FB12969716

FB12969716.swift

# FB12969716

// FB: `Form` and 
//
// Paste this into a fresh SwiftUI project's ContentView.swift to see a bug with `Form` views with
// conditional child views that use `@State`.
//
//  1. Tap "Toggle" to show the child view with a new model
//  2. Tap "+" in the child view to increment the model's state from 0 to another number
//  3. Tap "Toggle" two times to hide the child and then show it again
//
// Note that the child view does not show "0", as expected with the newly assigned model, but
// instead shows the previous incremented count. The child view's `@State` is still holding onto the
// old stale model, _not_ the model that lives in the parent `ContentView`.
//
// Workarounds:
//
//   * Don't use `Form`. If you swap `Form` out for a `VStack`, the child view uses the freshest
//     model for state.
//   * Drop the use of `@State`.

import SwiftUI

@Observable
class Model {
  var count = 0
}

struct ContentView: View {
  @State var model: Model?

  var body: some View {
    Form {  // 👈 Change this to 'VStack' to fix the problem
      Button("Toggle") {
        model = model == nil ? Model() : nil
      }
      if let model = model {
        ChildView(model: model)
      }
    }
  }
}

struct ChildView: View {
  @State var model: Model  // 👈 Or remove '@State'

  var body: some View {
    HStack {
      Text("\(model.count)")
      Button("+") {
        model.count += 1
      }
    }
  }
}

#Preview {
  ContentView()
}