Trouble with #Preview

[kevvdevv]

Hi, I had a question about prepareDependencies in the context of a #Preview block. I believe that prepareDependencies is being properly accessed, because I can see the logs of Sharing-GRDB setting up my database and populating seed data. I'm still ending up with the error 🟣 StructuredQueriesGRDBCore/DefaultDatabase.swift:42: A blank, in-memory database is being used. To set the database that is used by 'SharingGRDB' in a preview, use a tool like 'prepareDependencies'

My code runs as expected in the simulator and in the test environment (which runs the @FetchAll). It just isn't running as expected in the Preview environment (zero ListCollections are fetched even though I've seeded the database with one). Welcome any thoughts or ideas folks have.

Here's my #Preview code. The appDatabase function runs pretty similarly to the example in the Reminders app.

#Preview {
    let _ = prepareDependencies {
        $0.defaultDatabase = try! appDatabase()
    }
    @FetchAll(ListCollection.order(by: { $0.createdDate.desc() })) var listCollections
    
    Form {
        ForEach(listCollections) { collection in
            Text(collection.name)
        }
    }
}

[mbrandonw]

Hi @kevvdevv, can you try using @Previewable in front of @FetchAll? That pushes the property wrapper into the inner view that #Preview creates, which will help with it observing the state properly.

[kevvdevv]

Hi @mbrandonw, Thanks for that suggestion. I added the @Previewable in front of the @FetchAll, and I'm still getting the same error message about the blank, in-memory database. Here's my current code (the count is showing zero):

#Preview {
    @Previewable @FetchAll(ListCollection.order(by: { $0.createdDate.desc() })) var listCollections
    
    let _ = prepareDependencies {
        $0.defaultDatabase = try! appDatabase()
    }
    
    Form {
        Text("Count: \(listCollections.count)")
        ForEach(listCollections) {collection in
            Text(collection.name)
        }
    }
}

[mbrandonw]

Hi @kevvdevv, sorry I led you astray. The problem is that in a preview like this:

#Preview {
  @Previewable @FetchAll var values = …
  let _ = prepareDependencies { … }
  SomeView()
}

…is transformed into something like this:

struct WrapperView: View {
  @FetchAll var values = …
  var body: some View {
    let _ = prepareDependencies { … }
    SomeView()
  }
}

And so you can see that @FetchAll is unfortunately accessed before prepareDependencies.

The only workaround I can think of is to extract your view into a separate view so that it can delay when @FetchAll is actually accessed:

#Preview {
  let _ = prepareDependencies {
    $0.defaultDatabase = try! appDatabase()
  }
  InnerView()
}
private struct InnerView: View {
  @FetchAll(ListCollection.order(by: { $0.createdDate.desc() })) var listCollections
  var body: some View {
    …
  }
}

I just don't think there is a way to properly sequence the set up of the database and the access of the state. Theoretically this is what preview traits should be for, but we haven't had any luck in using them yet. They just don't seem to execute when we expect...

[kevvdevv]

Hi @mbrandonw. Thanks for working with me through this. I'm still not having success. The database is created and being seeded, and the defaultDatabase is being set, all before the 🟣 StructuredQueriesGRDBCore/DefaultDatabase.swift:42: A blank, in-memory database is being used. error message appears in the log.

Here's my current code:

#Preview {
    let _ = prepareDependencies {
        $0.defaultDatabase = try! appDatabase()
    }
    InnerView()
}

fileprivate struct InnerView: View {
    @FetchAll(ListCollection.order(by: { $0.createdDate.desc() })) var listCollections
    var body: some View {
        Form {
            Text("Count: \(listCollections.count)")
            ForEach(listCollections) {collection in
                Text(collection.name)
            }
        }
    }
}

[kevvdevv]

It turned out that there was a configuration error in my project that made things not work. Thanks for helping me out, @mbrandonw.