Optional reducer migrations

[natemann]

A majority of my reducers are optional. Looking at the docs and migration guides, I do not see how to convert an optional ReducerProtocol into an AnyRuder for migration purposes. Has anyone been successful in doing this?

[jshier]

Yes, you an adapt your new reducer into an AnyReducer and use the same optional and pullback modifiers you did before.

In my project, something like this:

expenseDomainReducer
    .pullback(state: \Identified.value, action: .self)
    .optional()
    .pullback(state: \SignedInState.selectedExpense,
              action: /SignedInAction.expense,
              environment: { ExpenseDomain.Environment(api: $0.api) })

became this:

AnyReducer { environment in
    ExpenseDomain(environment: .init(api: environment.api))
}
.pullback(state: \Identified.value, action: .self)
.optional()
.pullback(state: \SignedInState.selectedExpense,
          action: /SignedInAction.expense,
          environment: { ExpenseDomain.Environment(api: $0.api) })

[stephencelis]

In addition, when you fully migrate away from AnyReducer (it's soft-deprecated), you will end up with something like:

struct SignedIn: ReducerProtocol {
  struct State {
    // ...
  }
  enum Action {
    // ...
  }
  // ...
  var body: some ReducerProtocol<State, Action> {
    Reduce { state, action in
      // core feature logic
    }
    .ifLet(\.selectedExpense, action: /Action.expense) {
      Scope(state: \Identified.value, action: .self) {
        ExpenseDomain() // API propagated via @Dependency
      }
    }
  }
}

Navigation in this example is definitely clunky at the moment, but we have some very promising tools in development!

[mbrandonw]

We just updated our migration article to describe how to step-by-step migrate optional+pullback reducers:

Please let us know if you have any feedback on that.

[jshier]

May want to include environment passing or extracting values from the environment in that example, given your other examples move them into properties. Or just ignore the environment value in the example.

[mbrandonw]

Ah yes, good point. Just updated the doc, thanks!

[natemann]

These examples only seem to work if the parent and child ReducerProtocols are in the same module. When the child ReducerProtocol is in a separate module, things do not compile. For example

public static let reducer = AnyReducer<State, Action, Environment>.combine(

AnyReducer {
    RevenueSplitList(databaseClient: $0.databaseClient, uuid: $0.uuid)
}.optional()
    .pullback(
      state: \.revenueSplitList,
      action: /Action.revenueSplitList,
      environment: { 
        RevenueSplitList(
          databaseClient: $0.databaseClient,
          uuid: $0.uuid) }),

Does not compile. The compile thinks $0 in AnyReducer is RevenueSplitList. Even casting as Environment fails.

[jshier]

@natemann AnyReducer provides the environment in the initializer closure so there's no need to do anything to get it.

[mbrandonw]

Oh good point, that's pointing out a problem in our migration guide. By the time you have gotten to this point in the refactor there is no more child environment, and so you can actually just use a tuple:

environment: { 
  (
    databaseClient: $0.databaseClient,
    uuid: $0.uuid
  )
},

Then the RevenueSplitList can pluck off what it needs from the tuple.

We actually do quite a bit of reducer migration work in next week's episode, so hopefully seeing us do it live will help you see how these things go in complex applications. I will also update the migrate strategy to mention this.

[mbrandonw]

Oh yeah, and actually what @jshier is saying is even simpler. Just use environment: { $0 } to pass along the parent environment, and then inside AnyReducer you can pluck out whatever you need.

[natemann]

ok, this works

AnyReducer {
      RevenueSplitList(databaseClient: $0.1, uuid: $0.2)
    }.optional()
      .pullback(
        state: \.revenueSplitList,
        action: /Action.revenueSplitList,
        environment: { ($0.databaseClient, $0.uuid) }),

So is it correct to say the environment variable in the AnyReducer comes from the pullback environment

[natemann]

@mbrandonw yes, I think I just figured that out