Tests

Testing with Uber-FX makes it possible to test different logic while mocking parts of the DI graph.

Constructor per Type

While it’s possible to register several instances using one constructor function, If possible avoid this.

// Functions may also return multiple objects. For example, we could combine
// NewHandler and NewLogger into a single function:
//
//   func NewHandlerAndLogger() (*log.Logger, http.Handler, error)
//
// Fx also understands this idiom, and would treat NewHandlerAndLogger as the
// constructor for both the *log.Logger and http.Handler types. Just like
// constructors for a single type, NewHandlerAndLogger would be called at most
// once, and both the handler and the logger would be cached and reused as
// necessary.

Let’s explain the reason behind that. During tests, sometimes you will want to swap a real dependency with a fake/mocked one. Currently, the real practical way to do it is NOT calling the real constructor function that creates this dependency, but call a different one that returns the same type. If your constructor function will return several types, you can’t really swap only one of them. You will need to swap all of them since you can’t call the real constructor.