This question is about testable software design based on mostly value objects and services.
Services that have static dependencies are straightforward to instantiate or configure when using a DI container. However, in some cases, services require dependencies that are known at runtime only.
Say, imagine a simple FileSystemDataStore with some CRUD methods in it for managing files in a directory. This service will need a directory name as one of its constructor parameters. That name could be known at runtime only and will have to be provided by its collaborators.
This seems to be somewhat of a problem because you can’t configure such service in a DI container because of its dynamic nature. You’ll probably have to use a factory to create such services. However, this will result in a quirk in the unit tests of the service’s clients. You will have to mock the factory to return a mock of the service. This adds additional complexity to unit tests. Mocks returning mocks is often considered a test smell.
What is your opinion about this problem? Is it even a problem in your experience? Should such services be instead refactored to be more “pure”?
As a general observation, when services depend on run-time values, an Abstract Factory is indeed the appropriate response.
However, as pointed out in the question, this does have an impact on the maintainability of the tests, so if you can redesign the API to avoid such situations, you should do that. It’s not always possible, though.