onChange not work for state change by setter

[zpbc007]

Description

onChange not work for state change by setter.

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

onChange should trigger when the parentNumber changed

Actual behavior

onChange only trigger when the parentNumber changed by ParentFeature，when ChildFeature change it's number, ParentFeature.State.number changed but onChange not fire

Steps to reproduce

struct ChildFeature: Reducer {
    struct State: Equatable {
        var number: Int
    }
    
    enum Action: Equatable {
        case addNum
    }
    
    func reduce(into state: inout State, action: Action) -> Effect<Action> {
        switch action {
        case .addNum:
            state.number += 1
            return .none
        }
    }
}
struct ChildView: View {
    let store: StoreOf<ChildFeature>
    
    var body: some View {
        WithViewStore(self.store, observe: { $0 }) {viewStore in
            HStack {
                Text("child num: \(viewStore.number)")
                Button("+") {
                    viewStore.send(.addNum)
                }
            }
        }
    }
}

struct ParentFeature: Reducer {    
    struct State: Equatable {
        var parentNumber: Int
        
        var childState: ChildFeature.State {
            get {.init(number: parentNumber)}
            set {parentNumber = newValue.number}
        }
        
        var latestInfo = ""
    }
    
    enum Action {
        case add
        case child(ChildFeature.Action)
    }
    
    var body: some ReducerOf<Self> {
        Scope(state: \.childState, action: /Action.child) { 
            ChildFeature()
        }
        
        Reduce {state, action in
            switch action {
            case .add:
                state.parentNumber += 1
                return .none
            case .child:
                return .none
            }
        }
        .onChange(of: \.parentNumber) { oldValue, newValue in
            Reduce { state, action in
                state.latestInfo = "oldValue: \(oldValue), newValue: \(newValue)"
                return .none
            }
        }
    }
}
struct ContentView: View {
    let store: StoreOf<ParentFeature>
    
    var body: some View {
        WithViewStore(self.store, observe: { $0 }) { viewStore in
            VStack(alignment: .leading, spacing: 10) {
                Text("latestInfo: \(viewStore.latestInfo)")
                
                HStack {
                    Text("Parent num: \(viewStore.parentNumber)")
                    Button("+") {viewStore.send(.add)}
                }
                
                ChildView(store: self.store.scope(state: \.childState, action: { .child($0) }))
            }
        }
    }
}

The Composable Architecture version information

No response

Destination operating system

No response

Xcode version information

No response

Swift Compiler version information

No response

[stephencelis]

@zpbc007 The onChange modifier only applies to the reducer you chain onto it with. So in this case. the Reduce. If you want it to apply to the all of the child reducers, you must first combine them using CombineReducers (or in their own conformance/builder):

+CombineReducers {
   Scope(state: \.childState, action: /Action.child) {
     ChildFeature()
   }

   Reduce {state, action in
     switch action {
     case .add:
       state.parentNumber += 1
       return .none
     case .child:
       return .none
     }
   }
+}
 .onChange(of: \.parentNumber) { oldValue, newValue in
   Reduce { state, action in
     state.latestInfo = "oldValue: \(oldValue), newValue: \(newValue)"
     return .none
   }
 }

Since this isn't a bug with the library, I'm going to convert it to a discussion.

[zpbc007]

It worked!.Thanks a lot.I'm a newbie to swift and TCA.
I think adding more explanations about CombineReducers will help novices get started and solve similar problems.