Question to understand data flow in TodosViewModel

[magicwave]

I've just read Introducing: identified collections, where example code is given of a function, func todoCheckboxToggled(at id: Todo.ID) async that mutates the state of an element in the published todos IdentifiedArray.

The first line in the function, self.todos[id: id]?.isComplete.toggle(), appears to directly mutate the todo element state, which I assume will cause the SwiftUI view driven by the viewModel to update. I understand this as mutating local viewModel data for the purpose of immediately updating the todo list view?

Next is a do/try/catch block where presumably persistent todo data is mutated asynchronously via an apiClient instance.

Further, I assume that the viewModel implements an observer for changes to the persistent data, something along the lines of:

init() {
    self.apiClient.$publishedToDos
        .assign(to: &$todos)
}

With these assumptions, I wonder why we toggle local todo state in addition to updating the (again presuming) more canonical data source via apiClient? In that scenario it seems we have two sources of truth: local and persistent. I might expect that we would not change the local data source in the ToDoViewModel toggle function, and simply update, and let the list view react to, the change in the persistent data source via the publisher.

[kamcma]

Further, I assume that the viewModel implements an observer for changes to the persistent data, something along the lines of:

init() {
    self.apiClient.$publishedToDos
        .assign(to: &$todos)
}

This is a big assumption! For the purposes of the example, I think it's far more reasonable to assume apiClient is a simple CRUD HTTP client with no observability mechanism.

The problem of keeping local memory state and remote server state in sync is simply a higher-level problem than what this library exists to solve, and I don't think the example code was attempting to comment on it.

Edit: your question seems to assume apiClient is holding onto todos internally. That would actually be three sources of the truth: the remote API, whatever apiClient is holding onto internally, and the TodosViewModel. The example really only assumes the remote API and TodosViewModel, and apiClient is more stateless. And again: keeping those in sync is outside the scope of the library.