IfLetStore .onAppear or .task gotcha

[tgrapperon]

I've encountered a setup where IfLetStore's were not behaving as I would expect in the first place. This is not a bug per se, but the documentation can maybe be extended to mention this configuration.

In my setup, I have a list of optional states, each corresponding to the value of some enum (the states need to stay alive once initialized, even if the case changes, so I'm not using SwitchStore, but it works in the same spirit).

At the beginning, all the states are nil and some onAppear sends an action to initialize the correct state corresponding to the current enum's value. This non nil state then activate some IfLetStore and the view is populated. In the body of the view, I had something like:

Group {
  switch viewStore.kind {
    case .first: IfLetStore(store.scope…
    case .second: IfLetStore(store.scope…
  }
}
.onAppear { viewStore.send(.initialize) }

The onAppear method wasn't called. I understand why now, but I assumed it would be.
If some else: view is provided on any of the IfLetStore, onAppear is called as expected.

In other words, Group of IfLetStore's are not triggering .onAppear or .task when all the states are nil. I think this is naturally expected for individual IfLetStore's, but this can be surprising at first for Groups.

I don't know if it's worth adding a note in the documentation of IfLetStore. I didn't find a way to explain it concisely.

[mbrandonw]

Hey @tgrapperon, yeah this is definitely tricky, but it is indeed how SwiftUI and TCA is expected to work, and this exact thing would happen in vanilla SwiftUI.

It's tripped me up a number of times, but you have to think of view modifiers applied to Group as being applied to every view inside. So:

Group {
  IfLetStore(...)
  IfLetStore(...)
}
.onAppear { ... }

Is really:

IfLetStore(...)
  .onAppear { ... }
IfLetStore(...)
  .onAppear { ... }

Further, IfLetStore is really just:

if condition {
  content
}

so your original code further transforms into something like this:

if condition {
  content
    .onAppear { ... }
}
if condition {
  content
    .onAppear { ... }
}

And so from this it becomes more clear that .onAppear is only going to be called when something inside the IfLetStore is actually shown.

If you really need to kick off the initialization work without there being a view visible on the screen you might have to resort to some hacky things, such as .background(...) with a hidden view that has the .onAppear.

[tgrapperon]

I think it works the same for ForEach. I've just looked at Group documentation, and they mention this case exactly. I've should have checked this before opening the issue.
I've solved my problem by initializing some substate manually if needed when it is derived from the root state.
I think replacing the Group by a ZStack would have been enough otherwise.
Thanks for the response!

[mbrandonw]

Also I'm going to convert this to a discussion since it's not really a bug with the library.