Ordering of Scope(s) and Reduce in ReducerProtocol body

[mluisbrown]

Just starting on migration our project to ReducerProtocol and have a question about ordering. Previously child reducers were combined through pullbacks and necessarily came before the parent reducer, and this made changes in the child state visible to the parent reducer. Is this still the case with RedcuerProtocol? ie:

Should we do this:

var body: some ReducerProtocol {
  Scope(state: \.child1, action: /Action.child1) {
    Child1()
  }
  Scope(state: \.child2, action: /Action.child2) {
    Child2()
  }
  Reduce { state, action in 
    // parent reducer switch 
  }
}

or this:

var body: some ReducerProtocol {
  Reduce { state, action in 
    // parent reducer switch 
  }
  Scope(state: \.child1, action: /Action.child1) {
    Child1()
  }
  Scope(state: \.child2, action: /Action.child2) {
    Child2()
  }
}

The documentation has examples of both, although the documentation for Scope seems to imply that Scopes should come before the parent reducer.

Related to this question, we use the onChange reducer from IsoWords quite a lot and I was very pleased to see that there is version for ReducerProtocol in the IsoWords repo (https://github.com/pointfreeco/isowords/blob/main/Sources/TcaHelpers/OnChange.swift) which we are using.

My question is, with the following code:

var body: some ReducerProtocol {
  Scope(state: \.child1, action: /Action.child1) {
    Child1()
  }
  Scope(state: \.child2, action: /Action.child2) {
    Child2()
  }
  Reduce { state, action in 
    // parent reducer switch 
  }
  .onChange(of: \.child1.foo) { ... }
}

will the onChange reducer detect changes made only by the parent Reduce or also changes made with the Scoped child reducers? It seems to be only detecting changes made in the parent Reduce but I haven't definitely been able to confirm that yet.

[mluisbrown]

It seems to be only detecting changes made in the parent Reduce but I haven't definitely been able to confirm that yet.

I have now tested it and can confirm that onChange is only detecting changes made in the Reduce to which it is attached, not any changes made in the Child1() reducer.

[Alex293]

No worries mark yourself as the answer ! I would have suggested

@ReducerBuilder<State, Action>
var coreReducers: some ReducerProtocol {
}

but indeed there is a nicer tool in the lib for that

[mluisbrown]

Plot twist: if I use CombineReducers the compiler gives up with The compiler is unable to type-check this expression in reasonable time; try breaking up the expression into distinct sub-expressions. Using a @ReducerBuilder computed property doesn't have this issue. FWIW this is a huge reducer 😄

[tgrapperon]

@mluisbrown There was an issue with this reducer on Xcode 14 beta, and a trick was used to help the compiler. Are you using the latest version of Xcode?

[mluisbrown]

@tgrapperon we're using Xcode 14.1, soon switching to 14.2.

[tgrapperon]

@mluisbrown OK, thank! I'll try to reproduce then.