occasional crash when using the new async observation APIs and performing a swipe to delete on a SwiftUI List view

[yqiang]

I'm seeing an occasional crash when using the swipe-to-delete affordance created from the onDelete(perform: ...) modifier.

The stack trace is:

NSInternalInconsistencyException
Invalid update: invalid number of items in section 1. The number of items contained in an existing section after the update (6) must be equal to the number of items contained in that section before the update (6), plus or minus the number of items inserted or deleted from that section (0 inserted, 1 deleted) and plus or minus the number of items moved into or out of that section (0 moved in, 0 moved out).

My setup is that I have a SwiftUI List view and I start a ValueObservation using the .task modifier on the View, something like this:

  func observeData() async {
    let observation = ValueObservation
      .tracking { db -> ViewModelData in
        // my code to fetch data
        return ViewModelData(..)
      }
      .values(in: appDatabase.reader, scheduling: .immediate)

    do {
      for try await viewModelData in observation {
        // code to create a newSections
        sections = newSections
      }
    } catch {
      reportIssue(error)
    }
  }

The function to delete the item in question is something like this:

  func delete(indices: IndexSet) {
    do {
      try appDatabase.writer.write { db in
        for index in indices {
          var goal = goals[index]
          try goal.updateChanges(db) { goal in
            goal.deletionDate = try db.transactionDate
          }
        }
      }
    } catch {
      log.error("\(error)")
    }
  }

I think what might be happening is that when you do the swipe to delete, the item gets immediately removed from the view and then when the collection view backing the SwiftUI list view queries the datasource, it's seeing the old data, i.e., the try await hasn't run yet.

Does this hypothesis make sense? If so, how would one go about fixing this using the new async API?

[groue]

Hello @yqiang, thanks for the report!

I can not reproduce. Would you mind providing a minimal reproducible example?

[yqiang]

For sure - will try.

[yqiang]

I haven't been able to recreate the crash with a minimal example yet, but while trying to understand the crash further, I discovered that changing:

.values(in: appDatabase.reader, scheduling: .immediate) to .values(in: appDatabase.reader) seems to resolve the crash.

Will keep trying to create a repro project.

[yqiang]

No luck reproducing so far. Going to close this for now and reopen if I have any new information to provide.

[groue]

OK, @yqiang. I will be happy to help when a repro is ready.