How to compose redux reducers with dependent state

Question

I am working on a React/Redux application that allows for \"widgets\" to be added to a page and manipulated in 2D space. It is a requirement that multiple widgets can be selecte

Answer 1

You can try to use:

redux-named-reducers

Which allows you to get state anywhere in your code like so:

const localState1 = getState(reducerA.state1)
const localState2 = getState(reducerB.state2)

Or to assign external dependent state to your reducers like so:

reducerA.extState1 = reducerB.state1

Then access it like so:

getState(reduerA.extState1)

Answer 2

This is a good use case for Reselect:

Create a selector to access your state and return derived data.

As an example:

import { createSelector } from 'reselect'

const widgetsSelector = state => state.widgets;
const selectedWidgetsSelector = state => state.selection.widgets;

function minCoordinateSelector(widgets, selected) {
  const x_list = selected.map((widget) => {
    return widgets[widget].x;
  });

  const y_list = selected.map((widget) => {
    return widgets[widget].y;
  });

  return {
    x: Math.min(...x_list),
    y: Math.min(...y_list)
  };
}

const coordinateSelector = createSelector(
  widgetsSelector,
  selectedWidgetsSelector,
  (widgets, selected) => {
    return minCoordinateSelector(widgets, selected);
  }
);

The coordinateSelector now provides access to your min x and y properties. The above selector will only be updated when either the widgetsSelector or selectedWidgetsSelector state changes, making this a very performant way to access the properties you are after and avoids duplication in your state tree.

Answer 3

I am using this:

CommonReducer.js

export default function commonReducer(state = initialState.doesNotMatter, payload) {
   switch (payload.type) {
     case "JOINED_TYPE": {
       let newState = Object.assign({}, state);
       return newState;
     }
     default:
       return state;
  }
}

SomeOtherReducer.js

import commonReducer from './commonReducer';

export default function someReducer(state = initialState.somePart, payload) {
       switch (payload.type) {
         case "CUSTOM_TYPE": {
           let newState = Object.assign({}, state);
           return newState;
         }
         default:{
           return commonReducer(state, payload);
         }
      }
}

Answer 4

If you use the thunk middleware (https://github.com/gaearon/redux-thunk) you can make the SET_SELECTION action a thunk, which will allow it to read entire state before making the dispatch that will be received by your reducer.

// action creator
function setSelection(selectedWidgetId) {
    return (dispatch, getState) => {
        const {widgets} = this.getState();
        const coordinates = getSelectionCoordinates(widgets, selectedWidgetIds);

        dispatch({
            type: SET_SELECTION,
            payload: {
                widgets: selectedWidgets,
                x: coordinates.x,
                y: coordinates.y
            }
        });
}

This way you get all the information you need in your selection reducer without having to pass along the list of all widget-objects to your action.