Runtime issues with dynamic properties in App types

[carson-katri]

Here's a small example:

struct TokamakDemoApp: App {
  @ObservedObject var count = Count(value: 0)

  var body: some Scene {
    WindowGroup("Tokamak Demo") {
        Text("\(count.value)")
    }
  }
}

crashes with:

RuntimeError: memory access out of bounds
    at strlen (<anonymous>:wasm-function[47490]:0xb8643d)
    at $sSS7cStringSSSPys4Int8VG_tcfCTf4nd_n (<anonymous>:wasm-function[20390]:0x556bae)
    at $sSS7cStringSSSPys4Int8VG_tcfC (<anonymous>:wasm-function[20389]:0x556b68)
    at $s7Runtime19NominalMetadataTypePAAE11mangledNameSSyF (<anonymous>:wasm-function[8313]:0x2150cb)
    at $s7Runtime13ClassMetadataV10toTypeInfoAA0eF0VyF (<anonymous>:wasm-function[8168]:0x20e6a9)
    at $s7Runtime13ClassMetadataVAA19TypeInfoConvertibleA2aDP02todE0AA0dE0VyFTW (<anonymous>:wasm-function[8192]:0x20f2e3)
    at $s7Runtime8typeInfo2ofAA04TypeC0VypXp_tKF (<anonymous>:wasm-function[8660]:0x223bcd)
    at $s11OpenCombine16ObservableObjectPA2A0cD9PublisherC0d10WillChangeE0RtzrlE06objectfG0AEvg (<anonymous>:wasm-function[1705]:0x88986)
    at $s11TokamakDemo5CountC11OpenCombine16ObservableObjectAadEP16objectWillChange0giJ9PublisherQzvgTW (<anonymous>:wasm-function[16762]:0x443ce3)
    at $s11TokamakCore14ObservedObjectV16objectWillChange11OpenCombine12AnyPublisherVyyts5NeverOGvg (<anonymous>:wasm-function[11415]:0x2b6422)

This occurs in Views as well.

Update:

@State also has some issues. Here's a small example:

struct TokamakDemoApp: App {
  @State private var count: Int = 0
  var body: some Scene {
    WindowGroup("Tokamak Demo") {
      Button("\(count)") { count += 1 }
    }
  }
}

The count starts with the number 207884. 🤨

[MaxDesiatov]

Thanks, I'm looking into it.

[MaxDesiatov]

The state becomes corrupt within the injectEnvironment function. Making it generic instead of operating on Any fixes it for the first injection, which is called from makeMountedApp where the generic type of the app still known, but as soon as it's wrapped in _AnyApp it's erased completely. I'm looking for a solution/workaround now, but lmk if you have any ideas.

[MaxDesiatov]

The current signature I use is func injectEnvironment<T>(from environmentValues: EnvironmentValues, into element: inout T), it doesn't require changing anything in the body.

[MaxDesiatov]

I'm still not sure if it's a bug in the Runtime library or Swift runtime metadata itself. I'm seeing weird behavior with type(of:) API too, which is a part of the standard library. I haven't got good ideas how to diagnose or reproduce it outside of Tokamak for a good test case, but thinking of a workaround, I wonder if adding another generic argument to StackReconciler of App type would help. We then would always have the app type known statically. It is already known, we just type-erase it when wrapping in _AnyApp. We probably wouldn't even need _AnyApp type to exist then.

What remains a mystery to me is why this doesn't happen to AnyView and View types. The bigger problem will be if this is reproducible for _AnyScene and DynamicProperty fields on custom scenes, I'll try to verify this in my mounted-scene branch tomorrow.

[carson-katri]

I actually have seen this issue with ObservedObject in Views while working on Toolbars, but I haven’t been able to replicate it on a smaller scale.