Swift Dependency Injection

When to invoke

• Designing a new service boundary (CloudKit, networking, clock, RNG, notifications).
• Asking "how do I make this testable", "how do I inject X", or "should I use a singleton here".
• Establishing a composition root for a new app target or module.
• Reviewing code that reaches out to global state, URLSession.shared, Date(), or UUID().
• Choosing between constructor injection and SwiftUI environment injection.

Scope

Owns how a seam is shaped and injected (protocol / struct witness / environment / task-local) and how a fake is written. Does NOT own the test framework, snapshot tooling, or where shared fake types live → swift-testing-baseline (<Project>KitTesting); nor the target layout that hosts the composition root → swiftpm-modularization.

Inject via

Three routing decisions, front-loaded (details in the sections below):