swiftui_bugged_preferences.swift

struct PKey: PreferenceKey {
  static var defaultValue: Int?
  static func reduce(value: inout Int?, nextValue: () -> Int?) {
    print(#function, "value:", value as Any, "nextValue:", nextValue() as Any)
    value = nextValue()
  }
}

struct ContentView: View {
  let condition = true

  var body: some View {
    VStack
      .init {
        Text("A")
          .anchorPreference(
            key: PKey.self,
            value: .bounds,
            transform: { _ in 42 }
          )

        // `Optional` view with `.some(...)` case.
        Color?.some(.red)

        // This triggers yet another call to `reduce`
        if condition {
          Circle()
        }

        // This is a true NOP
        if condition == false {
          Rectangle()
        }

        // This does not trigger `reduce`
        Color?.none

        // This triggers another call to `reduce`
        Circle?.some(Circle())
      }
      .overlayPreferenceValue(PKey.self) { value -> Text in
        Text("Test").foregroundColor(Color.green)
      }
  }
}

// UNEPEXTED OUTPUT:
// reduce(value:nextValue:) value: Optional(42) nextValue: nil
// reduce(value:nextValue:) value: nil nextValue: nil
// reduce(value:nextValue:) value: nil nextValue: nil

// EXPECTED BEHAVIOR:
// `reduce` should not be called even once in this example.