How to update child state from parent reducer by trigger child action

[vvisionnn]

Hey guys, I have been using composable architecture with UIKit for few months, and I still cannot find a better way to figure it out that how to update the child state from the parent reducer

things is this, think we have a Parent Feature and a Child Feature, and for the Parent and Child, both of them got an action called refresh like below

struct Parent: ReducerProtocol {
    struct State: Equatable {
        ......
        var child: Child.State?
    }

    enum Action: Equatable {
        case refresh
    }

......
    var Body: some ReducerProtocol<State, Action> { Reduce { state, action in
        switch action {
        case .refresh:
            /* some parent state update */
            // then update the child state by trigger the action called `Child.Action.refresh`
        return .none
    }
}

struct Child: ReducerProtocol {
    @Dependency(\.database) var db

    struct State: Equatable {
        ......
    }

    enum Action: Equatable {
        case refresh
    }

......
    var Body: some ReducerProtocol<State, Action> { Reduce { state, action in
        switch action {
        case .refresh:
            /* some child state update from db */
            state.a = db.findA()
            state.b = db.findB()
        return .none
    }
}

things is, some times there may have no directly connection from parent state to child state, which means that I cannot using the compute property to calculate the new child state

like we have modified some database rows out of the Parent, but we want to update the whole parent state because some (not one) child states inside parent should reflect the database value which we have modified before, the child refresh action will handle this but I cannot trigger it from parent refresh action

I have discovered several ways to solve this,

1. first is I will modify the child state directly from the parent refresh action, that will work but I cannot guarantee that will cover all the property inside child state
2. second is I will trigger the action by create a new reducer then call the reduce(into:action:) method, like Child().reduce(&state.child, action: .refresh) that works, in some way, but if the child refresh returns the side effect, like .merge([.refreshA, .refreshB]) or just .refreshC, I can only loop refresh it, or somedays before that I just missed it

I think non of these solved my problem in some way, and I just want to know if there is an elegant and simple method to help me fix this, if so that would be helpful

[Alex293]

To make the parent send actions to the child, the parent feature should be composed with the child feature.

For example you can add a case child(ChildFeature.Action) to the parent actions. Make the parent reduce func or body run the child with ifLet or Scope. Then In your parent refresh action return Effect(value: .child(.refresh)) instead of .none.

There are multiples ways of doing this depending on the relation of you features. If they are unrelated you could have a third reducer that compose your features and relay the action

[vvisionnn]

Thanks! that is correct,I don't know why that I have tried this before and won't work, maybe something I have misconfigured and I don't know, this is really helpful, thanks @Alex293 !