BindableState + SharedState strategy reseting binding

[derekc00]

Hi, I'm attempting to use a BindableState property inside a state that is working on a subset of a parent state. I am experiencing a compiled file, but when I change the property's value, it resets back to its original value.

To provide more context, consider the 01-GettingStarted-SharedState case study.

I added a simple BindableState Bool to the Profile State, added an action, BindingReducer, and toggle object.
I placed the changes in this gist.

In this gif, I am repeatedly clicking the toggle button. This is the console output:

  Root.Action.shared(
    SharedState.Action.profile(
      SharedState.Profile.Action.binding(
        BindingAction.set(
          WritableKeyPath<State, BindableState<Bool>>,
          true
        )
      )
    )
  )
  (No state changes)

However, I am expecting state changes to the isOn BindableState property.

I'm 1 month into learning TCA and a few months into learning SwiftUI - And having a blast most of the time. I'm all ears to any hints or directions.

[tgrapperon]

Hey @derekc00, as you can see:

struct SharedState: ReducerProtocol {
  struct State: Equatable {
    var counter = Counter.State()
    var currentTab = Tab.counter

    var profile: Profile.State {
      get {
        Profile.State(
          currentTab: self.currentTab,
          count: self.counter.count,
          maxCount: self.counter.maxCount,
          minCount: self.counter.minCount,
          numberOfCounts: self.counter.numberOfCounts
        )
      }
      set {
        self.currentTab = newValue.currentTab
        self.counter.count = newValue.count
        self.counter.maxCount = newValue.maxCount
        self.counter.minCount = newValue.minCount
        self.counter.numberOfCounts = newValue.numberOfCounts
      }
    }
  }
  …

This means that the Profile.State value that is sent to your feature is a computed property. A new value will be derived each time, but you've not externalized the storage of your new flag, so the updated isOn value is lost. One thing you can do is adding a private storage in SharedState.State:

struct SharedState: ReducerProtocol {
  struct State: Equatable {
    var counter = Counter.State()
    var currentTab = Tab.counter
+   var isOnStorage: Bool = false
    var profile: Profile.State {
      get {
        Profile.State(
          currentTab: self.currentTab,
          count: self.counter.count,
          maxCount: self.counter.maxCount,
          minCount: self.counter.minCount,
          numberOfCounts: self.counter.numberOfCounts,
+         isOn: self.isOnStorage
        )
      }
      set {
        self.currentTab = newValue.currentTab
        self.counter.count = newValue.count
        self.counter.maxCount = newValue.maxCount
        self.counter.minCount = newValue.minCount
        self.counter.numberOfCounts = newValue.numberOfCounts
+       self.isOnStorage = newValue.isOn
      }
    }
  }
  …

When there are many internal fields that you want to track, and when you're able to create a State value in absolute, without local parameters¹, an alternative is to use a complete private Profile.State storage that you update instead of recreating a new value:

 struct State: Equatable {
    var counter = Counter.State()
    var currentTab = Tab.counter

    private var profileStorage = Profile.State()

    var profile: Profile.State {
      get {
        var state = profileStorage
        state.currentTab = self.currentTab
        state.count = self.counter.count
        state.maxCount = self.counter.maxCount
        state.minCount = self.counter.minCount
        state.numberOfCounts = self.counter.numberOfCounts
        return state
      }
      set {
        self.profileStorage = newValue
        // Synchronize other properties
        self.currentTab = newValue.currentTab
        self.counter.count = newValue.count
        self.counter.maxCount = newValue.maxCount
        self.counter.minCount = newValue.minCount
        self.counter.numberOfCounts = newValue.numberOfCounts
      }
    }
  }
  …

In both cases, the state of the isOn flag is persisted in the SharedState.State in some form or another, and your feature should work.

As an aside, I don't think that SharedAction needs to be BindableAction here. You're only using the Profile feature. You can probably remove all the Bindings stuff from the SharedState domain.

Footnotes

1. It won't work here, as you can't create a Profile.State() like that, but I didn't want to file the missing arguments in the init with placeholder values to make the presentation clearer. Imagine that each property in Profile.State has a default value and that you can create Profile.State(), or that you create var profileStorage = Profile.State(currentTab: .profile, count: 0, … instead. ↩

[derekc00]

Wow, thanks for a great explanation and quick reply, Thomas! I understand, now, how the computed property for the profile struct causes my binding to reset.

Cheers,
Derek