Logging `lift` operations ?

[npvisual]

@luizmb, I am finally getting to look at the log output from the LoggerMiddleware. It's pretty awesome so far !

Issue

I was debugging a state change for a modal sheet that was supposed to be presented when a button / icon was pressed.. but was not 🤨.

...
    var body: some View {
        NavigationView {
            List {
                ...
            }
            .navigationBarItems(trailing: profileButton)
            .sheet(isPresented: viewModel.binding[\.isShowingUserProfile]) { Text("User Profile") }
        }
    }

    var profileButton: some View {
        Button(action: { viewModel.dispatch(.showUserProfile) }) {
            Image(systemName: viewModel.state.iconName)
                .imageScale(.large)
                .accessibility(
                    label: Text(viewModel.state.iconAccessibilityLabel.localizedCapitalized)
                )
                .padding()
        }
    }

Initially I was seeing the appropriate actions being triggered :

2020-08-27 15:07:11.374696-0500 App[7300:453348]
🕹 navigation(App.NavigationAction.showUserProfile)
🎪 Home.swift:34 profileButton

However AppState was never changing. Took me 5 min (!) to figure out that the Reducer for that Navigation part had never been added to the global Reducer.app to lift the NavigationAction :

extension Reducer where ActionType == AppAction, StateType == AppState {
    static let app =
<reducer1>
          <>
<reducer2>
          <>
<reducer3>
---- missing --->
          <> Reducer<NavigationAction, AppState>.navigation.lift(
            action: \AppAction.navigation
---- missing ---<
        ) <> Reducer<AppLifecycleAction, AppLifecycle>.lifecycle.lift(
            action: \AppAction.appLifecycle,
            state: \AppState.appLifecycle
        )
}

Once this was added, things worked great. However the trace output was no different than what I had before (except for the State that had not changed).

Request

Would it be possible to have a trace that highlights the "part" reducers getting lifted all the way up to the top Reducer (i.e. seeing the chain) so that, in my case for example, I would have seen that while the

🕹 navigation(App.NavigationAction.showUserProfile)

was indeed being triggered, it was going nowhere.

[luizmb]

We could wrap every reducer in Logger Reducer which would be something like this:

let loggerReducer: (String, Reducer<AppAction, AppState>) -> Reducer<AppAction, AppState> = { name, reducer in
    return Reducer { action, state in
        print("Reducing \(action) on \(name)")
        reducer(action, &state)
    }
}

// Usage
loggerReducer("SomeReducer", someReducer)
<> loggerReducer("NavigationReducer", Reducer<NavigationAction, AppState>.navigation.lift(action: \AppAction.navigation))

Written without compiler, it may need some adjustments.
My only problem with that is the fact that reducer is performing side-effect (print), which is not a big deal.
If you plan to print more that action, for example state diff, then I recommend dispatching to a low-QoS queue to avoid overloading your main queue.

[luizmb]

If you make this a func generic over ActionType and StateType, then you can wrap it before lifting, which will be great because will discard any action that the inner reducer doesn't actually handle.

[npvisual]

I agree that having side-effects on the Reducer isn't ideal. The wrapper would be a good start. I was also looking at potentially adding logging inside the Reducer's reduce func. Not sure yet which way I'll go.

[luizmb]

The wrapper can easily become an extension such as:

Reducer<NavigationAction, AppState>
  .navigation
  .logging("Navigation")
  .lift(action: \AppAction.navigation)

Adding to the "reduce" function makes it baked into your app logic and hard to reuse, imo.
".logging" extension may also have a #if DEBUG condition so you don't log that in production.

extension Reducer {
    func logging(_ name: String) -> Reducer {
        Reducer { action, state in
            print("Reducing \(action) on \(name)")
            self.reduce(action, &state)
        }
    }
}