Observing stops collecting when body becomes an EmptyView()

[valentinilk]

Hey,

I'm observing the following issue. Whenever the only child of Observing in SwiftUI becomes an EmptyView(), SKIE's Observing view will stop collecting my flow.

Steps to reproduce:

// Some ViewModel in .kt
val viewState = flowOf(listOf("foo", "bar"))
    .map { ScreenState.Available(it) }
    .stateIn(viewModelScope, SharingStarted.Lazily, ScreenState.Initial)

// ScreenState
sealed interface ScreenState {
    data object Initial : ScreenState
    data class Available(val states: List<String>) : ScreenState
}

// Some View in .swift
private let viewModel = SomeViewModel()

var body: some View {
    VStack {
        // State independent UI
        Observing(viewModel.viewState) { viewState in
            switch onEnum(of: viewState) {
            case .initial: EmptyView()
            case .available(let state): Text("Available")
            }
        }
    }
    .navigationTitle("Some Title")
}

This will never display the text Available on my screen. Not even the logging statements in my flows get fired, as if the flows aren't being collected.

Funnily enough, I can fix it by simply replacing the EmptyView() with a simple Text.

// Some View in .swift
private let viewModel = SomeViewModel()

var body: some View {
    VStack {
        // State independent UI
        Observing(viewModel.viewState) { viewState in
            switch onEnum(of: viewState) {
            case .initial: Text("Loading") // <--------------- EmptyView() replaced with the text
            case .available(let state): Text("Available")
            }
        }
    }
    .navigationTitle("Some Title")
}

Same behavior can be observed with a ForEach, which might return an EmptyView() under the hood, if the list I'm handing in is empty?!

// Some ViewModel in .kt
val viewState = flowOf(listOf("foo", "bar"))
    .stateIn(viewModelScope, SharingStarted.Lazily, emptyList())

// Some View in .swift
private let viewModel = SomeViewModel()

var body: some View {
    VStack {
        // State independent UI
        Observing(viewModel.viewState) { viewState in
            ForEach(viewState) { state in
                Text(state)
            }
        }
    }
    .navigationTitle("Some Title")
}

So the sample above won't display my list. But the following code works:

// Some View in .swift
private let viewModel = SomeViewModel()

var body: some View {
    VStack {
        // State independent UI
        Observing(viewModel.viewState) { viewState in
            Text("Hello") // <--------------- Added
            ForEach(viewState) { state in
                Text(state)
            }
        }
    }
    .navigationTitle("Some Title")
}

[leonardog89]

I have faced the same behavior in my app

[TadeasKriz]

Thanks for the report. This is because .task on EmptyView never does anything. We didn't want to add any extra views in the hierarchy for performance reasons, but perhaps we should just to make sure this never happens. I'll see if we can add a check if the view is EmptyView and wrap it in a ZStack in that case.

[valentinilk]

Thanks for the quick reply. That's what I assumed was going on, but wasn't sure, since I'm coming from Android.
Maybe adding that info to the docs would already be sufficient, at least it would have helped me.

Otherwise Observing is incredibly helpful. Thanks for creating those helper. :)