Prepare or Initialise hook?

[jimmyff]

I'm using GetIt for dependency injection. Is there a prepare/initialise method I could hook in to configure the DI or should I do this in the function body of each @CloudFunction()?

Thanks

[mtwichel]

I think this could be super cool! I almost wonder if supporting middleware type functions could solve this usecause - ie functions that run before each function call. That way you could do thinks like initializing without writing to each method, but you could also verify authentication tokens, log requests to a custom server, ect.

I'm not sure how it would be defined, maybe add an @Middleware annotation to a function; then, when we build server.dart, we call these before calling the actual cloud function.

Or, they could be defined in the @CloudFunction() annotation itself? I'm thinking something like

Response middleware(Request request) {
    // do middleware stuff
}

@CloudFunction(
    middleware: [
        // functions here run before the function
        middleware
    ],
)
Response function(Request request) {
    // do function
}

Just ideas :) Happy to discuss further!

[mtwichel]

Hey @jimmyff!

I just wanted to share a partial solution to Di in Dart Functions Framework using GetIt. We wrote an extension on the GetIt instance to register a singleton if not already set:

/// Extension on GetIt to register a singleton if not alreay set
extension RegisterIfNotSet on GetIt {
  /// Registers a singleton for type T if it doesn't already exist.
  /// Creates singleton with `create` function.
  Future<T> registerIfNotSet<T extends Object>({
    required FutureOr<T> Function() create,
  }) async {
    if (!GetIt.I.isRegistered<T>()) {
      GetIt.I.registerSingleton<T>(await create());
    }

    return GetIt.I.get<T>();
  }
}

You still have to create a new instance in your function code, but you do so like this:

final secretClient = await GetIt.I.registerIfNotSet<SecretClient>(
    create: () => SecretClient(),
  );

This is super nice for testing because you can mock a dependency using something like mockito (although I prefer mocktail these days), then in your test code register it as a singleton.

final SecretClient secretClient = MockSecretClient();
when(...).thenAnswer(...); // stubing methods as needed
GetIt.I.registerSingleton<SecretClient>(secretClient);

Then when your test code runs the actual function, it won't create a new dependency and instead will use the mocked one! Super handy!

I'd love to know if you have any feedback or similar ideas to this. I think it'd be cool if as a community we could build some best practices together around using this package as we're all doing some testing and as it inevitably grows. 😄

[jimmyff]

Wow this is great, thanks @mtwichel. I will definetly gibe your solution a try. I also use GetIt for the same reasons - it makes testing much easier. I'll have a go implementing this and let you now how I get on.

It would be quite nice to have something in the functions_framework to do this kind of thing, something along the lines of:

@Prepare()
void prepare(RequestContext context) { ... }

I imagine most people would not use it but if you do include it then it gets called right before your @CloudFunction()

[nickmeinhold]

Just to throw in my 2c - I think the service locator pattern works really well here where we often want global state to cache expensive objects across function invocations, and lazy initialization to maximise cold start performance.

Personally I prefer to just add a simple Locator class rather than add an extra dependency.

There are of course good reasons not to use global state and I think that should probably be part of the conversation, but I'll just link to a good resource that discusses that:
https://gameprogrammingpatterns.com/service-locator.html#when-to-use-it

Thanks!