CoreData / SwiftUI List selection question

The sample code is able to pass an optional managed object type Trip? to hold the selection rather than the ID type. When I try to replicate that behavior, I can't.

Show the code you tried that didn't work. What is the problem you have with the code you tried? Do you get a build error? If so, what is the error? Do you get unexpected selection behavior? If so, state what you expect to see and what you see with the code you changed.

The code:

@FetchRequest(sortDescriptors: []) private var players: FetchedResults<PlayerEntity>
@State private var selectedPlayer : PlayerEntity?

var body: some View {
    
    NavigationSplitView {
        List(players, selection: $selectedPlayer) { player in
            Text(player.shortName ?? "")
        }
        .navigationTitle("Players")
    } detail: {
        if let player = selectedPlayer {
            Text("Do Something")
        }
    }
}

The list appears, but is not selectable, and the detail view doesn't get presented. I also tried using the same List/ForEach style of the sample code.

Hi @tdc ,

For selection in NavigationSplitView I recommend holding the selection with the ID since there is less overhead in checking the UUID vs the entire object.

That being said, for the selection variable to update correctly when not using ID, your selection variable must be of type PlayerEntity? and the PlayerEntity type must conform to Identifiable. You'll also want to use \.self as the id in your List initializer:

@State private var selectedPlayerID : PlayerEntity?
...
List(players,  id: \.self, selection: $selectedPlayer) {
..
}

Passing in an explicit id of self passes the entire object as the selection, which means that the selection variable and the id parameter of List now match. This should allow you do the selection in the same way as the sample, but again I would stick with the ID. The reason you did not need to pass an explicit id parameter in your original example is because PlayerEntity conforms to Identifiable and your selection variable is a UUID, so the id is inferred.

Ah yes, the CoreData Identifiable, which I forgot to mention above. Since you do not directly conform to Identifiable with CoreData entities, the only way to conform to it is to include an attribute of type UUID. You'll notice in that sample that there is no attribute of id (or any other name) of type UUID, which means Trip is not Identifiable and therefore does not infer an id in the List. This is why they do not need to pass an explicit self id parameter. If you play around with the sample and add an id attribute of type UUID, you'll see that the selection stops working. Conversely, in your code, if you remove the UUID attribute, you probably will not need to pass in an explicit id either.

Switching over to another entity that doesn't have an explicit uuid : UUID attribute didn't make a difference. And from what I can tell, by default CoreData entries conform to Identifiable so that doesn't make sense to me.

OK, so I have figured out what was going on in SampleTrip. It turns out that TripListItem includes a NavigationLink so copying that pattern

var body: some View {
		NavigationSplitView {
			List(players, selection: $selectedPlayer) { player in
				NavigationLink(value: player) {
					Text(player.shortName ?? "")
				}
			}
			.navigationTitle("Players")
		} detail: {
			if let p = selectedPlayer {
				Text("detail \(p.shortName ?? "")")
			} else {
				Text("detail empty")
			}
		}

Now works. So in this case, I don't need to pass in id. Any thoughts on whether using the NavigationLink here is the preferred approach? At least part of the mystery is resolved!