What is the recommended way to inject a mock service for unit testing?

[ballcoach12]

I am working on a service that needs to be provided by a mock class for unit testing and by a production service when running in production mode. So I need to inject that mock class for testing cases but inject the real class for production. My plan is to define the service in the AddedServices for the language, but I don't want to have to switch between the mock service and the production service by changing code. Is there a recommended way for injecting services that is more sophisticated than just changing the module to use a different service, or should I just define something like this?:

// mylanguage-module.ts
export type MyAddedServices = {
 validation:  {
   MyValidator: MyLanguageValidatior,
   MyTestScopeProvider: MyLanguageTestScopeProvider,
   MyProductionScopeProvider: MyLanguageProductionScopeProvider
  }
}

[msujew]

I actually don't believe we had the need to mock something in our languages yet. So no general recommendations. However, what I would probably do is add my to-be-mocked services to the dependency injection module context. See this, that we use for Langium by default. That way you could decide what and how to mock on the caller side instead of needing to build multiple different modules.

[ballcoach12]

OK. Thanks, @msujew. That's essentially what I am doing. Just wanted to make sure that I'm on the right track. The production system will have an import mechanism that hits a remote API endpoint to retrieve code to load into a Scope, so we just want to define that service for now in such a way as to delegate the responsibility for code loading to a mock object and not hit the API.

[msujew]

Yeah, that definitely makes sense 👍