provide the updated parent state to each reducer

[chriddyp]

hi folks!

In my app I've found that some reducers occasionally need access to other parts of the state in order to compute their own state.

My fork just passes in the parent state for each reducer to (optionally) use.

This probably isn't the best way to do this as combineReducers aims to keep each slice of the store logically independent. I'm curious to hear how other folks have solved this.

[d6u]

I found it most of the time I can solve this by making derived values computed when connecting with components by using https://github.com/reactjs/reselect. For two reason:

• Store structure can be minimal this way, no dependencies inside store
• Storing dependencies inside store can be confusing. Imagine this scenario: you make two API calls to the server, say tags, and posts, posts has tag_id on it. So in order to display the tag name on post you need to link posts with tags. If you calculate that in reducer, you could ended up having to handle the case when one arrived but the other didn't twice, one inside posts reducer, one in tags reducer. This duplication in large app is hard to manage.

[chriddyp]

Good suggestion @d6u !

I've solved this for my particular use case by constructing an additional "fullState" reducer outside of combine reducers:

slicedReducers = combineReducers({stateSlice1, stateSlice2, stateSlice3});

module.exports = function(fullState, action) {
    let nextState = slicedReducers(fullState, action);
    return fullStateReducer(nextState, action);
}

[gaearon]

Yep, both approaches work and I’m not sure whether we want to be opinionated here.
See also discussion in #1315.