Feat: adds clear function to HitsSource

[NicFontana]

Summary
This PR adds a clear() function to HitsSource protocol that can be used to clear the data source's memory, if desired.

Motivation:
HitsObservableController has a @Published public var hits: [Hit?] property that can be set empty to show an empty result list.
For instance, we can set it empty if we want to clear the results list for any reason, and not just when a query becomes empty by leveraging the showItemsOnEmptyQuery property of HitsInteractor.Settings.

Result
As a result we have a reduced memory consumption when we assign an empty array to HitsObservableController's hits and whenever desired.

I also added a couple of tests that cover the new introduced code.

I hope this can be useful and the same result is not achievable in another way, otherwise, I apologize.

Please let me know if I missed something.

Thank you

[VladislavFitz]

Thank your for your PR, @NicFontana !

HitsObservableController has a @published public var hits: [Hit?] property that can be set empty to show an empty result list.

This assumption is not consistent with the InstantSearch architecture. The source of truth containing the information about the current state of Hits is the HitsInteractor. You are not supposed to change the @Published public var hits: [Hit?] property manually. HitsObservableController is just a "read-only" adapter ensuring the compatibility with SwiftUI, but unfortunately this cannot be ensured via the type system.

If you want to clear your hits view manually, you might do it by changing the HitsInteractor state. The HitsInteractor instance will automatically update all the connected HitsController implementations, including HitsObservableController instances.

However, there is actually no simple way to reset the HitsInteractor state. So, my suggestion is to keep the clear methods for HitsInteractor and Paginator classes and to cancel changes in the HitsSource and HitsObservableController controller classes. Wdyt?

[NicFontana]

Hello @VladislavFitz

Thanks for your feedback, I followed your suggestions in the last commit.

I also marked with private(set) the HitsObservableController @Published properties. This should help making HitsObservableController consistent with InstantSearch's architecture:

The source of truth containing the information about the current state of Hits is the HitsInteractor. You are not supposed to change the @published public var hits: [Hit?] property manually. HitsObservableController is just a "read-only" adapter ensuring the compatibility with SwiftUI, but unfortunately this cannot be ensured via the type system.

[VladislavFitz]

By removing this conditional compiling clause, you make it fail while compiling for macOS. So, the test step fails.

[NicFontana]

Sorry, my bad. Fixed

[VladislavFitz]

What's the goal of this change? I purposely used String as a simplest type to simplify testing as much as possible.

[NicFontana]

By declaring @Published private(set) public var hits: [Hit?] I wasn't capable anymore to set hits directly, so I had to use a sample HitsInteractor with sample data

[VladislavFitz]

Suggested change

	@Published private(set) public var hits: [Hit?]
	@Published public var hits: [Hit?]

This field is set from the InstantSearchCore module, so it cannot be declared as private(set) without compilation failure.

[NicFontana]

I didn't have any compilation failure targeting macOS and iOS. Am I missing something?