Error handling

[stephencelis]

@Shared persistence strategies can fail to load or save, but we have generally silently ignored such failures. This PR explores introducing loadError and saveError properties on the projected value, similar to SwiftData's @Query's fetchError.

@Shared(.fileStorage(.syncUps)) var syncUps: [SyncUp] = []

var body: some View {
  if let loadError = $syncUps.loadError {
    ContentUnavailableView {
      Label("Failed to load sync-ups", systemImage: "xmark.circle")
    } description: {
      Text(loadError.localizedDescription)
    }
  } else {
    ForEach(syncUps) { syncUp in
      // ...
    }
  }
}

[VilemKurz]

Hi, thank you for the pr, much needed and appreciated 👍. Wondering, will we be able to mock (pre-set) load/save errors in previews and tests?

[stephencelis]

@VilemKurz As long as you mock the shared key's strategy to make it throw, then you should be able to test that $shared.loadError or $shared.saveError is populated.

Feel free to give this branch a spin and report back! Ideally we have some feedback from the community before merging.