Basic guidance on how to use with TCA

[jaredh159]

Thanks so much for the library, already using it in a prototype and loving it.

I'm having trouble figuring out how I would integrate this library and approach with TCA. I'm certain it's straightforward to do so, but for some reason the mental model of how the pieces would fit together just won't click for me. I looked for a case study involving TCA, but don't see any.

Would you be willing to give just a few sentences, or a few short code snippets of how you would imagine this library being integrated into a reducer-driven TCA app? Just enough to put me on the scent would be great.

Many thanks!

[stephencelis]

When we use observable models in our demos, they mostly map to TCA reducers:

• The view holds onto a store instead of the model,
• It accesses store state instead of model properties, and
• It sends actions to the store instead of invoking model methods

Otherwise things look mostly the same. So you could take the SyncUps demo, which uses observable models, and consider how it would be ported.

But the bottom line is you can use @FetchAll (and @FetchOne and @Fetch) wherever you want, or wherever @Shared properties are held. This includes reducer state, effects, even directly in a view if you want. And you can use @Dependency(\.defaultDatabase) to write to the database.

Let us know if you hit any particular stumbling blocks and we'll see if we can help!

[lukesutton]

I have a similar question, and I've managed to integrate SharingGRDB with TCA, but I'm struggling to find a way to have dynamic queries work.

Specifically, I have defined a @Fetch property on my State, which should load data via a FetchKeyRequest e.g.:

@Fetch(Albums(), animation: .default) var albums = Albums.Value()

Which I then update as required in the reducer:

return .run { _ in 
  state.$albums.load(Albums())
}

The issue I'm having is that I'm not able to call state.$albums.load(...) in my reducer, because the closure passed to .run captures the state variable. I understand why this is an error, but I'm stuck on trying to resolve it.

Do you have any guidance on what I could do in this scenario? Thanks!

[mbrandonw]

The issue I'm having is that I'm not able to call state.$albums.load(...) in my reducer, because the closure passed to .run captures the state variable. I understand why this is an error, but I'm stuck on trying to resolve it.

You can always capture the pieces of state you need in .run:

return .run { [albums = state.$albums] in 
  albums.load(Albums())
}

[lukesutton]

Well… I just learnt something new about Swift! Thank you.

[dominikmayer]

I'm struggling a bit with testing SharingGRDB in the Composable Architecture.

Normally the TestStore would detect changes to properties, even shared ones.

But with a shared database this seems not to work. So I have to assert on the state of the database, which means I could miss a change.

An alternative would be to treat the database as a dependency and create a DatabaseClient similar to, let's say, a FileSystemClient, where I would assert on the overridden test methods.

But then all the @Fetch properties in the reducer would not update as I'm not really interacting with the (in-memory) test database.

What's the best way to test SharingGRDB?

[mbrandonw]

But with a shared database this seems not to work. So I have to assert on the state of the database, which means I could miss a change.

This is just how things have to be done if you are using @FetchAll. There really is no way to recover exhaustive testing for database queries, but we think that's ok. All other state in your features can be exhaustive asserted on.

[dominikmayer]

Okay, thanks for the explanation. So I'm not missing anything.

[OddMagnet]

Sorry to dig this up again, but I believe that my problem is very similar, so I hope this is the right place to ask for help (rather than creating a new discussion). While I have something that works for me, it doesn't quite feel like the right way.

In my Reducer the State looked like this:

public struct State {
    var filter: Filter
    @FetchAll var issues: [Issue]

    init(filter: Filter) {
        self.filter = filter
        _issues = FetchAll(issuesQuery, animation: .default)
    }

    var issuesQuery: some StructuredQueriesCore.Statement<Issue> {
        Issue
            .where { ... }
            .select { ... }
    }
}

Then in my Reducer's body I tried a couple of ways of updating the query

return .run { [issues = state.$issues, query = state.issueQuery] _ in
    try await issues.load(query)
}

Problem: Capture of 'query' with non-Sendable type 'some Statement' in a '@sendable' closure

Next I tried this:

return .run { [state] _ in
    try await issues.load(query)
}

Problem: Capture of 'state' with non-Sendable type 'Content.State' in a '@sendable' closure
I could make State Sendable, but that doesn't feel right

Finally, I moved the var issueQuery out of state as a static function on my reducer, that takes the filter as an argument (not shown in code block)

return .run { [issues = state.$issues, filter = state.filter] _ in
    try await issues.load(Self.issuesQuery(for: filter))
}

This seems to work, but I really don't like needing a static function for this.
Is a sendable state the correct solution here, or am I missing something?

[mbrandonw]

Hi @OddMagnet, the error is that your issuesQuery needs to be sendable, and so you can just state that it is in its declaration:

var issuesQuery: some StructuredQueriesCore.Statement<Issue> & Sendable {
  …
}

the Statement protocol does not force sendability because that is not appropriate, but often statements are sendable and so you can just use & Sendable to indicate that to the compiler.

[OddMagnet]

Thanks for the fast reply!

I also noticed that I was able to capture state without making it sendable, somehow my Reducer was public (I created a snippet in Xcode and only later corrected it to not make my boilerplate reducer's public, so I guess from that).

That solution seems even better, now I can minimally capture only the issues and issueQuery properties! :D