为 ViewModel 注入依赖

If a ViewModel creates its own network client inside itself, tests are stuck hitting the real server. That tight coupling makes it hard to swap later.

Dependency injection means passing in what a ViewModel needs from outside, rather than letting it build those services itself.

Describe the work with a protocol, not a concrete type. The ViewModel depends on the abstraction and never on a specific implementation.

Take the dependency through the init. Now any object that conforms to the protocol can be handed to the ViewModel.

Inside methods, call the stored service rather than a hardcoded client. The ViewModel no longer cares who actually does the work.

Your production type conforms to the protocol and talks to the network. You inject it when building the real screen.

For tests, write a mock that returns fixed data instantly. Inject it to verify ViewModel logic without any real network calls.

With a mock injected, a test just calls a method and checks the result. No waiting, no flakiness, only fast and predictable runs. ✅

Give init a default real service so everyday code stays short, while tests can still override it with a mock when needed.

For app-wide services, pass them through SwiftUI @Environment instead of every initializer, keeping call sites clean.

Injection makes ViewModels flexible and isolated. You can swap real, fake, or offline services without touching the view at all.

You learned to inject services through a protocol, swap real for mock in tests, and even use @Environment for shared dependencies. That completes MVVM.