Memory churn when collecting CStateFlow of List due to === test.

[darronschall]

I have a viewModel that exposes a CStateFlow of List that I'm using in SwiftUI. It looks like this:

private val _items = MutableStateFlow<List<Item>>(listOf()).cMutableStateFlow()
var items: CStateFlow<List<Item>> = _items.cStateFlow()

In my SwiftUI view, I use the state helper to subscribe to the stateFlow:

struct ExampleView: View {
  @ObservableObject viewModel = ExampleViewModel()

  var body: some View {
    List(viewModel.state(\.items)) { (item: Item) in
      Text("Item: \(item.id)")
    }
  }
}

This setup is enough to trigger memory churn. Even though items never changes in the viewModel in this example, there's extremely high CPU usage and memory allocations for what should be an idle view.

I did some digging and have a general idea of the root cause. From what I can tell, asking for stateFlow.value inside the state method allocates a new array, every time. I don't know why this happens, perhaps it has something to do with the Kotlin/Swift(ObjC) bridge. You can see this for yourself by adding some simple logging inside of the method:

var value1 = stateFlow.value
var value2 = stateFlow.value
var value3 = stateFlow.value
            
print("value1 = \(value1)")
print("value2 = \(value2)")
print("value3 = \(value3)")
print("stateFlow.value = \(stateFlow.value)")
print("stateFlow.value = \(stateFlow.value)")
print("same instances = \(stateFlow.value === stateFlow.value)")

This will output, in an infinite loop, something like this:

value1 = Optional(<SharedKListAsNSArray 0x600001987900>(

)
)
value2 = Optional(<SharedKListAsNSArray 0x6000019be500>(

)
)
value3 = Optional(<SharedKListAsNSArray 0x6000019aba80>(

)
)
stateFlow.value = Optional(<SharedKListAsNSArray 0x6000019abcc0>(

)
)
stateFlow.value = Optional(<SharedKListAsNSArray 0x6000019f76c0>(

)
)
same instances = false

The solution is to change the equality test of NSArray state helper at https://github.com/icerockdev/moko-mvvm/blob/develop/mvvm-flow/apple/xcode/mokoMvvmFlowSwiftUI/ViewModelState.swift#L86.

Rather than equals: { $0 === $1 }, it should use isEqual(to:) to compare array contents, equals: { $0!.isEqual(to: $1 as! Array<T>) },

After making this change locally, the memory churn goes away.