Retrieve Named/Provided Dependency in DI Graph

[kevinrobayna]

Hi,

I'm currently writing an app and I wanted to have some integration tests where I could call the dependencies built with fx.

In a Java world with Guice I would do something like this.

injector.getInstance (Class<T> type);

Is there a way to achieve the same with Fx?

I saw this discussion #903 and I saw this piece of code

func TestServiceB(t *testing.T) {
  app := fxtest.New(t,
    opts(),
    fx.Decorate(newFakeServiceA), // returns ServiceA
  )

  // Starts the app right away, and defers a stop when the test ends.
  defer app.RequireStart().RequireStop()

  // TODO: make a request to the app
}

But this would be only if I put an HTTP server to which I do a request. How can I just call the service methods?

This is what I wish I could do.

func TestServiceX(t *testing.T) {
  app := fxtest.New(t, opts())
  defer app.RequireStart().RequireStop()

  x, _ := app.GetProvided/Named(X)  
  result := x.Foo()
  
  //TODO: Assert result contains the expected result
}

[abhinav]

Hello! You can use the fx.Populate API to request objects from the container.

var x MyFooer
app := fxtest.New(t,
  opts(),
  fx.Populate(&x),
)
defer app.RequireStart().RequireStop()

result := x.Foo()

Note that the argument must be a pointer to the type of value you're looking up.
So if the value inside the container is a *Foo, the pointer must be a **Foo.

var foo *Foo
fxtest.New(t,
  fx.Provide(NewFoo()), // returns *Foo
  fx.Populate(&foo), // **Foo
)

You can provide any number of pointers to fill to fx.Populate.

Pointers will be filled when fx.New/fxtest.New returns.
However, we recommend calling app.Start/app.RequireStart before using them,
because the object might have a dependency that needs to be started before it's usable.

---

Design consideration: Why is fx.Populate an argument to fx.New/fxtest.New?

Fx intentionally does not allow looking up objects in the App after creation.
We want to avoid an anti-pattern: passing the App object to business logic, and looking up values inside it on the request path.

[kevinrobayna]

Oh that makes sense!

tbh i agree with you making it accessible on “prod” code isn’t great and leads to anti patterns as you said.

thank you very much!