@Table Draft initializer in #Preview

[jacksonarich]

When I compile this code:

@Table struct Item {
  let id: Int
}

#Preview {
  let x = Item.Draft()
}

...I get this compile error: "Missing argument for parameter #1 in call." The error stems from the use of the Draft initializer inside the Preview macro. If I move the declaration of x out of the #Preview it compiles normally.

I thought perhaps it was related to the macro-declared Draft type being initialized inside another macro, but it compiles normally inside a swift-testing #expect or #require statement.

I tried importing the other libraries like StructuredQueriesCore just to see if there's a piece I'm missing, but that didn't help any either.

There is a workaround where you declare the item outside the #Preview and then reference it from inside:

extension Item {
  static let draft = Item.Draft()
}

#Preview {
  let x = Item.draft
}

This compiles normally, but obviously it's just extra boilerplate. Has anyone else seen this same issue?

[mbrandonw]

Hi @jacksonarich,

I thought perhaps it was related to the macro-declared Draft type being initialized inside another macro, but it compiles normally inside a swift-testing #expect or #require statement.

It definitely is macro-related, but it may be more of an Xcode bug with #Preview than a general macro bug. You can reproduce this same problem with @TaskLocal, which is now a macro:

import SwiftUI
@TaskLocal var count = 0
#Preview {
  $count.withValue(0) {  // 🛑
    Text("\(count)")
  }
}

Another work around is to just use PreviewProvider instead of #Preview. It works just fine with macros.

We have filed a feedback about this issue, and we have asked others that bring up this problem to dupe the feedback. It's the only way to get it fixed someday in the future.