[Proposal] Utilizing primary associated types to inject view stores to views

[Pearapps]

What it Does

Shows example of a proposed new best practice of injecting view stores into views.

Dependent on the PR to add primary associated types to the ViewStore protocol.

This injection has several advantages:

• Timing: We now have MockViewStore, which can be injected by SwiftUI previews and always immediately returns the data. We no longer rely on each individual view store to send an initial value with correct timing for previews and (theoretical) snapshot tests – we can now guarantee it. Control over timing in this manner is a prerequisite for using async/await & AsyncAlgorithms in view stores in place of Combine.
• Mocks: We only have to mock each store's view state now, which contains a subset of the data we were mocking before. For example, we don't need the whole provider for SwiftUI previews, we only use the Photos (even though we are, for now, just using the provider to generate those photos).
• Extensibility: Since we are injecting an object conforming to the ViewStore protocol, we can now conditionally return different view stores, compose them together, or do any number of combinations and pass in the result to a view. This means that if we wanted to add automatic analytics logging, for example, we could write a small AnalyticsViewStore (see below example) wrapper and inject a view store, passing that to a view.

AnalyticsViewStore example

final class AnalyticsViewStore<V, A: Loggable>: ViewStore {
    let otherViewStore: any ViewStore<V, A>

    init(otherViewStore: any ViewStore<V, A>) {
        self.otherViewStore = otherViewStore
    }

    var viewState: V {
        return otherViewStore.viewState
    }

    func send(_ action: A) {

        let log = action.log
        // log this to crashlytics or whatever ^
        
        otherViewStore.send(action)
    }
}

For something like this, you could transparently pass this view store instead of the PhotoListViewStore in this project!

let viewStore/*: PhotoListViewStoreType*/ = AnalyticsViewStore(otherViewStore: PhotoListViewStore(provider: /* ....*/))

How I Tested

• Made sure the app built
• Made sure the tests pass

Notes

Plan is to discuss this in the next engineering meeting!

[Pearapps]

@autoclosure to avoid any problems with having stores being incorrectly created multiple times. See the initializer on StateObject as well

[Cordavi]

Out of curiosity, why did you move these onto the type over the store?

[Pearapps]

You have to, since its generic in the view you can't talk to the concrete store type directly - its also an unintended advantage because it is a bit more strict with what you can do and protects the ViewStore concept with extra Published properties and the like

[Cordavi]

With this change, does this really even need to be a provider? Couldn't you just pass the photos from say Photo+SampleData extension etc?

[Pearapps]

This is used for the unit tests, still -- in the future we will probably move this into a test target or a test helper target, to your point, though.