manifest:

dependencies: [
    .package(
        url: "https://github.com/modern-swift-dev/skein-swift",
        from: "1.0.1"
    )
]

Then depend on the Skein product:

.product(name: "Skein", package: "skein-swift")

Vapor 4 applications can instead add the SkeinVapor product alongside Skein. The integration is available on macOS 15 and Linux and pins Vapor 4.122.0.

Quick start

Bindings declare their exposed type and receive a Resolver for their own dependencies. A single is created once on first use; a factory creates a new value for every resolution.

import Skein

protocol HTTPClient { }
final class URLSessionClient: HTTPClient { }

final class UserService {
    init(client: any HTTPClient) { }
}

let appModule = module {
    single((any HTTPClient).self, provider: { _ in URLSessionClient() })
    factory(UserService.self, using: UserService.init)
}

try startSkein {
    appModule
}

let service: UserService = try get()
stopSkein()

All setup and resolution operations that can fail use Swift error handling, so call them with try and handle failures appropriate to your application.

Documentation

Start with the hosted guides:

• Overview
• Documentation index
• Getting started
• Examples

The site publishes separate API references for each library product:

• Skein API
• SkeinSwiftUI API
• SkeinVapor API

Each stable GitHub release includes Skein-Documentation.zip, containing the three matching .doccarchive bundles. Download and unzip it, then open each bundle in Xcode to add it to the Documentation browser.

Build the same archive locally with:

make documentation

Runnable examples

The Examples package contains independent, executable use cases for:

• singleton and factory lifetimes;
• dependency chains and module composition;
• protocol-oriented registrations;
• qualified bindings;
• lifecycle and provider-error handling;
• MainActor-first services, binding-owned roots, and async startup validation;
• test isolation and hand-written fakes.

Run any example directly from the repository root:

swift run --package-path Examples BasicUsage
swift run --package-path Examples ModularComposition
swift run --package-path Examples QualifiedBindings
swift run --package-path Examples ErrorHandling
swift run --package-path Examples MainActorValidation
swift test --package-path Examples

These examples depend on the local checkout of Skein, so they also serve as compile-checked usage references while the API evolves.

Contributing

See CONTRIBUTING.md for website development and release instructions.